Skip to main content
Engineering LibreTexts

21.8: String Methods

  • Page ID
    16991
  • A method is similar to a function—it takes arguments and returns a value—but the syntax is different. For example, the method upper takes a string and returns a new string with all uppercase letters:

    Instead of the function syntax upper(word), it uses the method syntax word.upper().

    >>> word = 'banana'
    >>> new_word = word.upper()
    >>> print new_word
    BANANA
    

    This form of dot notation specifies the name of the method, upper, and the name of the string to apply the method to, word. The empty parentheses indicate that this method takes no argument.

    A method call is called an invocation; in this case, we would say that we are invoking upper on the word.

    As it turns out, there is a string method named find that is remarkably similar to the function we wrote:

    >>> word = 'banana'
    >>> index = word.find('a')
    >>> print index
    1
    

    In this example, we invoke find on word and pass the letter we are looking for as a parameter.

    Actually, the find method is more general than our function; it can find substrings, not just characters:

    >>> word.find('na')
    2
    

    It can take as a second argument the index where it should start:

    >>> word.find('na', 3)
    4
    

    And as a third argument the index where it should stop:

    >>> name = 'bob'
    >>> name.find('b', 1, 2)
    -1
    

    This search fails because b does not appear in the index range from 1 to 2 (not including 2).

    Exercise \(\PageIndex{1}\)

    There is a string method called count that is similar to the function in the previous exercise. Read the documentation of this method and write an invocation that counts the number of as in 'banana'.

    Exercise \(\PageIndex{2}\)

    Read the documentation of the string methods at http://docs.python.org/2/library/stdtypes.html#string-methods. You might want to experiment with some of them to make sure you understand how they work. strip and replace are particularly useful.

    The documentation uses a syntax that might be confusing. For example, in find(sub[, start[, end]]), the brackets indicate optional arguments. So sub is required, but start is optional, and if you include start, then end is optional.