Lesson 19 · Objects

Special (dunder) methods

Why can you write a + b, len(x) or print(x) for your own classes? Because Python turns that ordinary syntax into calls of special methods, the ones named with double underscores. Follow a small Coins class line by line as Coins(3) calls __init__, a + b calls __add__ and print(c) calls __repr__, then see what breaks when a special method is missing.

14 min9 steps5 questionsIntermediate

Fig 19 · Interactive

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5 quick questions

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01A class defines len(self) to return 3. What does Python call when you write len(box)?

len(box) asks the object for its length by calling the special method __len__ on its class. That is why len() works on any class that defines __len__.

02In coins.py, what does c = a + b run?

For a + b Python calls the left object's __add__ with the right object as its argument: a.__add__(b), strictly type(a).__add__(a, b). Here it returns a new Coins(7).

03After c = a + b in coins.py, what is a.n?

__add__ reads self.n and o.n and builds a brand-new object with Coins(self.n + o.n). It never changes a or b, so a.n is still 3.

04A class has neither str nor repr. What does print() show for one of its objects?

Every class inherits a default __repr__ from object: the module and class name plus a memory address. print() still works; it just is not very readable.

05Coins has no add. What happens at c = a + b ?

a + b needs type(a).__add__. Without it Python raises TypeError and stops at that line. (A __radd__ would not help here: Python only tries it when the two operands are of different types.)

Key terms

Glossary

Special method: A method with a reserved name such as __init__ or __add__ that Python calls for you when you use ordinary syntax: Coins(3), a + b, len(x), print(x).
Dunder: Short for "double underscore": the two underscores on each side of a special method's name, as in __init__ (said "dunder init").
__init__: The special method that sets up a new object. Coins(3) makes the object, then calls __init__ with it as self and 3 as n.
Attribute: A name stored inside an object, such as n in self.n = n. Each object keeps its own attributes.
__add__: The special method behind +. a + b calls a.__add__(b); it should return the result, usually a new object.
__repr__: The special method that returns an object's text form for programmers. print() and str() fall back to it when there is no __str__.
__str__: The special method for a friendly text form. print(x) and str(x) use it when a class defines it.
TypeError: The error Python raises when an operation does not support a type, such as + between two objects with no __add__.

Fine print

Notes and sources

"Dunder" is short for "double underscore". The Python docs call these special methods; some books call them magic methods. Never invent new __names__ of your own: Python reserves that form for itself.
Strictly, Python looks the special method up on the class, not on the object: a + b runs type(a).__add__(a, b). If that is missing (or returns NotImplemented) and b is of a different type, Python tries type(b).__radd__(b, a) before raising TypeError. For two Coins, __radd__ is never tried.
Coins(3) first calls __new__, which makes the empty object, and then __init__, which fills it in. You almost never need to write __new__ yourself.
print() and str() use __str__ when a class defines one, and fall back to __repr__ otherwise. The >>> prompt always shows __repr__. Without either, every object inherits the <... object at 0x...> form from object.
The figure shortens the traceback (it leaves out the File line and the ~~^~~ marker) and wraps the long TypeError line to fit its small terminal; the Try it panel shows the full text. $ is the terminal's own prompt.
Other common special methods: __len__ for len(x), __eq__ for ==, __lt__ for <, __getitem__ for x[i], __iter__ for for-loops, __contains__ for in, and __str__ for a friendlier print().

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Special (dunder) methods

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01A class defines __len__(self) to return 3. What does Python call when you write len(box)?

02In coins.py, what does c = a + b run?

03After c = a + b in coins.py, what is a.n?

04A class has neither __str__ nor __repr__. What does print() show for one of its objects?

05Coins has no __add__. What happens at c = a + b ?

01A class defines len(self) to return 3. What does Python call when you write len(box)?

04A class has neither str nor repr. What does print() show for one of its objects?

05Coins has no add. What happens at c = a + b ?