Creating Python classes

We don't have to write much Python code to realize that Python is a very clean language. When we want to do something, we can just do it, without having to set up a bunch of prerequisite code. The ubiquitous hello world in Python, as you've likely seen, is only one line.

Similarly, the simplest class in Python 3 looks like this:

class MyFirstClass: 
    pass 

There's our first object-oriented program! The class definition starts with the class keyword. This is followed by a name (of our choice) identifying the class, and is terminated with a colon.

The class name must follow standard Python variable naming rules (it must start with a letter or underscore, and can only be comprised of letters, underscores, or numbers). In addition, the Python style guide (search the web for PEP 8) recommends that classes should be named using CapWords notation (start with a capital letter; any subsequent words should also start with a capital).

The class definition line is followed by the class contents, indented. As with other Python constructs, indentation is used to delimit the classes, rather than braces, keywords, or brackets, as many other languages use. Also in line with the style guide, use four spaces for indentation unless you have a compelling reason not to (such as fitting in with somebody else's code that uses tabs for indents).

Since our first class doesn't actually add any data or behaviors, we simply use the pass keyword on the second line to indicate that no further action needs to be taken.

We might think there isn't much we can do with this most basic class, but it does allow us to instantiate objects of that class. We can load the class into the Python 3 interpreter, so we can interactively play with it. To do this, save the class definition mentioned earlier in a file named first_class.py and then run the python -i first_class.py command. The -i argument tells Python to run the code and then drop to the interactive interpreter. The following interpreter session demonstrates a basic interaction with this class:

>>> a = MyFirstClass()
>>> b = MyFirstClass()
>>> print(a)
<__main__.MyFirstClass object at 0xb7b7faec>
>>> print(b)
<__main__.MyFirstClass object at 0xb7b7fbac>
>>>  

This code instantiates two objects from the new class, named a and b. Creating an instance of a class is a simple matter of typing the class name, followed by a pair of parentheses. It looks much like a normal function call, but Python knows we're calling a class and not a function, so it understands that its job is to create a new object. When printed, the two objects tell us which class they are and what memory address they live at. Memory addresses aren't used much in Python code, but here, they demonstrate that there are two distinct objects involved.