magicmethods.md

Magic methods

In the class tutorial we learned to define a class like this:

class Website:

    def __init__(self, url, founding_year, free_to_use):
        self.url = url
        self.founding_year = founding_year
        self.free_to_use = free_to_use

    def info(self):
        print("URL:", self.url)
        print("Founding year:", self.founding_year)
        print("Free to use:", self.free_to_use)

After doing that we can create a new Website object like Website('https://github.com/', 2008, True). Python first creates the Website object, and then calls __init__ with the arguments we passed to Website to set it up. Methods that have a name __like_this__ and a special meaning are called magic methods or special methods.

Most magic methods define what the object has or what it can do, like "does it have a length" or "can we for loop over it". There are other magic methods that do other things also, like __init__.

Some magic methods have a default implementation that is used if the class doesn't define anything else. For example, if we don't define an __init__ then our class will take no arguments and it won't have any attributes by default. We'll learn more about this when we'll talk about inheritance.

TODO: write a classes2.md.

Custom length

Let's get started by defining an object that has a length:

>>> class Thing:
...     def __len__(self):
...         return 5
...
>>> t = Thing()
>>> t
<__main__.Thing object at 0x7f05e4597198>
>>> t.__len__()
5
>>> len(t)
5
>>>

This is what most magic methods are like. So far we have learned to use len() with lists, strings and other built-in types, but now we can call len() on our own Thing object. Many things can be fully customized with magic methods.

Note that magic methods like __len__ need to be defined in the class, just attaching an attribute called __len__ doesn't work:

>>> class EmptyThing:
...     pass
...
>>> def length():
...     return 5
...
>>> e = EmptyThing()
>>> e.__len__ = length
>>> e.__len__()
5
>>> len(e)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object of type 'EmptyThing' has no len()
>>>

You don't really need to worry about why Python works like this, but it's explained here if you want to know more about it.

String representations

You have probably noticed that typing something to the interactive >>> prompt is not quite the same thing as printing it. For example, strings behave like this:

>>> 'hello'
'hello'
>>> print('hello')
hello
>>>

If you want to print something the way it's displayed on the >>> prompt you can use the repr() function. Here "repr" is short for "representation".

>>> message = 'hello'
>>> print("the message is", repr(message))
the message is 'hello'
>>>

Combining repr() with string formatting is also easy.

>>> print(f"the message is {repr(message)}")
the message is 'hello'
>>>

The __repr__ magic method can be used to customize this. For example, we can do this:

>>> class Website:
...     def __repr__(self):
...         return '<a Website object>'
...
>>> w = Website()
>>> w.__repr__()
'<a Website object>'
>>> str(w)
'<a Website object>'
>>> print(w)
<a Website object>
>>> w
<a Website object>
>>>

The __repr__ method can return any string, but usually you should follow one of these styles:

1. A piece of code that describes how another, similar object can be created.

   >>> class Website:
   ...     def __init__(self, name, founding_year):
   ...         self.name = name
   ...         self.founding_year = founding_year
   ...     def __repr__(self):
   ...         return f'Website(name={repr(self.name)}, founding_year={repr(self.founding_year)})'
   ...
   >>> github = Website('GitHub', 2008)
   >>> github
   Website(name='GitHub', founding_year=2008)
   >>>

   This is useful for simple data containers like this Website class.

2. A description of the object wrapped between < and >.

   >>> class Website:
   ...     def __init__(self, name, founding_year):
   ...         self.name = name
   ...         self.founding_year = founding_year
   ...     def __repr__(self):
   ...         return f'<Website {repr(self.name)}, founded in {repr(self.founding_year)}>'
   ...
   >>> github = Website('GitHub', 2008)
   >>> github
   <Website 'GitHub', founded in 2008>
   >>>

   This style is good when you want to tell more about the object than you can by showing the __init__ arguments. Python's built-in things also use this style more:

   >>> import random
   >>> random
   <module 'random' from '/some/path/random.py'>
   >>>

Other magic methods

There are many more magic methods, and I don't see any reason to list them all here. The official documentation has more information about magic methods if you need it. We'll go through using the most important magic methods in the rest of this tutorial, so if you just keep reading you'll learn more about them.

When should we use magic methods?

There's nothing wrong with using __init__ everywhere, but other than that, magic methods are usually not needed. website.has_user(user) and user in website.userlist are way better than something weird that we could do with magic methods like user @ website. People expect website.has_user(user) check if a user has registered on the website, but nobody can guess what user @ website does. Explicit is better than implicit, and simple is better than complex.

On the other hand, using magic methods when needed can turn something good into something great. Especially the __repr__ method is useful because people can get a good idea of what an object is by just looking at it on the >>> prompt or printing it. I recommend using __repr__ methods in things that other people will import and use in their projects, but __repr__ methods aren't worth it for simple scripts that are not meant to be imported.

Summary

• Magic methods define what instances of a class can do and how, like "does it have a length" or "what does it look like when I print it".
• Python uses magic methods to implement many things internally, and we can customize everything by implementing the magic methods ourselves.
• Defining custom __repr__ methods is often a good idea when making things that other people will import and use in their own projects, and the __init__ method is very useful for many things. Other than that, magic methods are usually not worth it.

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You may use this tutorial freely at your own risk. See LICENSE.

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