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The Python 2.5 ternary operator - 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 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.

Numeric Operations

Python supplies the usual numeric operations, as we've just seen in Table 4-2. Numbers are immutable objects: when you perform numeric operations on number objects, you always produce a new number object and never modify existing ones. You can access the parts of a complex object z as read-only attributes z.real and z.imag. Trying to rebind these attributes on a complex object raises an exception.

A numbers optional + or - sign, and the + that joins a floating-point literal to an imaginary one to make a complex number, are not part of the literals' syntax. They are ordinary operators, subject to normal operator precedence rules (see Table 4-2). For example, -2**2 evaluates to -4: exponentiation has higher precedence than unary minus, so the whole expression parses as -(2**2), not as (-2)**2.

Numeric Conversions

You can perform arithmetic operations and comparisons between any two numbers of Python built-in types. If the operands' types differ, coercion applies: Python converts the operand with the "smaller" type to the "larger" type. The types, in order from smallest to largest, are integers, long integers, floating-point numbers, and complex numbers.

You can request an explicit conversion by passing a noncomplex numeric argument to any of the built-in number types: int, long, float, and complex. int and long drop their argument's fractional part, if any (e.g., int(9.8) is 9). You can also call complex with two numeric arguments, giving real and imaginary parts. You cannot convert a complex to another numeric type in this way, because there is no single unambiguous way to convert a complex number into, e.g., a float.

Each built-in numeric type can also take a string argument with the syntax of an appropriate numeric literal, with small extensions: the argument string may have leading and/or trailing whitespace, may start with a sign, and, for complex numbers, may sum or subtract a real part and an imaginary one. int and long can also be called with two arguments: the first one a string to convert, and the second the radix, an integer between 2 and 36 to use as the base for the conversion (e.g., int('101', 2) returns 5, the value of '101' in base 2).

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