Python Cookbook

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(  )

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 similar, but it uppercases the first letter of each word:

>>> 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 if a string object is already in a given case form with the methods isupper, islower, and istitle, which all return 1 if the string is nonempty and already meets the uppercase, lowercase, or titlecase constraints. There is no iscapitalized method, but we can code it as a function:

def iscapitalized(s):
    return s[:1].isupper() and s[1:].islower(  )

This may not be exactly what you want, because each of the is methods returns 0 for an empty string, and the three case-checking ones also return 0 for strings that, while not empty, contain no letters at all. This iscapitalized function does not quite match these semantics; rather, it accepts ...