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| -rw-r--r-- | Doc/faq/programming.rst | 46 |
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diff --git a/Doc/faq/programming.rst b/Doc/faq/programming.rst index 56cd406f41..f7ea25aab1 100644 --- a/Doc/faq/programming.rst +++ b/Doc/faq/programming.rst @@ -769,52 +769,6 @@ Yes, this feature was added in Python 2.5. The syntax would be as follows:: For versions previous to 2.5 the answer would be 'No'. -.. XXX remove rest? - -In many cases you can 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: - if not isfunction(on_true): - return on_true - else: - return on_true() - else: - if not isfunction(on_false): - return on_false - else: - return on_false() - -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)``. - -It has been asked *why* Python has no 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 has 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. - Is it possible to write obfuscated one-liners in Python? -------------------------------------------------------- |