This is an old copy of the Python FAQ. The information here may be outdated.

Is there an equivalent of C's "?:" ternary operator?

Yes. In Python 2.5 and later, you can use:

<true-value> if <condition> else <false-value>

Note that the condition is in the middle of the expression, not at the front. This syntax is similar to the list comprehension syntax.

In earlier versions, you can often mimic “a ? b : c” with “a and b or c”, but there’s a flaw: if b is zero (or empty, or None — anything that tests false) then c will be selected instead. In many cases you can prove by looking at the code that this can’t happen (e.g. because b is a constant or has a type that can never be false), but in general this can be a problem.

Tim Peters (who wishes it was Steve Majewski) suggested the following solution:

(a and [b] or [c])[0].

Because [b] is a singleton list it is never false, so the wrong path is never taken; then applying [0] to the whole thing gets the b or c that you really wanted. Ugly, but it gets you there in the rare cases where it is really inconvenient to rewrite your code using ‘if’.

The best course is usually to write a simple if…else statement. Another solution is to implement the “?:” operator as a function:

def q(cond,on_true,on_false):
    if cond:
        result = on_true
        result = on_false
    if callable(result):
        return result()
    return result

In most cases you’ll pass b and c directly: q(a,b,c). To avoid evaluating b or c when they shouldn’t be, encapsulate them within a lambda function, e.g.: q(a,lambda: b, lambda: c).

History #

It has been asked why Python didn’t have an if-then-else expression. There are several answers: many languages do just fine without one; it can easily lead to less readable code; no sufficiently “Pythonic” syntax had been discovered; a search of the standard library found remarkably few places where using an if-then-else expression would make the code more understandable.

In 2002, PEP 308 was written proposing several possible syntaxes and the community was asked to vote on the issue. The vote was inconclusive. Most people liked one of the syntaxes, but also hated other syntaxes; many votes implied that people preferred no ternary operator rather than having a syntax they hated.


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