How does Python know where the end of a function is?

I'm just learning python and confused when a "def" of a function ends?

I see code samples like:

def myfunc(a=4,b=6):
    sum = a + b
    return sum

myfunc()

I know it doesn't end because of the return (because I've seen if statements... if FOO than return BAR, else return FOOBAR). How does Python know this isn't a recursive function that calls itself? When the function runs does it just keep going through the program until it finds a return? That'd lead to some interesting errors.

Thanks

Solution

In Python whitespace is significant. The function ends when the indentation becomes smaller (less).

def f():
    pass # first line
    pass # second line
pass # <-- less indentation, not part of function f.

Note that one-line functions can be written without indentation, on one line:

def f(): pass

And, then there is the use of semi-colons, but this is not recommended:

def f(): pass; pass

The three forms above show how the end of a function is defined syntactically. As for the semantics, in Python there are three ways to exit a function:

Using the return statement. This works the same as in any other imperative programming language you may know.
Using the yield statement. This means that the function is a generator. Explaining its semantics is beyond the scope of this answer. Have a look at Can somebody explain me the python yield statement?
By simply executing the last statement. If there are no more statements and the last statement is not a return statement, then the function exists as if the last statement were return None. That is to say, without an explicit return statement a function returns None. This function returns None:
```
def f():
    pass
```
And so does this one:
```
def f():
    42
```