Think Python, 2nd Edition

Chapter 7. Iteration

This chapter is about iteration, which is the ability to run a block of statements repeatedly. We saw a kind of iteration, using recursion, in “Recursion”. We saw another kind, using a for loop, in “Simple Repetition”. In this chapter we’ll see yet another kind, using a while statement. But first I want to say a little more about variable assignment.

Reassignment

As you may have discovered, it is legal to make more than one assignment to the same variable. A new assignment makes an existing variable refer to a new value (and stop referring to the old value).

>>> x = 5
>>> x
5
>>> x = 7
>>> x
7

The first time we display x, its value is 5; the second time, its value is 7.

Figure 7-1 shows what reassignment looks like in a state diagram.

At this point I want to address a common source of confusion. Because Python uses the equal sign (=) for assignment, it is tempting to interpret a statement like a = b as a mathematical proposition of equality; that is, the claim that a and b are equal. But this interpretation is wrong.

First, equality is a symmetric relationship and assignment is not. For example, in mathematics, if a=7 then 7=a. But in Python, the statement a = 7 is legal and 7 = a is not.

Also, in mathematics, a proposition of equality is either true or false for all time. If a=b now, then a will always equal b. In Python, an assignment statement can make two variables equal, but they don’t have to stay that way:

>>> a = 5 >>> b = a # a and b are now equal >>> ...