all Python 3OOPfunction

__setitem__

magic methods

The method behind item assignment: obj[k] = v calls obj.__setitem__(k, v).

Syntax

def __setitem__(self, key, value): ...

Without __setitem__ item assignment raises TypeError: 'object does not support item assignment' — the way tuples and strings behave. The companion __delitem__ handles del obj[k]. __setitem__ is a natural place to validate the value before storing it.

Examples

class Grades:
    def __init__(self):
        self._data = {}
    def __setitem__(self, name, score):
        if not 0 <= score <= 100:
            raise ValueError(f'оценка {score} вне 0..100')
        self._data[name] = score
    def __getitem__(self, name):
        return self._data[name]
g = Grades()
g['Анна'] = 95
print(g['Анна'])  # → 95
g['Борис'] = 120  # → ValueError: оценка 120 вне 0..100

See also

Common errors

Open in the full glossarydocs.python.orgReport an inaccuracy