Order of execution and style of coding in Python

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

Python is executed from top to bottom, but executing a "def" block doesn't immediately execute the contained code. Instead it creates a function object with the given name in the current scope. Consider a Python file much like your example:

def func2():
    print "func2"

def func1():
    func2()

def func():
    func1()

if __name__ == '__main__':
    func()

What happens when this script is executed is as follows:

Firstly, a function object is created and bound to the name "func2" in the global scope. Then a function object is created and bound to the name "func1" in the global scope. Then one called "func". Then the "if" statement is executed, the condition is true and the "func()" statement is executed. At this point "func" is a function object found in the global scope, so it is invoked and its code runs. That code contains the "func1()" statement, which is resolved to a call to the "func1" function, and so on.

If you put the "if" statement at the top then when it executes there would not yet be anything defined with the name "func", so you would get an error. The important thing to recognise is that the "def" statement is itself a statement that is executed. It is not like in some other languages where definitions are a separate sort of declaration with no order of execution.

Also note that so long as the "if __name__..." bit is at the end of the file, it doesn't really matter what order the other declarations are in, since by the time any of them are called all of the "def"s will have already been executed.