Python Cookbook

Simulating the Ternary Operator in Python

Credit: Jürgen Hermann, Alex Martelli, Oliver Steele, Lloyd Goldwasser, Chris Perkins, and Brent Burley

Problem

You want to express in Python the equivalent of C’s so-called ternary operator ?: (as in, condition?iftrue:iffalse).

Solution

There are many ways to skin a ternary operator. An explicit if/else is most Pythonic, but somewhat verbose:

for i in range(1, 3):
    if i == 1:
        plural = ''
    else:
        plural = 's'
    print "The loop ran %d time%s" % (i, plural)

Indexing is compact if there are no side effects in the iftrue and iffalse expressions:

for i in range(1, 3):
    print "The loop ran %d time%s" % (i, ('', 's')[i != 1])

For the specific case of plurals, there’s also a neat variant using slicing:

for i in range(1, 3):
    print "The loop ran %d time%s" % (i, "s"[i==1:])

Short-circuited logical expressions can deal correctly with side effects:

for i in range(1, 3):
    print "The loop ran %d time%s" % (i, i != 1 and 's' or '')

The output of each of these loops is:

The loop ran 1 time
The loop ran 2 times

However, the short circuit (which is necessary when either or both of iftrue and iffalse have side effects) fails if turned around:

for i in range(1, 3):
    print "The loop ran %d time%s" % (i, i == 1 and '' or 's')

Since '' evaluates as false, this snippet outputs:

The loop ran 1 times
The loop ran 2 times

So generally, when iftrue and iffalse are unknown at coding time (either could have side effects or be false), we need:

for i in range(1, 3): print "The loop ran %d ...