Python Pocket Reference, 3rd Edition

Specific Statements

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39

Augmented assignment

A set of additional assignment statement formats, listed in

Table 13, are available. Known as augmented assignments,

these formats imply a binary expression plus an assignment.

For instance, the following two formats are roughly equivalent:

X = X + Y

X += Y

However, the reference to target X in the second format needs

to be evaluated only once, and in-place operations can be

applied for mutables as an optimization (e.g.,

list1 += list2

automatically calls list1.extend(list2), instead of the slower

concatenation operation implied by

+). Classes can overload

in-place assignments with method names that begin with an

i

(e.g., __iadd__ for +=, __add__ for +). The format X //= Y

(floor division) is new as of Version 2.2.

Expressions

expression

function([value, name=value, ...])

object.method([value, name=value, ...])

Any expression can appear as a statement (but statements

cannot appear as expressions). Expressions are commonly

used for calling functions and for interactive-mode printing.

In function and method calls, actual arguments are sepa-

rated by commas and are normally matched to arguments in

function

def headers by position. Calls can optionally list

specific argument names in functions to receive passed val-

ues by using the

name=value keyword syntax.

Table 13. Augmented assignment statements

X += Y X &= Y X -= Y X |= Y

X *= Y X ^= Y X /= Y X >>= Y

X %= Y X <<= Y