How functions work

• When a function definition is executed (run), the parameters and the body of the function are stored with the function name.

  def compose2(func1, func2, arg):
      """Returns the result of applying func1 to the result of applying func2 to arg.
  
      func1: a function of one argument
      func2: a function of one argument
      arg:   something that can be passed to func2 as an argument
      """
      return func1(func2(arg))

• After this we can call the function. Each time we call it, the first thing that happens is that each of the parameters (a variable) gets bound to the values of the arguments in the function call. So in the following function call,

  compose2(len, str, 10+20)

  this first step is equivalent to the following (temporary) assignments

  func1 = len
  func2 = str
  arg = 10+20

  Note that the values of the parameters are the values of the expressions on the right-hand sides, that is, the function len in the first case and 30 in the third case.
• Then each statement in the body of the definition is executed, with the variable binding holding. In the example, this is equivalent to doing the following:

  return len(str(10))

  Because this is a return statement, the resulting value is returned by the function call

  >>> compose2(len, str, 10+20)
  2
  

• After the function returns, the bindings of the parameters are no longer in effect.

  >>> func1
  Traceback (most recent call last):
    File "<pyshell#13>", line 1, in <module>
      func1
  NameError: name 'func1' is not defined

Defining functions: divide and conquer

• When defining all but the most trivial functions, break what they're supposed to do into smaller parts, and work on these separately. Sometimes it's a good idea to write separate ("helper") functions for the parts.
• Example
  • Write a function that gets two integers from the user and returns the number of places (digits) in the product of the two integers.
  • The function does three things: gets the integers from the user, multiplies them, and finds the number of digits in the answer.
  • Let's write a function that gets an integer from the user.
  • Multiplying integers is easy; we don't need a new function for that.
  • Let's write a function that calculates the number of places in an integer.
  • Now let's put the pieces together.
  • Let's see how this would have worked with a single function.

Answers

Answers to problems in these notes