Chapter 14. Execution Environment

Chapter Topics

• Callable Objects

• Code Objects

• Statements and Built-in Functions

• Executing Other Programs

• Terminating Execution

• Miscellaneous Operating System Interface

• Related Modules

There are multiple ways in Python to run other pieces of code outside of the currently executing program, i.e., run an operating system command or another Python script, or execute a file on disk or across the network. It all depends on what you are trying to accomplish. Some specific execution scenarios could include:

• Remain executing within our current script

• Create and manage a subprocess

• Execute an external command or program

• Execute a command that requires input

• Invoke a command across the network

• Execute a command creating output that requires processing

• Execute another Python script

• Execute a set of dynamically generated Python statements

• Import a Python module (and executing its top-level code)

There are built-ins and external modules that can provide any of the functionality described above. The programmer must decide which tool to pick from the box based on the application that requires implementation. This chapter sketches a potpourri of many of the aspects of the execution environment within Python; however, we will not discuss how to start the Python interpreter or the different command-line options. Readers seeking information on invoking or starting the Python interpreter should review Chapter 2.

Our tour of Python's execution environment consists of looking at "callable" objects and following up with a lower-level peek at code objects. We will then take a look at what Python statements and built-in functions are available to support the functionality we desire. The ability to execute other programs gives our Python script even more power, as well as being a resource-saver because certainly it is illogical toreimplement all this code, not to mention the loss of time and manpower. Python provides many mechanisms to execute programs or commands external to the current script environment, and we will run through the most common options. Next, we give a brief overview of Python's restricted execution environment, and finally, the different ways of terminating execution (other than letting a program run to completion). We begin our tour of Python's execution environment by looking at "callable" objects.