Modules, Classes, and Methods
In Importing Modules, we saw that a module is a kind of object, one that can contain functions and other variables. There is another kind of object that is similar to a module: a class. You’ve been using classes all along, probably without realizing it: a class is how Python represents a type.
You may have called built-in function help on int, float, bool, or str. We’ll do that now with str (notice that the first line says that it’s a class):
| | >>> help(str) |
| | Help on class str in module builtins: |
| | |
| | class str(object) |
| | | str(object='') -> str |
| | | str(bytes_or_buffer[, encoding[, errors]]) -> str |
| | | |
| | | Create a new string object from the given object. If encoding or |
| | | errors is specified, then the object must expose a data buffer |
| | | that will be decoded using the given encoding and error handler. |
| | | Otherwise, returns the result of object.__str__() (if defined) |
| | | or repr(object). |
| | | encoding defaults to sys.getdefaultencoding(). |
| | | errors defaults to 'strict'. |
| | | |
| | | Methods defined here: |
| | | |
| | | __add__(self, value, /) |
| | | Return self+value. |
| | | |
| | | __contains__(self, key, /) |
| | | Return key in self. |
| | |
| | [Lots of other names with leading and trailing underscores not shown here.] |
| | |
| | | capitalize(...) |
| | | S.capitalize() -> str |
| | | |
| | | Return a capitalized version of S, i.e. make the first character |
| | | have upper case and the rest lower case. |
| | | |
| | | casefold(...) |
| | | S.casefold() -> str |
| | | |
| | | Return a version of S suitable for caseless comparisons. |
| | | |
| | | center(...) |
| | | S.center(width[, fillchar]) -> str |
| | | |
| | | Return S centered in a string of length width. Padding is |
| | | done using the specified fill character (default is a space) |
| | | |
| | | count(...) |
| | | S.count(sub[, start[, end]]) -> int |
| | | |
| | | Return the number of non-overlapping occurrences of substring sub in |
| | | string S[start:end]. Optional arguments start and end are |
| | | interpreted as in slice notation. |
| | |
| | [There are many more of these as well.] |
Near the top of this documentation is this:
| | | str(object[, encoding[, errors]]) -> str |
| | | |
| | | Create a new string object from the given object. |
That describes how to use str as a function: we can call it to create a string. For example, str(17) creates the string ’17’.
We can also use str to call a method in class str, much like we call a function in module math. The main difference is that every method in class str requires a string as its first argument:
| | >>> str.capitalize('browning') |
| | 'Browning' |
This is how methods are different from functions: the first argument to every string method must be a string, and the parameter is not described in the documentation for the method. This is because all string methods require a string as the first argument, and more generally, all methods in a class require an object of that class as the first argument. Here are two more examples, this time using the other two string methods from the code. Both of these also require a string as the first argument.
| | >>> str.center('Sonnet 43', 26) |
| | ' Sonnet 43 ' |
| | >>> str.count('How do I love thee? Let me count the ways.', 'the') |
| | 2 |
The first method call produces a new string that centers ’Sonnet 43’ in a string of length 26, padding to the left and right with spaces.
The second method call counts how many times ’the’ occurs in ’How do I love thee? Let me count the ways.’ (once in the word thee and once as the penultimate word in the string).