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Diffstat (limited to 'Doc/faq')
| -rw-r--r-- | Doc/faq/programming.rst | 74 |
1 files changed, 20 insertions, 54 deletions
diff --git a/Doc/faq/programming.rst b/Doc/faq/programming.rst index a56f3f3..1e94c34 100644 --- a/Doc/faq/programming.rst +++ b/Doc/faq/programming.rst @@ -679,61 +679,21 @@ are not truly operators but syntactic delimiters in assignment statements. Is there an equivalent of C's "?:" ternary operator? ---------------------------------------------------- -Yes, this feature was added in Python 2.5. The syntax would be as follows:: +Yes, there is. The syntax is as follows:: [on_true] if [expression] else [on_false] x, y = 50, 25 - small = x if x < y else y -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:: +Before this syntax was introduced in Python 2.5, a common idiom was to use +logical operators:: - 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)``. + [expression] and [on_true] or [on_false] -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. +However, this idiom is unsafe, as it can give wrong results when *on_true* +has a false boolean value. Therefore, it is always better to use +the ``... if ... else ...`` form. Is it possible to write obfuscated one-liners in Python? @@ -852,15 +812,21 @@ the :ref:`string-formatting` section, e.g. ``"{:04d}".format(144)`` yields How do I modify a string in place? ---------------------------------- -You can't, because strings are immutable. If you need an object with this -ability, try converting the string to a list or use the array module:: +You can't, because strings are immutable. In most situations, you should +simply construct a new string from the various parts you want to assemble +it from. However, if you need an object with the ability to modify in-place +unicode data, try using a :class:`io.StringIO` object or the :mod:`array` +module:: >>> s = "Hello, world" - >>> a = list(s) - >>> print(a) - ['H', 'e', 'l', 'l', 'o', ',', ' ', 'w', 'o', 'r', 'l', 'd'] - >>> a[7:] = list("there!") - >>> ''.join(a) + >>> sio = io.StringIO(s) + >>> sio.getvalue() + 'Hello, world' + >>> sio.seek(7) + 7 + >>> sio.write("there!") + 6 + >>> sio.getvalue() 'Hello, there!' >>> import array |