Creating objects: constructors

To create an object, you must use a constructor. Python offers shortcuts for many of its objects: for str, we just use 'string notation', and list and dict have their own notations, too. But user-defined classes can be instantiated into objects using a constructor. Here ‘constructor’ is a fancy word for “a function that returns a new object”.

We’ve already seen constructors used: when we raise an exception, we write raise ValueError('something went wrong'). The argument to the raise keyword is a call to the ValueError constructor.

In Python, a type/class and its constructor are one and the same thing. The functions str and list and dict we’ve seen are the type names for those types, but they’re also the constructors, i.e., calling them will create new strings and lists and dictionaries.

>>> str
<class 'str'>
>>> list
<class 'list'>
>>> dict
<class 'dict'>
>>> str('hi')
'hi'
>>> str(5)
'5'
>>> list("hello there")
['h', 'e', 'l', 'l', 'o', ' ', 't', 'h', 'e', 'r', 'e']
>>> list()
[]

Notice that the constructors here are very versatile. Since these types are special, it’s perhaps clearer to see an example with the various classes of exception and error we’ve seen:

>>> v = ValueError('hi there')
>>> v
ValueError('hi there')
>>> v2 = ValueError('another one')
>>> v2
ValueError('another one')
>>> v3 = ValueError()
>>> v3
ValueError()