Python for Programmers: None

• None is Python's null value, used to indicate the lack of something. It's similar to the null or nil values in many other languages, or undefined in JavaScript.

• We use value is None to decide whether a value is None. It returns a boolean.

• >

  None is None

  Result:

  True

• >

  "something" is None

  Result:

  False

• >

  address = None
  if address is None:
    message = "no address"
  else:
    message = "the address is " + address
  message

  Result:

  'no address'

• This works because there's only one None value. No matter where we refer to None, we get exactly the same value, stored at the same place in memory.

• It's also possible to check with value == None, but that's considered bad style.

• >

  # This isn't recommended! Use `is` instead.
  None == None

  Result:

  True

• As we saw in an earlier lesson on identity, we can also use the is not comparison to ask "are these two different values, stored at different locations in memory?" This is especially common when comparing against None. value is not None gives us the same result as not (value is None). However, value is not None is generally considered more readable.

• >

  not (3 is None)

  Result:

  True

• >

  3 is not None

  Result:

  True

• Functions that lack a return statement automatically return None. (They have to return something!) An unexpected None often means that we forgot the return statement.

• >

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

  Result:

  None

• One final note about None. If you're familiar with JavaScript, note that Python doesn't share JavaScript's distinction between null and undefined. Like most languages, Python only has one "null" or "empty" value: None.