An expression is a phrase of code that Python evaluates to produce a value. The simplest expressions are literals and identifiers. You build other expressions by joining subexpressions with the operators and/or delimiters in Table 4-2. This table lists operators in decreasing order of precedence, higher precedence before lower. Operators listed together have the same precedence. The third column lists the associativity of the operator: L (left-to-right), R (right-to-left), or NA (nonassociative).
Table 4-2. Operator precedence in expressions
Operator | Description | Associativity |
---|---|---|
| String conversion | NA |
| Dictionary creation | NA |
| List creation | NA |
| Tuple creation or just parentheses | NA |
| Function call | L |
| Slicing | L |
| Indexing | L |
| Attribute reference | L |
| Exponentiation ( | R |
| Bitwise NOT | NA |
| Unary plus and minus | NA |
| Multiplication, division, truncating division, remainder | L |
| Addition, subtraction | L |
| Left-shift, right-shift | L |
| Bitwise AND | L |
| Bitwise XOR | L |
| Bitwise OR | L |
| Comparisons (less than, less than or equal, greater than, greater than or equal, inequality, equality)^{[a]} | NA |
| Identity tests | NA |
| Membership tests | NA |
| Boolean NOT | NA |
| Boolean AND | L |
| Boolean OR | L |
| Anonymous simple function | NA |
^{[a] } |
In Table 4-2, expr
, key
, f
, index
, x
, and y
indicate any expression, while attr
and arg
indicate any identifier. The notation ,...
means commas join zero or more repetitions, except for string conversion, where you need one or more repetitions. A trailing comma is allowed and innocuous in all such cases, except for string conversion, where it’s forbidden. The string conversion operator, with its quirky behavior, is not recommended; use built-in function repr
(covered in repr
in Built-in Functions) instead.
You can chain comparisons, implying a logical and
. For example:
a < b <= c < d
has the same meaning as:
a < b and b <= c and c < d
The chained form is more readable and evaluates each subexpression once at the most.
Operators and
and or
short-circuit their operands’ evaluation: the righthand operand evaluates only if its value is needed to get the truth value of the entire and
or or
operation.
In other words, x
and
y
first evaluates x
. If x
is false, the result is x
; otherwise, the result is y
. Similarly, x
or
y
first evaluates x
. If x
is true, the result is x
; otherwise, the result is y
.
and
and or
don’t force their results to be True
or False
, but rather return one or the other of their operands. This lets you use these operators more generally, not just in Boolean contexts. and
and or
, because of their short-circuiting semantics, differ from all other operators, which fully evaluate all operands before performing the operation. and
and or
let the left operand act as a guard for the right operand.
Python 2.5 introduces another short-circuiting operator, the ternary operator if
/else
:
whentrue if condition else whenfalse
Each of whentrue
, whenfalse
, and condition
is an arbitrary expression. condition
evaluates first. If condition
is true, the result is whentrue
; otherwise, the result is whenfalse
. Only one of the two subexpressions whentrue
and whenfalse
evaluates, depending on the truth value of condition
.
The order of the three subexpressions in this new ternary operator may be a bit confusing. Also, a recommended style is to always place parentheses around the whole expression.
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