Question

I am new to Python so please don’t flame me if the question is too basic 🙂

I have read that Python is executed from top – to – bottom.

If this is the case, why do programs go like this:

def func2(): 
    pass

def func1():
    func2()

def func():
    func1()

if __name__ == '__main__':
    func()

So from what I have seen, the main function goes at last and the other functions are stacked on top of it.

Am I wrong in saying this? If no, why isn’t the main function or the function definitions written from top to bottom?

EDIT: I am asking why I can’t do this:

if __name__ == '__main__':
    func()

def func1():
    func2()

Isn’t this the natural order? You keep on adding stuff at the bottom, since it is executed from top to bottom.

Answer 1

The defs are just creating the functions. No code is executed, other than to parse the syntax and tie functions to those names.

The if is the first place code is actually executed. If you put it first, and call a function before it is defined, the result is a NameError. Therefore, you need to put it after the functions are defined.

Note that this is unlike PHP or JavaScript, where functions are ‘hoisted’ – any function definitions are processed and parsed before everything else. In PHP and JavaScript, it’s perfectly legal to do what you are saying and define functions in the source lower down than where they are called. (One detail in JS is that functions defined like function(){} are hoisted, while functions defined like var func1=function(){}; are not. I don’t know how it works with anonymous functions in PHP 5.3 yet).

See, in this, cat() will print correctly, and yip() gives you a NameError because the parser hasn’t gotten to the definition of yip() at the time you call it.

def cat():
  print 'meowin, yo'

cat()

yip()

def yip():
  print 'barkin, yall'

meowin, yo
Traceback (most recent call last):
File “cat.py”, line 5, in
yip()
NameError: name ‘yip’ is not defined