Section 19.8. PyForm: A Persistent Object Viewer


19.8. PyForm: A Persistent Object Viewer

Instead of going into additional database interface details that are freely available at Python.org, I'm going to close out this chapter by showing you one way to combine the GUI technology we met earlier in the text with the persistence techniques introduced in this chapter. This section presents PyForm, a Tkinter GUI designed to let you browse and edit tables of records:

  • Tables browsed are shelves, DBM files, in-memory dictionaries, or any other object that looks and feels like a dictionary.

  • Records within tables browsed can be class instances, simple dictionaries, strings, or any other object that can be translated to and from a dictionary.

Although this example is about GUIs and persistence, it also illustrates Python design techniques. To keep its implementation both simple and type-independent, the PyForm GUI is coded to expect tables to look like dictionaries of dictionaries. To support a variety of table and record types, PyForm relies on separate wrapper classes to translate tables and records to the expected protocol:

  • At the top table level, the translation is easyshelves, DBM files, and in-memory dictionaries all have the same key-based interface.

  • At the nested record level, the GUI is coded to assume that stored items have a dictionary-like interface too, but classes intercept dictionary operations to make records compatible with the PyForm protocol. Records stored as strings are converted to and from the dictionary objects on fetches and stores; records stored as class instances are translated to and from attribute dictionaries. More specialized translations can be added in new table wrapper classes.

The net effect is that PyForm can be used to browse and edit a wide variety of table types, despite its dictionary interface expectations. When PyForm browses shelves and DBM files, table changes made within the GUI are persistentthey are saved in the underlying files. When used to browse a shelve of class instances, PyForm essentially becomes a GUI frontend to a simple object database that is built using standard Python persistence tools.

19.8.1. Processing Shelves with Code

Before we get to the GUI, though, let's see why you'd want one in the first place. To experiment with shelves in general, I first coded a canned test datafile. The script in Example 19-19 hardcodes a dictionary used to populate databases (cast), as well as a class used to populate shelves of class instances (Actor).

Example 19-19. PP3E\Dbase\testdata.py

 # definitions for testing shelves, dbm, and formgui cast = {     'rob':   {'name': ('Rob', 'P'),   'job': 'writer', 'spouse': 'Laura'},     'buddy': {'name': ('Buddy', 'S'), 'job': 'writer', 'spouse': 'Pickles'},     'sally': {'name': ('Sally', 'R'), 'job': 'writer'},     'laura': {'name': ('Laura', 'P'), 'spouse': 'Rob',   'kids':1},     'milly': {'name': ('Milly', '?'), 'spouse': 'Jerry', 'kids':2},     'mel':   {'name': ('Mel', 'C'),   'job': 'producer'},     'alan':  {'name': ('Alan', 'B'),  'job': 'comedian'} } class Actor:                                       # unnested file-level class     def _ _init_ _(self, name=( ), job=''):            # no need for arg defaults,         self.name = name                           # for new pickler or formgui         self.job  = job     def _ _setattr_ _(self, attr, value):              # on setattr( ): validate         if attr == 'kids' and value > 10:          # but set it regardless             print 'validation error: kids =', value         if attr == 'name' and type(value) != type(( )):             print 'validation error: name type =', type(value)         self._ _dict_ _[attr] = value                # don't trigger _ _setattr_ _ 

The cast object here is intended to represent a table of records (it's really a dictionary of dictionaries when written out in Python syntax like this). Now, given this test data, it's easy to populate a shelve with cast dictionaries. Simply open a shelve and copy over cast, key for key, as shown in Example 19-20.

Example 19-20. PP3E\Dbase\castinit.py

 import shelve from testdata import cast db = shelve.open('data/castfile')      # create a new shelve for key in cast.keys( ):     db[key] = cast[key]                # store dictionaries in shelve 

Once you've done that, it's almost as easy to verify your work with a script that prints the contents of the shelve, as shown in Example 19-21.

Example 19-21. PP3E\Dbase\castdump.py

 import shelve db = shelve.open('data/castfile')        # reopen shelve for key in db.keys( ):                  # show each key,value     print key, db[key] 

Here are these two scripts in action, populating and displaying a shelve of dictionaries:

 ...\PP3E\Dbase>python castinit.py ...\PP3E\Dbase>python castdump.py alan {'job': 'comedian', 'name': ('Alan', 'B')} mel {'job': 'producer', 'name': ('Mel', 'C')} buddy {'spouse': 'Pickles', 'job': 'writer', 'name': ('Buddy', 'S')} sally {'job': 'writer', 'name': ('Sally', 'R')} rob {'spouse': 'Laura', 'job': 'writer', 'name': ('Rob', 'P')} milly {'spouse': 'Jerry', 'name': ('Milly', '?'), 'kids': 2} laura {'spouse': 'Rob', 'name': ('Laura', 'P'), 'kids': 1} 

So far, so good; but here is where you reach the limitations of manual shelve processing: to modify a shelve you need much more general tools. You could write little Python scripts that each perform very specific updates. Or you might even get by for awhile typing such update commands by hand in the interactive interpreter:

 >>> import shelve >>> db  = shelve.open('data/castfile') >>> rec = db['rob'] >>> rec['job'] = 'hacker' >>> db['rob'] = rec 

For all but the most trivial databases, though, this will get tedious in a hurryespecially for a system's end users. What you'd really like is a GUI that lets you view and edit shelves arbitrarily, and that can be started up easily from other programs and scripts, as shown in Example 19-22.

Example 19-22. PP3E\Dbase\castview.py

 import shelve from TableBrowser.formgui import FormGui    # after initcast db = shelve.open('data/castfile')           # reopen shelve file FormGui(db).mainloop( )                           # browse existing shelve-of-dicts 

To make this particular script work, we need to move on to the next section.

19.8.2. Adding a Graphical Interface

The path traced in the last section really is what led me to write PyForm, a GUI tool for editing arbitrary tables of records. When those tables are shelves and DBM files, the data PyForm displays is persistent; it lives beyond the GUI's lifetime. Because of that, PyForm can be seen as a simple database browser.

