Strict and non-strict evaluation

Functional programming's efficiency stems, in part, from being able to defer a computation until it's required. The idea of lazy or non-strict evaluation is very helpful. It's so helpful that Python already offers this feature.

In Python, the logical expression operators and, or, and if-then-else are all non-strict. We sometimes call them short-circuit operators because they don't need to evaluate all arguments to determine the resulting value.

The following command snippet shows the and operator's non-strict feature:

>>> 0 and print("right")
0
>>> True and print("right")
right

When we execute the preceding command snippet, the left-hand side of the and operator is equivalent to False; the right-hand side is not evaluated. When the left-hand side is equivalent to True, the right-hand side is evaluated.

Other parts of Python are strict. Outside the logical operators, an expression is evaluated eagerly...