In addition to its predefined attributes, a function object may have other arbitrary attributes. To create an attribute of a function object, bind a value to the appropriate attribute reference in an assignment statement after the def statement executes. For example, a function could count how many times it gets called:

  def counter():
      counter.count += 1
      return counter.count
  counter.count = 0

Note that this is not common usage. More often, when you want to group together some state (data) and some behavior (code), you should use the object-oriented mechanisms covered in Chapter 5. However, the ability to associate arbitrary attributes with a function can sometimes come in handy.

The return Statement

The return statement in Python is allowed only inside a function body and can optionally be followed by an expression. When return executes, the function terminates, and the value of the expression is the function's result. A function returns None if it terminates by reaching the end of its body or by executing a return statement that has no expression (or, of course, by executing return None).

As a matter of style, you should never write a return statement without an expression at the end of a function body. If some return statements in a function have an expression, all return statements should have an expression. return None should only be written explicitly to meet this style requirement. Python does not enforce these stylistic conventions, but your code will be clearer and more readable if you follow them.

Calling Functions

A function call is an expression with the following syntax:

  function-object(arguments)

function-object  may be any reference to a function (or other callable) object; most often, it's the function's name. The parentheses denote the function-call operation itself. arguments , in the simplest case, is a series of zero or more expressions separated by commas (,), giving values for the function's corresponding parameters. When the function call executes, the parameters are bound to the argument values, the function body executes, and the value of the function-call expression is whatever the function returns.

Note that just mentioning a function (or other callable object) does not call it. To call a function (or other object) without arguments, you must use () after the function's name.

The semantics of argument passing

In traditional terms, all argument passing in Python is by value. For example, if you pass a variable as an argument, Python passes to the function the object (value) to which the variable currently refers, not "the variable itself." Thus, a function cannot rebind the caller's variables. However, if you pass a mutable object as an argument, the function may request changes to that object because Python passes the object itself, not a copy. Rebinding a variable and mutating an object are totally disjoint concepts. For example:

  def f(x, y):
      x = 23
      y.append(42)
  a = 77
  b = [99]
  f(a, b)
  print a, b           # prints: 77 [99, 42]

The print statement shows that a is still bound to 77. Function f's rebinding of its parameter x to 23 has no effect on f's caller, nor, in particular, on the binding of the caller's variable that happened to be used to pass 77 as the parameter's value. However, the print statement also shows that b is now bound to [99, 42]. b is still bound to the same list object as before the call, but that object has mutated, as f has appended 42 to that list object. In either case, f has not altered the caller's bindings, nor can f alter the number 77, since numbers are immutable. However, f can alter a list object, since list objects are mutable. In this example, f mutates the list object that the caller passes to f as the second argument by calling the object's append method.