all Python 3copyfunction

__copy__

customization

The method copy.copy(obj) calls when it is defined: custom logic for a shallow copy.

Syntax

def __copy__(self): ...

Without __copy__ the copy module copies the object itself: it creates an instance of the same class and transfers its __dict__, leaving nested objects shared. A custom __copy__ is needed when part of the state must not or need not be copied: a connection, a cache, a counter. The method takes no arguments besides self.

Examples

import copy
class Doc:
    def __init__(self, text):
        self.text, self.cache = text, {'len': len(text)}
    def __copy__(self):
        return Doc(self.text)  # кэш у копии свой, а не общий
a = Doc('abc')
b = copy.copy(a)
print(b.text, b.cache is a.cache)  # → abc False

See also

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