2.3 Strict and non-strict evaluation

Functional programming’s efficiency stems, in part, from being able to defer a computation until it’s required. There are two similar concepts for avoiding computation. These are:

• Strictness: Python operators are generally strict and evaluate all sub-expressions from left to right. This means an expression like f(a)+f(b)+f(c) is evaluated as if it was (f(a)+f(b))+f(c). An optimizing compiler might avoid strict ordering to improve performance. Python doesn’t optimize and code is mostly strict. We’ll look at cases where Python is not strict below.

• Eagerness and laziness: Python operators are generally eager and evaluate all sub-expressions to compute the final answer. This means (3-3) * f(d) is fully evaluated even though the first part of the multiplication—the (3-3) sub-expression—is always zero, meaning the result is always zero, no matter what value is computed by the expression f(d). Generator...