Monday, January 28, 2008

Python note: the main entry of a python file

In a python module we could write

if __name__ == "__main__":
# the main program entry

and this is often taken as the main entry of the module. The fact may be misleading for C++ programmers, because python doesn't actually need a main entry when running a module.

Unlike C++, each python module is treated as a sequence of executable commands. No matter when it is imported or run as a program, python always executes the python file from the first line to the end. This is also the reason why python is called a script language. Generally, there are three types of commands in python

1. import commands: when python meets these commands, it checks sys.modules to see if the imported module is already in the list. If it is, python bypasses this command. Otherwise, python goes into the imported module and runs every command in it.

2. definition commands: these commands include "class" and "def"; when python meets them it doesn not do anything, but remembers the definitions in the corresponding place (normally in the __dir__ of the current module).

3. execution commands: all the other commands, including assignments, conditions and branching statements, function calls etc; python runs them according to their semantics.

When a module is loaded into python as the main program, python gives the module name __name__ the special value "__main__". The condition check for __name__ == "__main__" is used to make sure that the commands below are executed only when the module is run as the main program, but not as an inmported module. There can be as many such conditions as possible, and they can occur anywhere in a python module. There is not a particular function or statement that python takes as the main entry of modules.

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