Boolean Tests

When we want to make a decision in a program, we formulate the decision in the form:

if (something is true):
  do this thing
  and this other thing
  otherwise do this thing
  and some other thing

the something is true test checks the boolean truthiness of the expression. So an empty list, the number 0, an empty string, or any thing which is falsey will skip the if block and instead run the else block.

We can include math expressions:, boolean combinations, the results of functions, or any other expression in the test.

if Statements

  • only do a given “suite” of statements if the “check” matches

  • else is for when no other check matches (and is optional)

  • elif is short-form for else if which you will see in other languages such a javascript


In Python, we use indentation (the number of spaces at the start of the line) to indicate the structure of the program. Other languages will use {} characters, or even words such as if and fi to indicate the start and end of a suite of things that are done together.

>>> x = 32
>>> if x < 5:
...     print('hello')
... elif (x+4 > 33):
...     print('hello world')
... else:
...     print('world')
hello world


Technical Tidbit

Your computer is formed of tiny electrical switches where a current in one “wire” can prevent or allow a current from flowing in another “wire”. Below all the levels of abstraction, when the computer decides “if this is True” it is checking whether a value can flow through the second “wire”.


Recall that comparisons are boolean operators

  • == (are they equal) vs = (assign value)

  • >=, <=, != (not equal)

  • logical combinations allow you to string together boolean tests

    • and, or, not

>>> x = 23
>>> y = 42
>>> (x == y) or (x * 2 > y )
>>> (x == y) or (x > y)
>>> (x < y) and (y > 30)
>>> (x == y) or (not x > y)