We've already met all the GUI interfaces PyForm uses earlier in this book, so I won't go into all of its implementation details here (see the chapters in Part III for background details). Before we see the code at all, though, let's see what it does. Figure 19-1 shows PyForm in action on Windows, browsing a shelve of persistent instance objects, created from the testdata module's Actor class. It looks slightly different but works the same on Linux and Macs.

Figure 19-1. PyForm displaying a shelve of Actor objects


PyForm uses a three-window interface to the table being browsed; all windows are packed for proper window expansion and clipping, as set by the rules we studied earlier in this book. The window in the upper left of Figure 19-1 is the main window, created when PyForm starts; it has buttons for navigating through a table, finding items by key, and updating, creating, and deleting records (more useful when browsing tables that persist between runs). The table (dictionary) key of the record currently displayed shows up in the input field in the middle of this window.

The "index" button pops up the listbox window in the upper right, and selecting a record in either window at the top creates the form window at the bottom. The form window is used both to display a record and to edit itif you change field values and press "store," the record is updated. Pressing "new" clears the form for input of new values (fill in the Key=> field and press "store" to save the new record).

Field values are typed with Python syntax, so strings are quoted (more on this later). When browsing a table with records that contain different sets of field names, PyForm erases and redraws the form window for new field sets as new records are selected. To avoid seeing the window re-created, use the same format for all records within a given table.

19.8.3. PyForm GUI Implementation

On to the code; the first thing I did when writing PyForm was to code utility functions to hide some of the details of widget creation. By making a few simplifying assumptions (e.g., packing protocol), the module in Example 19-23 helps keep some GUI coding details out of the rest of the PyForm implementation.

Example 19-23. PP3E\Dbase\TableBrowser\guitools.py

 # added extras for entry width, calcgui font/color from Tkinter import * def frame(root, side, **extras):     widget = Frame(root)     widget.pack(side=side, expand=YES, fill=BOTH)     if extras: widget.config(**extras)                     # or apply(f, ( ), {})     return widget def label(root, side, text, **extras):     widget = Label(root, text=text, relief=RIDGE)     widget.pack(side=side, expand=YES, fill=BOTH)     if extras: widget.config(**extras)     return widget def button(root, side, text, command, **extras):     widget = Button(root, text=text, command=command)     widget.pack(side=side, expand=YES, fill=BOTH)     if extras: widget.config(**extras)     return widget def entry(root, side, linkvar, **extras):     widget = Entry(root, relief=SUNKEN, textvariable=linkvar)     widget.pack(side=side, expand=YES, fill=BOTH)     if extras: widget.config(**extras)     return widget 

Armed with this utility module, the file in Example 19-24 implements the rest of the PyForm GUI. It uses the GuiMixin module we wrote in Chapter 11, for simple access to standard pop-up dialogs. It's also coded as a class that can be specialized in subclasses or attached to a larger GUI. I run PyForm as a standalone program. Attaching its FormGui class really attaches its main window only, but it can be used to provide a precoded table browser widget for other GUIs.

This file's FormGui class creates the GUI shown in Figure 19-1 and responds to user interaction in all three of the interface's windows. Because we've already covered all the GUI tools that PyForm uses, you should study this module's source code listing for additional implementation details. Notice, though, that this file knows almost nothing about the table being browsed, other than that it looks and feels like a dictionary of dictionaries. To understand how PyForm supports browsing things such as shelves of class instances, you will need to look elsewhere (or at least wait for the next module).

