Section 20.9. Sorting Sequences

20.9. Sorting Sequences

Another staple of many systems is sorting: ordering items in a collection according to some constraint. The script in Example 20-24 defines a simple sort routine in Python, which orders a list of objects on a field. Because Python indexing is generic, the field can be an index or a keythis function can sort lists of either sequences or mappings.

Example 20-24. PP3E\Dstruct\Classics\sort1.py

 def sort(list, field):     res = []                                     # always returns a list     for x in list:         i = 0         for y in res:             if x[field] <= y[field]: break       # list node goes here?             i = i+1         res[i:i] = [x]                           # insert in result slot     return res if _ _name_ _ == '_ _main_ _':     table = [ {'name':'john', 'age':25}, {'name':'doe', 'age':32} ]     print sort(table, 'name')     print sort(table, 'age')     table = [ ('john', 25), ('doe', 32) ]     print sort(table, 0)     print sort(table, 1) 

Here is this module's self-test code in action:

 C:\...\PP3E\Dstruct\Classics>python sort1.py [{'age': 32, 'name': 'doe'}, {'age': 25, 'name': 'john'}] [{'age': 25, 'name': 'john'}, {'age': 32, 'name': 'doe'}] [('doe', 32), ('john', 25)] [('john', 25), ('doe', 32)] 

20.9.1. Adding Comparison Functions

Since functions can be passed in like any other object, we can easily allow for an optional comparison function. In the next version (Example 20-25), the second argument takes a function that should return TRue if its first argument should be placed before its second. A lambda is used to provide an ascending-order test by default. This sorter also returns a new sequence that is the same type as the sequence passed in, by applying the slicing techniques used in earlier sections: if you sort a tuple of nodes, you get back a tuple.

Example 20-25. PP3E\Dstruct\Classics\sort2.py

 def sort(seq, func=(lambda x,y: x <= y)):             # default: ascending     res = seq[:0]                                     # return seq's type     for j in range(len(seq)):         i = 0         for y in res:             if func(seq[j], y): break             i = i+1         res = res[:i] + seq[j:j+1] + res[i:]          # seq can be immutable     return res if _ _name_ _ == '_ _main_ _':     table = ({'name':'doe'}, {'name':'john'})     print sort(list(table),  (lambda x, y: x['name'] > y['name']))     print sort(tuple(table), (lambda x, y: x['name'] <= y['name']))     print sort('axbyzc') 

This time, the table entries are ordered per a field comparison function passed in:

 C:\...\PP3E\Dstruct\Classics>python sort2.py [{'name': 'john'}, {'name': 'doe'}] ({'name': 'doe'}, {'name': 'john'}) abcxyz 

This version also dispenses with the notion of a field altogether and lets the passed-in function handle indexing if needed. That makes this version much more general; for instance, it's also useful for sorting strings.