Python for Programmers: Functions

• We can define functions with the def keyword. Note the : after the argument list!

• >

  def double(x):
    return 2 * x
  
  double(2)

  Result:

  4

• When a function has no explicit return value, it returns None, which is Python's null value. Another lesson will cover None in more detail. To avoid this, remember to use the return keyword!

• >

  def double(x):
    2 * x
  
  double(2)

  Result:

  Run Code

• >

  def add(a, b):
    # We forgot the `return` keyword!
    a + b
  
  add(2, 3)

  Result:

  None

• Python functions are "first-class values", which means that we can use them in the same way that we use any other value, like 4 or [1, 2, 3].

• All Python values have attributes that we can inspect. Even functions have attributes, like .__name__ for example. By default, this is the same name we gave the function, so the function double has the name 'double'.

• >

  def double(x):
    return 2 * x
  
  double.__name__

  Result:

  'double'

• Because functions are first-class values, they can be passed as arguments. In the next few examples, we write an apply_twice function that takes any function f, then does f(f(x)).

• >

  def apply_twice(f, value):
    return f(f(value))
• Note: this code example reuses elements (variables, etc.) defined in earlier examples.

  >

  def add1(x):
    return x + 1
  
  apply_twice(add1, 3)

  Result:

  Run Code
• Note: this code example reuses elements (variables, etc.) defined in earlier examples.

  >

  def double(x):
    return x * 2
  
  apply_twice(double, 3)

  Result:

  12
• Note: this code example reuses elements (variables, etc.) defined in earlier examples.

  >

  def append_s(some_string):
    return some_string + "s"
  
  apply_twice(append_s, "happine")

  Result:

  'happiness'

• In future lessons, we'll see first-class functions used idiomatically in Python code.

• Finally, a note on terminology. Strictly speaking, functions accept "parameters", but we pass "arguments" when calling them. However, in Python, the word "argument" is normally used for both ideas: functions accept arguments, and we pass arguments to functions. We'll follow that convention in this course, using the term "argument" for both ideas.