Example 19-24. PP3E\Dbase\TableBrowser\formgui.py

 #!/usr/local/bin/python ############################################################################# # PyForm: a persistent table viewer GUI. Uses guimixin for std dialogs. # Assumes the browsed table has a dictionary-of-dictionary interface, and # relies on table wrapper classes to convert other structures as needed. # Store an initial record with dbinit script to start a dbase from scratch. # Caveat: doesn't do object method calls, shows complex field values poorly. ############################################################################# from Tkinter  import *                                 # Tk widgets from guitools import frame, label, button, entry       # widget builders from PP3E.Gui.Tools.guimixin import GuiMixin           # common methods class FormGui(GuiMixin, Frame):     def _ _init_ _(self, mapping):                       # an extended frame         Frame._ _init_ _(self)                           # on default top-level         self.pack(expand=YES, fill=BOTH)                  # all parts expandable         self.master.title('PyForm 2.0 - Table browser')         self.master.iconname("PyForm")         self.makeMainBox( )         self.table     = mapping                 # a dict, dbm, shelve, Table,..         self.index     = mapping.keys( )        # list of table keys         self.cursor    = -1                      # current index position         self.currslots = []                      # current form's (key,text)s         self.currform  = None                  # current form window         self.listbox   = None                  # index listbox window     def makeMainBox(self):         frm = frame(self, TOP)         frm.config(bd=2)         button(frm, LEFT, 'next',  self.onNext)       # next in list         button(frm, LEFT, 'prev',  self.onPrev)       # backup in list         button(frm, LEFT, 'find',  self.onFind)       # find from key         frm = frame(self, TOP)         self.keytext = StringVar( )                    # current record's key         label(frm, LEFT, 'KEY=>')                       # change before 'find'         entry(frm, LEFT,  self.keytext)         frm = frame(self, TOP)         frm.config(bd=2)         button(frm,  LEFT,  'store',  self.onStore)     # updated entry data         button(frm,  LEFT,  'new',    self.onNew)       # clear fields         button(frm,  LEFT,  'index',  self.onMakeList)  # show key list         button(frm,  LEFT,  'delete', self.onDelete)    # show key list         button(self, BOTTOM,'quit',   self.quit)        # from guimixin     def onPrev(self):         if self.cursor <= 0:             self.infobox('Backup', "Front of table")         else:             self.cursor -= 1             self.display( )     def onNext(self):         if self.cursor >= len(self.index)-1:             self.infobox('Advance', "End of table")         else:             self.cursor += 1             self.display( )     def sameKeys(self, record):                      # can we reuse the same form?         keys1 = record.keys( )                      # or map(lambda x:x[0], list)         keys2 = [x[0] for x in self.currslots]         keys1.sort(); keys2.sort( )                 # keys list order differs         return keys1 == keys2                        # if insertion-order differs     def display(self):         key = self.index[self.cursor]              # show record at index cursor         self.keytext.set(key)                      # change key in main box         record = self.table[key]                   # in dict, dbm, shelf, class         if self.sameKeys(record):             self.currform.title('PyForm - Key=' + repr(key))             for (field, text) in self.currslots:                 text.set(repr(record[field]))      # same fields? reuse form         else:                                      # repr(x) works like expr 'x'             if self.currform:                 self.currform.destroy( )            # different fields?             new = Toplevel( )                       # replace current box             new.title('PyForm - Key=' + repr(key))   # new resizable window             new.iconname("pform")             left  = frame(new, LEFT)             right = frame(new, RIGHT)             self.currslots = []                    # list of (field, entry)             for field in record.keys( ):                 label(left, TOP, repr(field))      # key,value to strings                 text = StringVar( )                     # we could sort keys here                 text.set( repr(record[field]) )                 entry(right, TOP, text, width=40)                 self.currslots.append((field, text))             self.currform = new             new.protocol('WM_DELETE_WINDOW', lambda:0)    # ignore destroy's         self.selectlist( )                                # update listbox     def onStore(self):         if not self.currform: return         key = self.keytext.get( )         if key in self.index:                    # change existing record             record = self.table[key]             # not: self.table[key][field]=         else:             record = {}                          # create a new record             self.index.append(key)               # add to index and listbox             if self.listbox:                 self.listbox.insert(END, key)    # or at len(self.index)-1         for (field, text) in self.currslots:             try:                                   # fill out dictionary rec                 record[field] = eval(text.get( )) # convert back from string             except:                 self.errorbox('Bad data: "%s" = "%s"' % (field, text.get( )))                 record[field] = None         self.table[key] = record                 # add to dict, dbm, shelf,...         self.onFind(key)                         # readback: set cursor,listbox     def onNew(self):         if not self.currform: return               # clear input form and key         self.keytext.set('?%d' % len(self.index))  # default key unless typed         for (field, text) in self.currslots:       # clear key/fields for entry             text.set('')         self.currform.title('Key: ?')     def onFind(self, key=None):         target = key or self.keytext.get( )            # passed in, or entered         try:             self.cursor = self.index.index(target)      # find label in keys list             self.display( )         except:             self.infobox('Not found', "Key doesn't exist", 'info')     def onDelete(self):         if not self.currform or not self.index: return         currkey = self.index[self.cursor]         del self.table[currkey]                      # table, index, listbox         del self.index[self.cursor:self.cursor+1]    # like "list[i:i+1] = []"         if self.listbox:             self.listbox.delete(self.cursor)         # delete from listbox         if self.cursor < len(self.index):             self.display( )                           # show next record if any         elif self.cursor > 0:             self.cursor = self.cursor-1              # show prior if delete end             self.display( )         else:                                        # leave box if delete last             self.onNew( )     def onList(self,evnt):         if not self.index: return                  # on listbox double-click         index = self.listbox.curselection( )            # fetch selected key text         label = self.listbox.get(index)            # or use listbox.get(ACTIVE)         self.onFind(label)                         # and call method here     def onMakeList(self):         if self.listbox: return                      # already up?         new = Toplevel( )                           # new resizable window         new.title("PyForm - Key Index")              # select keys from a listbox         new.iconname("pindex")         frm    = frame(new, TOP)         scroll = Scrollbar(frm)         list   = Listbox(frm, bg='white')         scroll.config(command=list.yview, relief=SUNKEN)         list.config(yscrollcommand=scroll.set, relief=SUNKEN)         scroll.pack(side=RIGHT, fill=BOTH)         list.pack(side=LEFT, expand=YES, fill=BOTH)    # pack last, clip first         for key in self.index:                         # add to list-box             list.insert(END, key)                      # or: sort list first         list.config(selectmode=SINGLE, setgrid=1)      # select,resize modes         list.bind('<Double-1>', self.onList)           # on double-clicks         self.listbox = list         if self.index and self.cursor >= 0:            # highlight position             self.selectlist( )         new.protocol('WM_DELETE_WINDOW', lambda:0)     # ignore destroy's     def selectlist(self):                              # listbox tracks cursor         if self.listbox:             self.listbox.select_clear(0, self.listbox.size( ))             self.listbox.select_set(self.cursor) if _ _name_ _ == '_ _main_ _':     from PP3E.Dbase.testdata import cast          # self-test code     for k in cast.keys( ): print k, cast[k]      # view in-memory dict-of-dicts     FormGui(cast).mainloop( )     for k in cast.keys( ): print k, cast[k]      # show modified table on exit 

The file's self-test code starts up the PyForm GUI to browse the in-memory dictionary of dictionaries called "cast" in the testdata module listed earlier. To start PyForm, you simply make and run the FormGui class object this file defines, passing in the table to be browsed. Here are the messages that show up in stdout after running this file and editing a few entries displayed in the GUI; the dictionary is displayed on GUI startup and exit:

 ...\PP3E\Dbase\TableBrowser>python formgui.py alan {'job': 'comedian', 'name': ('Alan', 'B')} sally {'job': 'writer', 'name': ('Sally', 'R')} rob {'spouse': 'Laura', 'job': 'writer', 'name': ('Rob', 'P')} mel {'job': 'producer', 'name': ('Mel', 'C')} milly {'spouse': 'Jerry', 'name': ('Milly', '?'), 'kids': 2} buddy {'spouse': 'Pickles', 'job': 'writer', 'name': ('Buddy', 'S')} laura {'spouse': 'Rob', 'name': ('Laura', 'P'), 'kids': 1} alan {'job': 'comedian', 'name': ('Alan', 'B')} jerry {'spouse': 'Milly', 'name': 'Jerry', 'kids': 0} sally {'job': 'writer', 'name': ('Sally', 'R')} rob {'spouse': 'Laura', 'job': 'writer', 'name': ('Rob', 'P')} mel {'job': 'producer', 'name': ('Mel', 'C')} milly {'spouse': 'Jerry', 'name': ('Milly', '?'), 'kids': 2} buddy {'spouse': 'Pickles', 'job': 'writer', 'name': ('Buddy', 'S')} laura {'name': ('Laura', 'P'), 'kids': 3, 'spouse': 'bob'} 

The last line represents a change made in the GUI. Since this is an in-memory table, changes made in the GUI are not retained (dictionaries are not persistent by themselves). To see how to use the PyForm GUI on persistent stores such as DBM files and shelves, we need to move on to the next topic.

