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.
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
>>> help(str.capitalize) Help on method_descriptor: capitalize(...) 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
upper(word), it uses the method syntax,
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
A method call is called an invocation; in this case, we
would say that we are invoking
upper on the
For example, there is a string method named
searches for the position of one string within another:
In this example, we invoke
pass the letter we are looking for as a parameter.
find method can find substrings as well as characters:
>>> word.find('na') 2
It can also take as a second argument the index where it should start looking:
>>> word.find('na', 3) 4
One common task is to remove white space (spaces, tabs, or newlines)
from the beginning and end of a string using the
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
method before we do any checking:
In the last example, the method
lower is called. Then, we
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
to count the number of times a double s (ss) appears in a word. There are 3 mistakes to fix.