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Arithmetic Operations - Python

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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Python arithmetic operations behave in rather obvious ways, with the possible exception of division and exponentiation.


If the right operand of /, //, or % is 0, Python raises a runtime exception. The // operator performs truncating division, which means it returns an integer result (converted to the same type as the wider operand) and ignores the remainder, if any. When both operands are integers (plain or long), the / operator behaves like // if the switch -Qold was used on the Python command line (-Qold is the default in Python 2.3, 2.4, and 2.5). Otherwise, / performs true division, returning a floating-point result (or a complex result if either operand is a complex number). To have / perform true division on integer operands in Python 2.3, 2.4, or 2.5, use the switch -Qnew on the Python command line, or begin your source file with the statement:

  from __future__ import division

This statement ensures that operator / (within the module that starts with this statement only) works without truncation on operands of any type.

To ensure that the behavior of division does not depend on the -Q switch, and on the exact version of Python you're using, always use // when you want truncating division. When you do not want truncation, use /, but also ensure that at least one operand is not an integer. For example, instead of using just a/b, code 1.*a/b to avoid making any assumption on the types of a and b. To check whether your program has version dependencies in its use of division, use the switch -Qwarn on the Python command line to get runtime warnings about all uses of / on integer operands.

The built-in divmod function takes two numeric arguments and returns a pair whose items are the quotient and remainder, so you don't have to use both // for the quotient and % for the remainder.


The exponentiation ("raise to power") operation, a**b , raises an exception if a  is less than zero and b is a floating-point value with a nonzero fractional part. The built-in pow(a, b) function returns the same result as a**b . With three arguments, pow(a, b, c) returns the same result as (a**b)%c  but faster.


All objects, including numbers, can be compared for equality (==) and inequality (!=). Comparisons requiring order (<, <=, >, >=) may be used between any two numbers, unless either operand is complex, in which case they raise runtime exceptions. All these operators return Boolean values (True or False).

Bitwise Operations on Integers

Integers and long integers can be taken as strings of bits and used with the bitwise operations shown in Table 4-2. Bitwise operators have lower priority than arithmetic operators. Positive integers are conceptually extended by an infinite string of 0 bits on the left. Negative integers are represented in two's complement notation, and therefore are conceptually extended by an infinite string of 1 bits on the left.

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