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Python Expressions and Operators

In this fourth part of a nine-part series on Python, you'll learn about expressions, operators, sequences, and more. This article is excerpted from chapter four of the book Python in a Nutshell, Second Edition, written by Alex Martelli (O'Reilly; ISBN: 0596100469). Copyright 2007 O'Reilly Media, Inc. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.

  1. Python Expressions and Operators
  2. The Python 2.5 ternary operator
  3. Arithmetic Operations
  4. Sequence Operations
By: O'Reilly Media
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October 02, 2008

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Expressions and Operators

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
`expr,...` String conversion NA
{key:expr,...} Dictionary creation NA
[expr,...] List creation NA
(expr,...) Tuple creation or just parentheses NA
f(expr,...) Function call L
x[index:index] Slicing L
x[index] Indexing L
x.att r Attribute reference L
x**y Exponentiation ( x to y th power) R
~x Bitwise NOT NA
+x , -x Unary plus and minus NA
x*y , x/y , x//y , x%y Multiplication, division, truncating division, remainder L
x+y , x-y Addition, subtraction L
x<<y , x>>y Left-shift, right-shift L
x&y Bitwise AND L
x^y Bitwise XOR L
x|y Bitwise OR L
x<y , x<=y , x>y , xy , x<<>>y , Comparisons (less than, less than or NA
x!=y , x==y equal, greater than, greater than or equal, inequality, equality) a  
x is y , x is not y Identity tests NA
x in y , x not in y Membership tests NA
not x Boolean NOT NA
x and y Boolean AND L
x or y Boolean OR L
lambda arg,...: expr Anonymous simple function NA

a <> and != are alternate forms of the same operator. != is the preferred version; <> is obsolete.

In Table 4-2, exprkey ,  findexx , 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 on page 166) instead.

Comparison Chaining

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.

Short-Circuiting Operators

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.

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