19.8.4. PyForm Table Wrappers

The following file defines generic classes that "wrap" (interface with) various kinds of tables for use in PyForm. It's what makes PyForm useful for a variety of table types.

The prior module was coded to handle GUI chores, and it assumes that tables expose a dictionary-of-dictionaries interface. Conversely, this next module knows nothing about the GUI but provides the translations necessary to browse nondictionary objects in PyForm. In fact, this module doesn't even import Tkinter at allit deals strictly in object protocol conversions and nothing else. Because PyForm's implementation is divided into functionally distinct modules like this, it's easier to focus on each module's task in isolation.

Here is the hook between the two modules: for special kinds of tables, PyForm's FormGui is passed an instance of the Table class coded here. The Table class intercepts table index fetch and assignment operations and uses an embedded record wrapper class to convert records to and from dictionary format as needed.

For example, because DBM files can store only strings, Table converts real dictionaries to and from their printable string representation on table stores and fetches. For class instances, Table exTRacts the object's _ _dict_ _ attribute dictionary on fetches and copies a dictionary's fields to attributes of a newly generated class instance on stores.[*] The end result is that the GUI thinks the table is all dictionaries, even if it is really something very different here.

[*] Subtle thing revisited: like the new pickle module, PyForm tries to generate a new class instance on store operations by simply setting a generic instance object's _ _class_ _ pointer to the original class; only if this fails does PyForm fall back on calling the class with no arguments (in which case the class must have defaults for any constructor arguments other than self). Assignment to _ _class_ _ can fail in restricted execution mode. See the class InstanceRecord in the source listing for further details.

While you study this module's listing, shown in Example 19-25, notice that there is nothing here about the record formats of any particular database. In fact, there was none in the GUI-related formgui module either. Because neither module cares about the structure of fields used for database records, both can be used to browse arbitrary records.

Example 19-25. PP3E\Dbase\formtable.py

 ############################################################################# # PyForm table wrapper classes and tests # Because PyForm assumes a dictionary-of-dictionary interface, this module # converts strings and class instance records to and from dicts.  PyForm # contains the table mapping--Table is not a PyForm subclass.  Note that # some of the wrapper classes may be useful outside PyForm--DmbOfString can # wrap a dbm containing arbitrary datatypes.  Run the dbinit scripts to # start a new database from scratch, and run the dbview script to browse # a database other than the one tested here.  No longer requires classes to # have defaults in constructor args, and auto picks up record class from the # first one fetched if not passed in to class-record wrapper.  Caveat: still # assumes that all instances in a table are instances of the same class. ############################################################################ ############################################################################# # records within tables ############################################################################# class DictionaryRecord:     def todict(self, value):         return value                   # to dictionary: no need to convert     def fromdict(self, value):         return value                   # from dictionary: no need to convert class StringRecord:     def todict(self, value):         return eval(value)             # convert string to dictionary (or any)     def fromdict(self, value):         return str(value)              # convert dictionary (or any) to string class InstanceRecord:     def _ _init_ _(self, Class=None):     # need class object to make instances         self.Class = Class     def todict(self, value):           # convert instance to attr dictionary         if not self.Class:             # get class from obj if not yet known             self.Class = value._ _class_ _         return value._ _dict_ _     def fromdict(self, value):         # convert attr dictionary to instance         try:             class Dummy: pass                         # try what new pickle does             instance = Dummy( )                      # fails in restricted mode             instance._ _class_ _ = self.Class         except:                                       # else call class, no args             instance = self.Class( )                 # init args need defaults         for attr in value.keys( ):             setattr(instance, attr, value[attr])    # set instance attributes         return instance                             # may run Class._ _setattr_ _ ############################################################################# # table containing records ############################################################################# class Table:     def _ _init_ _(self, mapping, converter):    # table object, record converter         self.table  = mapping                     # wrap arbitrary table mapping         self.record = converter                   # wrap arbitrary record types     def storeItems(self, items):                 # initialize from dictionary         for key in items.keys( ):               # do _ _setitem_ _ to xlate, store             self[key] = items[key]     def printItems(self):                      # print wrapped mapping         for key in self.keys( ):                    # do self.keys to get table keys             print key, self[key]               # do _ _getitem_ _ to fetch, xlate     def _ _getitem_ _(self, key):                  # on tbl[key] index fetch         rawval = self.table[key]               # fetch from table mapping         return self.record.todict(rawval)      # translate to dictionary     def _ _setitem_ _(self, key, value):           # on tbl[key]=val index assign         rawval = self.record.fromdict(value)   # translate from dictionary         self.table[key] = rawval               # store in table mapping     def _ _delitem_ _(self, key):                  # delete from table mapping         del self.table[key]     def keys(self):                            # get table mapping keys index         return self.table.keys( )     def close(self):         if hasattr(self.table, 'close'):       # call table close if has one             self.table.close( )                     # may need for shelves, dbm ############################################################################# # table/record combinations ############################################################################# import shelve, anydbm def ShelveOfInstance(filename, Class=None):     return Table(shelve.open(filename), InstanceRecord(Class)) def ShelveOfDictionary(filename):     return Table(shelve.open(filename), DictionaryRecord( )) def ShelveOfString(filename):     return Table(shelve.open(filename), StringRecord( )) def DbmOfString(filename):     return Table(anydbm.open(filename, 'c'), StringRecord( )) def DictOfInstance(dict, Class=None):     return Table(dict, InstanceRecord(Class)) def DictOfDictionary(dict):     return Table(dict, DictionaryRecord( )) def DictOfString(filename):     return Table(dict, StringRecord( )) ObjectOfInstance   = DictOfInstance           # other mapping objects ObjectOfDictionary = DictOfDictionary         # classes that look like dicts ObjectOfString     = DictOfString ############################################################################# # test common applications ############################################################################# if _ _name_ _ == '_ _main_ _':     from sys import argv     from formgui import FormGui                    # get dict-based GUI     from PP3E.Dbase.testdata import Actor, cast    # get class, dict-of-dicts     TestType   = 'shelve'                          # shelve, dbm, dict     TestInit   = 0                                 # init file on startup?     TestFile   = '../data/shelve1'                 # external filename     if len(argv) > 1: TestType = argv[1]     if len(argv) > 2: TestInit = int(argv[2])     if len(argv) > 3: TestFile = argv[3]     if TestType == 'shelve':                       # Python formtbl.py shelve?         print 'shelve-of-instance test'         table = ShelveOfInstance(TestFile, Actor)  # wrap shelf in Table object         if TestInit:             table.storeItems(cast)                 # Python formtbl.py shelve 1         FormGui(table).mainloop( )         table.close( )         ShelveOfInstance(TestFile).printItems( )    # class picked up on fetch     elif TestType == 'dbm':                        # Python formtbl.py dbm         print 'dbm-of-dictstring test'         table = DbmOfString(TestFile)              # wrap dbm in Table object         if TestInit:             table.storeItems(cast)                 # Python formtbl.py dbm 1         FormGui(table).mainloop( )         table.close( )         DbmOfString(TestFile).printItems( )         # dump new table contents 

