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

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