Python Cookbook 2Nd Edition Jun 1002005 [Electronic resources] نسخه متنی

Recipe 1.12. Controlling Case

Credit: Luther Blissett

Problem

You need to convert a string from uppercase to lowercase, or vice
versa.

Solution

That's what the
upper and lower methods of
string objects are for. Each takes no arguments and returns a copy of
the string in which each letter has been changed to upper- or
lowercase, respectively.

big = little.upper( )
little = big.lower( )

Characters that are not letters are copied unchanged.

s.capitalize is similar to s[:1].upper(
)+s[1:].lower( ): the first character is changed to
uppercase, and all others are changed to lowercase.
s.title is again similar, but it capitalizes the
first letter of each word (where a
"word" is a sequence of letters)
and uses lowercase for all other letters:

>>> print 'one tWo thrEe'.capitalize( )
One two three
>>> print 'one tWo thrEe'.title( )
One Two Three

Discussion

Case manipulation of strings is a very frequent need. Because of
this, several string methods let you produce case-altered copies of
strings. Moreover, you can also check whether a string object is
already in a given case form, with the methods
isupper, islower, and
istitle, which all return true
if the string is not empty, contains at least one letter, and already
meets the uppercase, lowercase, or titlecase constraints. There is no
analogous iscapitalized method, and coding it is
not trivial, if we want behavior that's strictly
similar to strings' is...
methods. Those methods all return False for an
"empty" string, and the three
case-checking ones also return False for strings
that, while not empty, contain no letters at
all.

The simplest and clearest way to code iscapitalized
is clearly:

def iscapitalized(s):
return s == s.capitalize( )

However, this version deviates from the boundary-case semantics of
the analogous is... methods, since it also returns
TRue for strings that are empty or contain no
letters. Here's a stricter one:

import string
notrans = string.maketrans('', '')  # identity "translation"
def containsAny(str, strset):
return len(strset) != len(strset.translate(notrans, str))
def iscapitalized(s):
return s == s.capitalize( ) and containsAny(s, string.letters)

Here, we use the function shown in Recipe 1.8 to ensure we return
False if s is empty or contains
no letters. As noted in Recipe 1.8, this means that this
specific version works only for plain strings, not for Unicode ones.

See Also

Library Reference and Python in a
Nutshell docs on string methods; Perl
Cookbook recipe 1.9; Recipe 1.8.