Question

I have a background in C++ and Java and Objective C programming, but i am finding it hard to learn python, basically where its “Main Function” or from where the program start executing. So is there any tutorial/book which can teach python to people who have background in C++ or Java. Basically something which can show if how you were doing this in C++ and how this is done in Python.

OK i think i did not put the question heading or question right, basically i was confused about the “Main” Function, otherwise other things are quite obvious from python official documentation except this concept.

Thanks to all

Answer 1

When you run a script through the Python interpreter (or import that script from another script), it actually executes all the code from beginning to end — in that sense, there is no “entry point” to a Python script.

So to work around this, Python automatically creates a __name__ variable and fills it with the value "__main__" when you are running a script by itself (as opposed to something else importing that script). That’s why you’ll see many scripts like:

def foo():
    print "Hello!"

if __name__ == "__main__":
   foo()

where all the function/class definitions are at the top, and there is a similar if-statement as the last thing in the script. You are guaranteed that Python will start executing the script from top-to-bottom, so it will read all of your definitions there. If you wanted, you could intermingle actual functional code inside all the function definitions.

If this script was named bar.py, you could do python bar.py at the command line and you would see the script print out "Hello!".

On the other hand, if you did import bar from another Python script, nothing would print out until you did bar.foo(), because __name__ was no longer "__main__" and the if-statement failed, thus foo was never executed.