1:mod:`copy` --- Shallow and deep copy operations
2================================================
3
4.. module:: copy
5   :synopsis: Shallow and deep copy operations.
6
7**Source code:** :source:`Lib/copy.py`
8
9--------------
10
11Assignment statements in Python do not copy objects, they create bindings
12between a target and an object. For collections that are mutable or contain
13mutable items, a copy is sometimes needed so one can change one copy without
14changing the other. This module provides generic shallow and deep copy
15operations (explained below).
16
17
18Interface summary:
19
20.. function:: copy(x)
21
22   Return a shallow copy of *x*.
23
24
25.. function:: deepcopy(x[, memo])
26
27   Return a deep copy of *x*.
28
29
30.. exception:: Error
31
32   Raised for module specific errors.
33
34.. _shallow_vs_deep_copy:
35
36The difference between shallow and deep copying is only relevant for compound
37objects (objects that contain other objects, like lists or class instances):
38
39* A *shallow copy* constructs a new compound object and then (to the extent
40  possible) inserts *references* into it to the objects found in the original.
41
42* A *deep copy* constructs a new compound object and then, recursively, inserts
43  *copies* into it of the objects found in the original.
44
45Two problems often exist with deep copy operations that don't exist with shallow
46copy operations:
47
48* Recursive objects (compound objects that, directly or indirectly, contain a
49  reference to themselves) may cause a recursive loop.
50
51* Because deep copy copies everything it may copy too much, such as data
52  which is intended to be shared between copies.
53
54The :func:`deepcopy` function avoids these problems by:
55
56* keeping a ``memo`` dictionary of objects already copied during the current
57  copying pass; and
58
59* letting user-defined classes override the copying operation or the set of
60  components copied.
61
62This module does not copy types like module, method, stack trace, stack frame,
63file, socket, window, array, or any similar types.  It does "copy" functions and
64classes (shallow and deeply), by returning the original object unchanged; this
65is compatible with the way these are treated by the :mod:`pickle` module.
66
67Shallow copies of dictionaries can be made using :meth:`dict.copy`, and
68of lists by assigning a slice of the entire list, for example,
69``copied_list = original_list[:]``.
70
71.. index:: module: pickle
72
73Classes can use the same interfaces to control copying that they use to control
74pickling.  See the description of module :mod:`pickle` for information on these
75methods.  In fact, the :mod:`copy` module uses the registered
76pickle functions from the :mod:`copyreg` module.
77
78.. index::
79   single: __copy__() (copy protocol)
80   single: __deepcopy__() (copy protocol)
81
82In order for a class to define its own copy implementation, it can define
83special methods :meth:`__copy__` and :meth:`__deepcopy__`.  The former is called
84to implement the shallow copy operation; no additional arguments are passed.
85The latter is called to implement the deep copy operation; it is passed one
86argument, the ``memo`` dictionary.  If the :meth:`__deepcopy__` implementation needs
87to make a deep copy of a component, it should call the :func:`deepcopy` function
88with the component as first argument and the memo dictionary as second argument.
89
90
91.. seealso::
92
93   Module :mod:`pickle`
94      Discussion of the special methods used to support object state retrieval and
95      restoration.
96
97