meeper34 wrote: > 4. Really basic question, and I'm sure I will learn this very quickly > with more reading, but I'm confused by the lack of an entry point in a > Python app, i.e. int main().
Keir's answer about testing for __main__ is good, but it might also help you to think a bit about what it actually means to run a Python program. Python executes code line by line from top to bottom. Almost every statement in Python is executed. This is quite different from a language like C where executable statements are safely tucked inside functions and execution starts at an arbitrary point (the 'main' function), or even C++ where global and static variable initialisations are also executable. In Python everything except the 'global' statement is executed at runtime (also comments and docstrings don't execute). That includes 'import', 'class' and 'def'. So, when you run a Python script it starts on the first line of the script. Usually that will be an 'import', so the interpreter will go off and find the referenced module, compile it if required, and then execute the module from top to bottom. If a module is imported more than once, it only starts executing the code in that module on the first import, and if you have circular imports you can end up accessing a module which has only partly executed. When it sees a class definition the interpreter executes its body in a new namespace and uses the result to create the class. When it sees a function definition it doesn't execute the body, but it does execute any default arguments and then it creates the function. What does this mean in practice? You cannot reference a class or function until after you have executed the relevant definition. You can stick a function definition inside a loop if you want: the name just gets rebound to a new object each time it is executed. The behaviour can be quite suprising at times for people who think of declaring functions, but it is extremely logical if you remember that everything has to execute. The __name__=='__main__' convention works because the main script executes in the module called '__main__', whereas every other module executes under its own name. But it isn't really the start of the program: rather it is the very last thing executed after all the modules have imported and done whatever initialisation they want, and after all the classes and functions have been defined. -- http://mail.python.org/mailman/listinfo/python-list