Besides the Table and record-wrapper classes, the module defines generator functions (e.g., ShelveOfInstance) that create a Table for all reasonable table and record combinations. Not all combinations are validDBM files, for example, can contain only dictionaries coded as strings because class instances don't easily map to the string value format expected by DBM. However, these classes are flexible enough to allow additional Table configurations to be introduced.

The only thing that is GUI related about this file at all is its self-test code at the end. When run as a script, this module starts a PyForm GUI to browse and edit either a shelve of persistent Actor class instances or a DBM file of dictionaries, by passing in the right kind of Table object. The GUI looks like the one we saw in Figure 19-1 earlier; when run without arguments, the self-test code lets you browse a shelve of class instances:

 ...\PP3E\Dbase\TableBrowser>python formtable.py shelve-of-instance test ...display of contents on exit... 

Because PyForm displays a shelve this time, any changes you make are retained after the GUI exits. To reinitialize the shelve from the cast dictionary in testdata, pass a second argument of 1 (0 means don't reinitialize the shelve). To override the script's default shelve filename, pass a different name as a third argument:

 ...\PP3E\Dbase\TableBrowser>python formtable.py shelve 1 ...\PP3E\Dbase\TableBrowser>python formtable.py shelve 0 ../data/shelve1 

To instead test PyForm on a DBM file of dictionaries mapped to strings, pass a dbm in the first command-line argument; the next two arguments work the same:

 ...\PP3E\Dbase\TableBrowser>python formtable.py dbm 1 ..\data\dbm1 dbm-of-dictstring test ...display of contents on exit... 

Finally, because these self-tests ultimately process concrete shelve and DBM files, you can manually open and inspect their contents using normal library calls. Here is what they look like when opened in an interactive session:

 ...\PP3E\Dbase\data>ls dbm1        myfile         shelve1 ...\PP3E\Dbase\data>python >>> import shelve >>> db = shelve.open('shelve1') >>> db.keys( ) ['alan', 'buddy', 'sally', 'rob', 'milly', 'laura', 'mel'] >>> db['laura'] <PP3E.Dbase.testdata.Actor instance at 799850> >>> import anydbm >>> db = anydbm.open('dbm1') >>> db.keys( ) ['alan', 'mel', 'buddy', 'sally', 'rob', 'milly', 'laura'] >>> db['laura'] "{'name': ('Laura', 'P'), 'kids': 2, 'spouse': 'Rob'}" 

The shelve file contains real Actor class instance objects, and the DBM file holds dictionaries converted to strings. Both formats are retained in these files between GUI runs and are converted back to dictionaries for later redisplay.[*]

[*] Note that DBM files of dictionaries use str and eval to convert to and from strings, but could also simply store the pickled representations of record dictionaries in DBM files instead using pickle. But since this is exactly what a shelve of dictionaries does, the str/eval scheme was chosen for illustration purposes here. Suggested exercise: add a new PickleRecord record class based upon the pickle module's loads and dumps functions described earlier in this chapter and compare its performance to StringRecord. See also the pickle file database structure in Chapter 14; its directory scheme with one flat-file per record could be used to implement a "table" here too, with appropriate Table subclassing.

19.8.5. PyForm Creation and View Utility Scripts

The formtable module's self-test code proves that it works, but it is limited to canned test-case files and classes. What about using PyForm for other kinds of databases that store more useful kinds of data?

Luckily, both the formgui and the formtable modules are written to be genericthey are independent of a particular database's record format. Because of that, it's easy to point PyForm to databases of your own; simply import and run the FormGui object with the (possibly wrapped) table you wish to browse.

The required startup calls are not too complex, and you could type them at the interactive prompt every time you want to browse a database; but it's usually easier to store them in scripts so that they can be reused. The script in Example 19-26, for example, can be run to open PyForm on any shelve containing records stored in class instance or dictionary format.

Example 19-26. PP3E\Dbase\dbview.py

 ################################################################## # view any existing shelve directly; this is more general than a # "formtable.py shelve 1 filename" cmdline--only works for Actor; # pass in a filename (and mode) to use this to browse any shelve: # formtable auto picks up class from the first instance fetched; # run dbinit1 to (re)initialize dbase shelve with a template. ################################################################## from sys import argv from formtable import * from formgui import FormGui mode = 'class' file = '../data/mydbase-' + mode if len(argv) > 1: file = argv[1]                  # dbview.py file? mode?? if len(argv) > 2: mode = argv[2] if mode == 'dict':     table = ShelveOfDictionary(file)              # view dictionaries else:     table = ShelveOfInstance(file)                # view class objects FormGui(table).mainloop( ) table.close( )                                     # close needed for some dbm 

The only catch here is that PyForm doesn't handle completely empty tables very well; there is no way to add new records within the GUI unless a record is already present. That is, PyForm has no record layout design tool; its "new" button simply clears an existing input form.

