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Running a Python Program

Specify Python Interpreter

Python programs with source code in files with .py extension can be compiled, but for general purpose use and transparency are distributed as text in the original files. A Python program is run by specifying the main Python program filename as the first command line parameter to the interpreter:

python hello-world.py

where the hello-world.py file contains:

print("Hello world")

Main Entry Point in Python Program

The interpreter will execute the Python program from top to bottom. If the code does not contain any functions and simply contains Python commands, then those commands are executed in sequence. This is appropriate for very simple programs or modules; however, in most cases the program will be complex enough to require functions.

When the Python interpreter reads a source file, it defines a few special variables. If the interpreter is running the program as the main file (as specified on the command line when starting Python), the interpreter sets a special __name__ variable to have a value of "__main__". If the module is imported and executed, then __name__ is set to the module name. This allows the following syntax to be used in the program file:

def main():
    '''
    Main program...
    '''
    # The following are not required...just an example
    # Parse command line parameters
    parseArgs() 
    # Initialize for run
    initialize()
    # Insert some logic here if not handled in called function
    # Close down
    shutdown()
    return


if ( __name__ == "__main__" ):
    # Run the main function
    main()

Graceful Exit

Need to discuss here graceful exit so that calling program can check the exit status for success or failure.

Create a Script to Run Python or a Python Program

In many cases, Python scripts can be run using the default Python that is recognized for an operating system (often the last one that was installed because the installation procedure will update the PATH environment variable). However, it may be appropriate to specify a different version for a task and in particular on Windows it may be necessary to use py to specify the Python version.

It often makes sense to create a script to use the correct Python version, specify the correct script to run, and provide command line parameters. This helps simplify command-line syntax for users. The following environment variables control the Python execution environment and can also be configured, for example to specify the location of custom modules:

  • PYTHONPATH - tells the Python interpreter where to locate the module files imported into a Python program. It should include the Python source library folder and the folders containing Python source code. This variable may be set by the installer.

  • PYTHONSTARTUP - specifies the path to an initialization file containing Python code that is executed every time the interpreter starts up.

It may be helpful to understand the startup environment. Run the following Python program to print environment information.

# Print PYTHONPATH folders
import sys
print sys.path

Example output from running python on Cygwin and executing the above is:

$ python
Python 2.7.10 (default, Jun  1 2015, 18:05:38)
[GCC 4.9.2] on cygwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> print sys.path
['', '/usr/lib/python2.7/site-packages/logilab_common-0.62.0-py2.7.egg', '/usr/lib/python27.zip', '/usr/lib/python2.7', '/usr/lib/python2.7/plat-cygwin', '/usr/lib/python2.7/lib-tk', '/usr/lib/python2.7/lib-old', '/usr/lib/python2.7/lib-dynload', '/usr/lib/python2.7/site-packages']
>>> quit()

From the above it can be seen that the supporting library files are installed in /usr/lib/python2.7.

A simple bash configuration/run script may be similar to the following.

#!/bin/bash
#
# Specify the Python interpreter to run
set PYTHON=/usr/sbin/python3
# Set the PYTHONPATH so that third party libraries can be found in the installation folder for the specific Python version:
set PYTHONPATH=/some/new/path:$PYTHONPATH
# Now run the python script and pass command line parameters
# - need to make sure file is found, perhaps by using absolute path or path from $HOME
%PYTHON% hello-world.py $*

where the hello-world.py file contains:

print("Hello world")

or, to use built-in system functionality to run the python3 interpreter automatically, which may be appropriate for simple scripts:

#!/usr/bin/python3

print("Hello world")