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authorAntoine Pitrou <solipsis@pitrou.net>2011-12-03 21:11:45 (GMT)
committerAntoine Pitrou <solipsis@pitrou.net>2011-12-03 21:11:45 (GMT)
commit3cb369dd0a4c4bbca5ec6fe05494917ec8efe748 (patch)
tree37d1535b9c989d1b97185b031d3c8a25b8818f10 /Doc/faq
parent73080b8d1822eda3d587d16dfa921559439736b7 (diff)
parentc5b266efb5fd9157befd92fc13466607ebe23eb9 (diff)
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Streamline FAQ entry about the ternary operator, and suggest using io.StringIO for a mutable unicode container.
Diffstat (limited to 'Doc/faq')
-rw-r--r--Doc/faq/programming.rst74
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