Because of that, to start a new database from scratch, you need to add an initial record that gives PyForm the field layout. Again, this requires only a few lines of code that could be typed interactively, but why not instead put it in generalized scripts for reuse? The file in Example 19-27 shows one way to go about initializing a PyForm database with a first empty record.

Example 19-27. PP3E\Dbase\dbinit1.py

 ###################################################################### # store a first record in a new shelve to give initial fields list; # PyForm GUI requires an existing record before you can add records; # delete the '?' key template record after real records are added; # change mode, file, template to use this for other kinds of data; # if you populate shelves from other datafiles you don't need this; # see dbinit2 for object-based version, and dbview to browse shelves. ###################################################################### import os from sys import argv mode = 'class' file = '../data/mydbase-' + mode if len(argv) > 1: file = argv[1]                          # dbinit1.py file? mode?? if len(argv) > 2: mode = argv[2] try:     os.remove(file)                                       # delete if present except: pass if mode == 'dict':     template = {'name': None, 'age': None, 'job': None}   # start dict shelve else:     from PP3E.Dbase.person import Person                  # one arg defaulted     template = Person(None, None)                         # start object shelve import shelve dbase = shelve.open(file)                                 # create it now dbase['?empty?'] = template dbase.close( ) 

Now, simply change some of this script's settings or pass in command-line arguments to generate a new shelve-based database for use in PyForm. You can substitute any fields list or class name in this script to maintain a simple object database with PyForm that keeps track of real-world information (we'll see two such databases in action in a moment).

The empty record created by this script shows up with the key ?empty? when you first browse the database in PyForm with dbview; replace it with a first real record using the PyForm store key, and you are in business. As long as you don't change the database's shelve outside of the GUI, all of its records will have the same fields format, as defined in the initialization script.

But notice that the dbinit1 script goes straight to the shelve file to store the first record; that's fine today, but it might break if PyForm is ever changed to do something more custom with its stored data representation. Perhaps a better way to populate tables outside the GUI is to use the Table wrapper classes it employs. The following alternative script, for instance, initializes a PyForm database with generated Table objects, not direct shelve operations (see Example 19-28).

Example 19-28. PP3E\Dbase\dbinit2.py

 ################################################################# # this works too--based on Table objects not manual shelve ops; # store a first record in shelve, as required by PyForm GUI. ################################################################# from formtable import * import sys, os mode = 'dict' file = '../data/mydbase-' + mode if len(sys.argv) > 1: file = sys.argv[1] if len(sys.argv) > 2: mode = sys.argv[2] try:     os.remove(file) except: pass if mode == 'dict':     table    = ShelveOfDictionary(file)     template = {'name': None, 'shoesize': None, 'language': 'Python'} else:     from PP3E.Dbase.person import Person     table    = ShelveOfInstance(file, Person)     template = Person(None, None)._ _dict_ _ table.storeItems({'?empty?': template}) table.close( ) 

19.8.5.1. Creating and browsing custom databases

Let's put the prior section's scripts to work to initialize and edit a couple of custom databases. Figure 19-2 shows one being browsed after initializing the database with a script and adding a handful of real records within the GUI.

Figure 19-2. A shelve of Person objects (dbinit1, dbview)


The listbox here shows the record I added to the shelve within the GUI. I ran the following commands to initialize the database with a starter record and to open it in PyForm to add records (that is, Person class instances):

 ...\PP3E\Dbase\TableBrowser>python dbinit1.py ...\PP3E\Dbase\TableBrowser>python dbview.py 

You can tweak the class name or fields dictionary in the dbinit scripts to initialize records for any sort of database you care to maintain with PyForm; use dictionaries if you don't want to represent persistent objects with classes (but classes let you add other sorts of behavior as methods not visible under PyForm). Be sure to use a distinct filename for each database; the initial ?empty? record can be deleted as soon as you add a real entry (later, simply select an entry from the listbox and press "new" to clear the form for input of a new record's values).

The data displayed in the GUI represents a true shelve of persistent Person class instance objectschanges and additions made in the GUI will be retained for the next time you view this shelve with PyForm. If you like to type, though, you can still open the shelve directly to check PyForm's work:

 ...\PP3E\Dbase\data>ls mydbase-class  myfile         shelve1 ...\PP3E\Dbase\data>python >>> import shelve >>> db = shelve.open('mydbase-class') >>> db.keys( ) ['emily', 'jerry', '?empty?', 'bob', 'howard'] >>> db['bob'] <PP3E.Dbase.person.Person instance at 798d70> >>> db['emily'].job 'teacher' >>> db['bob'].tax 30000.0 

Notice that bob is an instance of the Person class we met earlier in this chapter (see the section "Shelve Files"). Assuming that the person module is still the version that introduced a _ _getattr_ _ method, asking for a shelved object's tax attribute computes a value on the fly because this really invokes a class method. Also note that this works even though Person was never imported herePython loads the class internally when re-creating its shelved instances.

You can just as easily base a PyForm-compatible database on an internal dictionary structure, instead of on classes. Figure 19-3 shows one being browse after being initialized with a script and populated with the GUI.

Figure 19-3. A shelve of dictionaries (dbinit2, dbview)


Besides its different internal format, this database has a different record structure (its record's field names differ from the last example), and it is stored in a shelve file of its own. Here are the commands I used to initialize and edit this database:

 ...\PP3E\Dbase\TableBrowser>python dbinit2.py ../data/mydbase-dict dict ...\PP3E\Dbase\TableBrowser>python dbview.py ../data/mydbase-dict dict 

After adding a few records (that is, dictionaries) to the shelve, you can either view them again in PyForm or open the shelve manually to verify PyForm's work:

 ...\PP3E\Dbase\data>ls mydbase-class  mydbase-dict   myfile         shelve1 ...\PP3E\Dbase\data>python >>> db = shelve.open('mydbase-dict') >>> db.keys( ) ['tom', 'guido', '?empty?', 'larry', 'randal', 'mel'] >>> db['guido'] {'shoesize': 42, 'name': 'benevolent dictator', 'language': 'Python'} >>> db['mel']['shoesize'] {'left': 7.5, 'right': 7L} 

This time, shelve entries are really dictionaries, not instances of a class or converted strings. PyForm doesn't care, thoughbecause all tables are wrapped to conform to PyForm's interface, both formats look the same when browsed in the GUI.

