Chapter 7. Iteration
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).
The output of this program is
7, because the first time
bruce is printed, its value is 5, and the
second time, its value is 7. The comma at the end of the first
Figure 7-1 shows what multiple assignment looks like in a state diagram.
With multiple assignment it is especially important to distinguish
between an assignment operation and a statement of equality. Because
Python uses the equal sign (
assignment, it is tempting to interpret a statement like
a = b as a statement of equality. It is
First, equality is a symmetric relation and assignment is not. For
example, in mathematics, if a=7 then
7=a. But in Python, the statement
a = 7 is legal and
= a is not.
Furthermore, in mathematics, a statement 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 and b are now equal
# a and b are no longer equal
The third line changes the value of
a but does not change the value of
b, so they are no longer equal.
Although multiple assignment is frequently helpful, ...