should function be defined before it is used in python?

Question

should the functions be defined before it is used? but why the following code works:

def main():
    dog()

def dog():
    print(\"This is a dog.\")

if __name__         


        

Answer 1

Code within the blocks of functions/methods does not execute before the functions are called. And the CPython language implementation (that's the most common one and the one you are probably using) does not do name-checking at compile time like languages such as C. Since it is primarily an interpreted language, it checks if names are available in the namespace dynamically at runtime.

Furthermore something you should know about Python is that code within classes is executed at the time of class creation, but code with in methods/functions is not. A def statement simply adds the name of the function to the namespace. It does not run any code within the function itself until the function is called.

Consider the following legal Python code.

class Something:
    def __init__(self):
        self.puppy = Dog()

class Dog:
    def __init__(self):
        pass

inst = Something()

This is perfectly legal because __init__ is not run before Dog class gets defined. But what if we moved that constructor above the creation of class Dog?

This code would complain about a NameError because Dog isn't defined yet!

class Something:
    def __init__(self):
        self.puppy = Dog()

inst = Something()

class Dog:
    def __init__(self):
        pass

One final example...

This code would complain about the same NameError.

class Something:
    my_puppy = Dog()
    def __init__(self):
        self.puppy = Dog()

class Dog:
    def __init__(self):
        pass

inst = Something()

This is because all of the code inside Something is immediately executed and at that point in the execution, the namespace/scope does not have access to the name Dog.