19.8.6. Data as Code

Notice that the shoesize and language fields in the screenshot in Figure 19-3 really are a dictionary and a list. You can type any Python expression syntax into this GUI's form fields to give values (that's why strings are quoted there).

PyForm uses the Python built-in repr function to convert value objects for display (repr(x) is like the older 'x' expression and is similar to str(x) but yields an as-code display that adds quotes around strings). To convert from a string back to value objects, PyForm uses the Python eval function to parse and evaluate the code typed into fields. The key entry/display field in the main window does not add or accept quotes around the key string because keys must still be strings in things such as shelves (even though fields can be arbitrary types).

As we've seen at various points in this book, eval (and its statement cousin, exec) is powerful but dangerousyou never know when a user might type something that removes files, hangs the system, emails your boss, and so on. If you can't be sure that field values won't contain harmful code (whether malicious or otherwise), use the rexec restricted execution mode tools we met in Chapter 18 to evaluate strings. Alternatively, you can simply limit the kinds of expressions allowed and evaluate them with simpler tools (e.g., int, str) or store all data as strings.

19.8.7. Browsing Other Kinds of Objects with PyForm

Although PyForm expects to find a dictionary-of-dictionary interface (protocol) in the tables it browses, a surprising number of objects fit this mold because dictionaries are so pervasive in Python object internals. In fact, PyForm can be used to browse things that have nothing to do with the notion of database tables of records at all, as long as they can be made to conform to the protocol.

For instance, the Python sys.modules table we met in Chapter 3 is a built-in dictionary of loaded module objects. With an appropriate wrapper class to make modules look like dictionaries, there's no reason we can't browse the in-memory sys.modules with PyForm too, as shown in Example 19-29.

Example 19-29. PP3E\Dbase\TableBrowser\viewsysmod.py

 # view the sys.modules table in FormGui class modrec:     def todict(self, value):         return value._ _dict_ _     # not dir(value): need dict     def fromdict(self, value):         assert 0, 'Module updates not supported' import sys from formgui import FormGui from formtable import Table FormGui(Table(sys.modules, modrec())).mainloop( ) 

This script defines a class to pull out a module's _ _dict_ _ attribute dictionary (formtable's InstanceRecord won't do, because it also looks for a _ _class_ _). The rest of it simply passes sys.modules to PyForm (FormGui) wrapped in a Table object; the result appears in Figure 19-4.

Figure 19-4. FormGui browsing sys.modules (viewsysmod)


With similar record and table wrappers, all sorts of objects could be viewed in PyForm. As usual in Python, all that matters is that they provide a compatible interface.

19.8.8. Browsing Other Kinds of Databases with PyForm

In fact, with just a little creativity, we could also write table wrappers that allow the PyForm GUI to view objects in ZODB databases and records in SQL databases third-party systems we studied earlier in this chapter:

  • ZODB should be simple: it is an access-by-key storage medium with a dictionary-like interface similar to shelves. We would need to provide a close method that commits changes, though, since the table wrapper protocol expects one.

  • SQL databases would be more challenging, since they are composed of tables of rows, not objects stored under unique keys. We could, however, define a column to be the unique key values for records in a table and run SQL queries to fetch by key on indexing.

In deference to space, we'll leave the second of these extensions as a suggested exercise. The first is straightforward: Example 19-30 launches the PyForm GUI to browse the ZODB people database we used as an example earlier in this chapter. This script worksit allows you to use the GUI to browse and update persistent class instances stored in a ZODB object databasebut it suffers from some innate limitations in the GUI's design.

As coded, PyForm doesn't support instances of more than one class in the database, and it has no way to call class methods. More subtly, PyForm assumes that instances either are created from a class with no nondefault constructor arguments or support _ _class_ _ attribute assignments (its code tries both schemes to re-create the instance from its dictionary-based representation). The former of these constraints was not coded in the original class, and the latter did not work for classes derived from ZODB persistence classes when this script was tested.

Because of these constraints, the test script in Example 19-30 uses an empty class to initialize the database: since methods and derived subclasses aren't yet supported, classes in PyForm are little more than flat attribute namespaces. As currently coded, PyForm does not leverage the full power of Python classesany methods they contain may still be called by code outside the context of the PyForm GUI, but they have no purpose within it. We'll explore some of these design issues in more detail in the next section.

Perhaps just as remarkable as its flaws, though, is the fact that PyForm can be used on a ZODB database at allby encapsulating the database behind a common object interface, it supports any conforming object.

Example 19-30. PP3E\Database\ZODBscripts\viewzodb.py

 ########################################################## # view the person ZODB database in PyForm's FormGui; # FileDB maps indexing to db root, close does commit; # caveat 1: FormGui doesn't yet allow mixed class types; # caveat 2: FormGui has no way to call class methods; # caveat 3: Persistent subclasses don't allow _ _class_ _ # to be set: must have defaults for all _ _init_ _ args; # Person here works only if always defined in _ _main_ _; ########################################################## import sys filename = 'data/people-simple.fs' from zodbtools import FileDB from PP3E.Dbase.TableBrowser.formgui   import FormGui from PP3E.Dbase.TableBrowser.formtable import Table, InstanceRecord class Person: pass initrecs = {'bob': dict(name='bob', job='devel', pay=30),             'sue': dict(name='sue', job='music', pay=40)} dbtable = Table(FileDB(filename), InstanceRecord(Person)) if len(sys.argv) > 1:     for key in dbtable.keys( ):         del dbtable[key]                 # "viewzodb.py -" inits db     dbtable.storeItems(initrecs)         # "viewzodb.py" browses db FormGui(dbtable).mainloop( ) dbtable.printItems( ) dbtable.close( ) 

Run this code on your machine to see its windowsthey are exactly like those we've seen before, but the records browsed are objects that reside in a ZODB database instead of a shelve.

