Magic methods

Methods with double underscores (dunders) hook into Python's built-in behaviour:

__str__ controls what print(obj) and str(obj) show
__len__ makes len(obj) work
__eq__ defines ==

Define these methods with self, just like other methods: def __str__(self): returns text, and def __len__(self): returns a nonnegative integer.

For equality, Python passes the other value as a second argument:

def __eq__(self, other):
    if not isinstance(other, type(self)):
        return NotImplemented
    return self.value == other.value

isinstance(other, SomeClass) checks whether other belongs to that class (or a subclass). NotImplemented is a special return value, not an exception: it lets Python try the other object's comparison before falling back. Check the type before reading its attributes.

Without __str__ you get something ugly like <__main__.Book object at 0x...>.

See it run

class Money:
    def __init__(self, cents):
        self.cents = cents

    def __add__(self, other):
        return Money(self.cents + other.cents)

    def __str__(self):
        return f"{self.cents / 100:.2f}"

a = Money(250)
b = Money(175)
total = a + b
print(total)

Magic methods let Python syntax like + and print call your code.

Practice magic methods

Exercise 1

Give Book in the code editor below a __str__ that returns TITLE by AUTHOR using each book's attributes (e.g. "Dune by Frank Herbert"), and a __len__ that returns its page count.

Exercise 2

Build Playlist(name, songs) storing a list copy. len(playlist) gives the current song count and str(playlist) returns NAME (N tracks). Both must reflect later edits to playlist.songs, including empty playlists.

Exercise 3

Build Coordinate(x, y). str(point) is (x, y). Two Coordinates compare equal only when both axes match. For unrelated types, __eq__ returns NotImplemented so comparisons stay safe.

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Practice

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Memory · names → values

Variables appear here as Python creates them.

Output & plots