7.9. String methods

Strings are an example of Python objects. An object contains both data (the actual string itself) and methods, which are effectively functions that are built into the object and are available to any instance of the object.

Python has a function called dir which lists all the methods available for an object. The type function returns the type of an object.

While the dir function lists the methods, and you can use help to get some simple documentation on a method, a better source of documentation for string methods would be https://docs.python.org/library/stdtypes.html#string-methods.

>>> help(str.capitalize)
Help on method_descriptor:

    S.capitalize() -> str

    Return a capitalized version of S, i.e. make the first character
    have upper case and the rest lower case.

Calling a method is similar to calling a function (it takes arguments and returns a value), but the syntax is different. We call a method by typing the variable name, adding a period, and then adding the method call.

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().

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.

For example, there is a string method named find that searches for the position of one string within another:

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

The find method can find substrings as well as characters:

>>> word.find('na')

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

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

One common task is to remove white space (spaces, tabs, or newlines) from the beginning and end of a string using the strip method:

Some methods, such as startswith, return boolean values:

You will note that startswith requires case to match, so sometimes we’ll convert a line to lowercase using the lower method before we do any checking:

In the last example, the method lower is called. Then, we use startswith to see if the resulting lowercase string starts with the letter “h”. As long as we are careful with the order, we can make multiple method calls in a single expression.

Fix the following function. It should use the string method count to count the number of times a double s (ss) appears in a word. There are 3 mistakes to fix.

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