19.8.9. PyForm Limitations

Although the sys.modules and ZODB viewer scripts of the last two sections work, they highlight a few limitations of PyForm's current design:


Two levels only

PyForm is set up to handle a two-dimensional table/record-mapping structure only. You can't descend further into fields shown in the form, large data structures in fields print as long strings, and complex objects such as nested modules, classes, and functions that contain attributes of their own simply show their default print representation. We could add object viewers to inspect nested objects interactively, but they might be complex to code.


No big forms

PyForm is not equipped to handle a large number of record fieldsif you select the os module's entry in the index listbox in Figure 19-4, you'll get a huge form that is likely too big to even fit on your screen (the os module has lots and lots of attributes; it goes off my screen after about 40). We could fix this with a scroll bar, but it's unlikely that records in the databases that PyForm was designed to view will have many dozens of fields.


Data attributes only

PyForm displays record attribute values, but it does not support calling method functions of objects being browsed and cannot display dynamically computed attributes (e.g., the tax attribute in Person objects). Since some class methods require arguments to be passed, an additional interface would be necessary; required arguments could be extracted from the method function itself (hint: see built-in function and code attributes such as function.func_code.co_argcount).


One class per table

PyForm currently assumes all instances in a table are of the same class, even though that's not a requirement for shelves in general.


New style classes with _ _slots_ _ don't work

As coded, PyForm may not currently support some instances of new style classes. In particular, new style classes with a _ _slots_ _ attribute may not have a _ _dict_ _ namespace dictionary and so will not work in PyForm (slots save the space normally taken by the instance _ _dict_ _, and may be fetched quicker).

This same restriction currently exists in the Python pickle module, thougha class that defines _ _slots_ _ without defining _ _getstate_ _ (called to return a state to pickle) cannot be pickledso this is not an additional constraint imposed by the GUI. Supporting _ _slots_ _ in addition to _ _dict_ _ may be possible, but we leave this as an exercise (this may require a class tree climb to collect all _ _slot_ _ lists in all superclasses, or inspecting the result of a dir call).


Wrapping protocol alternatives

In some cases, it may be possible to avoid the to/from dictionary conversion for class instances browsed. The trick would be to wrap records rather than tables. This would almost allow us to get rid of the Table wrapper class completely for this use casethe GUI could browse either a shelve of instances or a shelve of dictionaries directly, with no conversions. It would not, however, handle other use cases (e.g., DBM files of evaluated strings), and it might turn out to be more complex than the current general dictionary-based scheme, due to extra-special cases.

The last item in the preceding list is a subtle design point, and it merits some addition explanation. PyForm current overloads table index fetch and assignment, and the GUI internally uses dictionaries to represent records. Fetches assume a dictionary-like object comes back, and stores make a new dictionary object (or use the current one), fill it out, and pass it off to the Table wrapper for conversion to the table's underlying record implementation. When browsing tables of instances, the fetch conversion is trivial (we use the instance's _ _dict_ _ directly), but stores must create and fill out a new instance.

It would be almost as easy to overload record field index fetch and assignment instead, to avoid converting dictionaries to instances, and possibly avoid the Table wrapper layer. In this scheme, records held in PyForm might be whatever object the table stores (not necessarily dictionaries), and each record field fetch or assignment in PyForm would be routed back to record wrapper classes.

For example, by wrapping instance records in a class that maps dictionary field indexing to class attributes with _ _getitem_ _ and _ _setitem_ _ overload methods, the GUI might browse actual class instance objects. These two overload methods would simply call the getattr and setattr built-in functions to access the attribute corresponding to the key by string name, and the keys call in the GUI used to extract field names could be mapped by the record wrapper to the instance _ _dict_ _.

The trickiest part of this scheme is that the GUI would have to know how to make a new empty record before filling its fieldsthis would likely require that the GUI have knowledge of the concrete type of the record (dictionary or instance, as well as the class if it is an instance) or use of a Table wrapper with a customizable method for creating a new empty record. By building and filling dictionaries, the GUI currently finesses this issue completely and delegates it to the customized table and record wrappers.

There are also a few substantial downsides to this approach. For one, PyForm could not browse any instance object unless it inherits from the record wrapper class or is wrapped up in one automatically by a Table interface class on fetches and stores. For another, Table also has some additional interfaces not provided by shelves, which we have to code elsewhere. This scheme might also preclude use of indexing overload methods in the record class itself, though the GUI itself does not support such operations anyhow.

Most significantly, this model would not transparently handle other use cases, such as string-based records. Cases requiring conversion with eval and str, for instance, would not fit the new model at allDBM files that map whole records to strings might require complex special case logic to handle field-at-a-time requests or fall back to converting from and to dictionaries on fetches and stores, as is currently done.

Because of such exceptions, we would probably wind up with a Table wrapper anyhow, unless we limit the GUI's use cases. Generating a new empty record just by itself varies so much per record kind that we need a class hierarchy to customize the operation. In the end, it may be easier to use dictionaries in all cases and convert from that where needed, as PyForm currently does.

In other words, there is room for improvement if you care to experiment. On the other hand, extensions in this domain are somewhat open-ended, so we'll leave them as suggested exercises. PyForm was designed to view mappings of mappings and was never meant to be a general Python object viewer.

But as a simple GUI for tables of persistent objects, it meets its design goals as planned. Python's shelves and classes make such systems both easy to code and powerful to use. Complex data can be stored and fetched in a single step, as well as augmented with methods that provide dynamic record behavior. As an added bonus, by programming such programs in Python and Tkinter, they are automatically portable among all major GUI platforms. When you mix Python persistence and GUIs, you get a lot of features "for free."




Programming Python
Programming Python
ISBN: 0596009259
EAN: 2147483647
Year: 2004
Pages: 270
Authors: Mark Lutz

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