CS61A: Print and None type

Forethought

Consider

>>> -2
-2
>>> print(-2)
-2

There is a difference in the two.

The print function and the None value

• Python has rules for automatically displaying values of expressions

The print function

• This function prints out the value. (Duh)
• It can also print out multiple values seperated by commas

  >>> print(1,2,3)
  1 2 3
  >>>
  

• The return value of print is a None!

None indicates that nothing is returned

• The None value represents the absence of a value (nothing). When we run it in the python interpreter, literally nothing happens. This is because None itself is not automatically displayed by the interpreter.

  >>> None
  >>>
  

• If we print None with the print function, we actually see the value

  >>> print(None)
  None
  >>>
  

• If a function is not defined to return a value, it will return None.

Two types of Functions

Pure functions

• Just return values
• Ex: abs(x), pow(a,b)
• They are closed systems

Non-Pure Functions

• Have other side effects
• Ex: Print returns None itself, but it also displays a value. (A consequence of calling the function)

Interpreting Nested Print Expressions

>>> print(print(1), print(2))
1
2
None None

1) We try to execute the outside print, which wants to print the return values of print(1) and print(2) seperated by white space. 2) We execute print(1) and print(2) to gain their return values 3) First, print(1) also prints “1” with a newline, then returns None. 4) Then, print(2) prints “2” with a newline, then returns None. 5) print(print(1), print(2)) effectively becomes print(None, None), and returns None. 6) None None is printed 7) None is not displayed according to the rules of the Python interpreter.