Count the number of occurrences of a character in a string

How do I count the number of occurrences of a character in a string?

e.g. 'a' appears in 'Mary had a little lamb' 4 times.

str.count(sub[, start[, end]])

Return the number of non-overlapping occurrences of substring sub in the range [start, end]. Optional arguments start and end are interpreted as in slice notation.

>>> sentence="Mary had a little lamb"
>>> sentence.count('a')
4