Why does the istitle() string method return false if the string is clearly in title-case?

Question

Of the istitle() string method, the Python 2.6.5 manual reads:

Return true if the string is a titlecased string and there is at least one character, for example uppercase characters may only follow uncased characters and lowercase characters only cased ones. Return false otherwise.

But in this case it returns false:

>>> book = 'what every programmer must know'
>>> book.title()
'What Every Programmer Must Know'
>>> book.istitle()
False

What am I missing?

Answer 1

Probably because you are still calling istitle() on the original book.

Try book.title().istitle() instead....

Answer 2

book.title() does not change the variable book. It just returns the string in title case.

>>> book.title()
'What Every Programmer Must Know'
>>> book             # still not in title case
'what every programmer must know'
>>> book.istitle()   # hence it returns False.
False
>>> book.title().istitle()   # returns True as expected
True

Answer 3

Do the following:

print book

after you do book.title(). You will see that book hasn't changed.

The reason is that book.title() creates a new string. The name book still refers to the original string.

Answer 4

The method title() doesn't mutate the string (strings are immutable in Python). It creates a new string which you must assign to your variable:

>>> book = 'what every programmer must know'
>>> book = book.title()
>>> book.istitle()
True