copy — Shallow and deep copy operations

Código-fonte: Lib/copy.py


Assignment statements in Python do not copy objects, they create bindings between a target and an object. For collections that are mutable or contain mutable items, a copy is sometimes needed so one can change one copy without changing the other. This module provides generic shallow and deep copy operations (explained below).

Sumário da Interface:

copy.copy(x)

Devolve uma cópia rasa de x.

copy.deepcopy(x[, memo])

Retorna uma cópia profunda de x.

exception copy.error

Raised for module specific errors.

The difference between shallow and deep copying is only relevant for compound objects (objects that contain other objects, like lists or class instances):

  • A shallow copy constructs a new compound object and then (to the extent possible) inserts references into it to the objects found in the original.

  • A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.

Two problems often exist with deep copy operations that don’t exist with shallow copy operations:

  • Recursive objects (compound objects that, directly or indirectly, contain a reference to themselves) may cause a recursive loop.

  • Because deep copy copies everything it may copy too much, such as data which is intended to be shared between copies.

The deepcopy() function avoids these problems by:

  • keeping a memo dictionary of objects already copied during the current copying pass; and

  • letting user-defined classes override the copying operation or the set of components copied.

Este módulo não copia tipos como módulo, método, stack trace (situação da pilha de execução), quadro de empilhamento, arquivo, soquete, janela, vetor ou outros tipos semelhantes. Ele “copia” funções e classes (rasas e profundas), devolvendo o objeto original inalterado; isso é compatível com a maneira que estes itens são tratados pelo módulo pickle.

Shallow copies of dictionaries can be made using dict.copy(), and of lists by assigning a slice of the entire list, for example, copied_list = original_list[:].

Classes can use the same interfaces to control copying that they use to control pickling. See the description of module pickle for information on these methods. In fact, the copy module uses the registered pickle functions from the copyreg module.

In order for a class to define its own copy implementation, it can define special methods __copy__() and __deepcopy__(). The former is called to implement the shallow copy operation; no additional arguments are passed. The latter is called to implement the deep copy operation; it is passed one argument, the memo dictionary. If the __deepcopy__() implementation needs to make a deep copy of a component, it should call the deepcopy() function with the component as first argument and the memo dictionary as second argument.

Ver também

Módulo pickle

Discussion of the special methods used to support object state retrieval and restoration.