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 1
 2:mod:`copy` --- Shallow and deep copy operations
 3================================================
 4
 5.. module:: copy
 6   :synopsis: Shallow and deep copy operations.
 7
 8
 9.. index::
10   single: copy() (in copy)
11   single: deepcopy() (in copy)
12
13This module provides generic (shallow and deep) copying operations.
14
15Interface summary::
16
17   import copy
18
19   x = copy.copy(y)        # make a shallow copy of y
20   x = copy.deepcopy(y)    # make a deep copy of y
21
22For module specific errors, :exc:`copy.error` is raised.
23
24The difference between shallow and deep copying is only relevant for compound
25objects (objects that contain other objects, like lists or class instances):
26
27* A *shallow copy* constructs a new compound object and then (to the extent
28  possible) inserts *references* into it to the objects found in the original.
29
30* A *deep copy* constructs a new compound object and then, recursively, inserts
31  *copies* into it of the objects found in the original.
32
33Two problems often exist with deep copy operations that don't exist with shallow
34copy operations:
35
36* Recursive objects (compound objects that, directly or indirectly, contain a
37  reference to themselves) may cause a recursive loop.
38
39* Because deep copy copies *everything* it may copy too much, e.g.,
40  administrative data structures that should be shared even between copies.
41
42The :func:`deepcopy` function avoids these problems by:
43
44* keeping a "memo" dictionary of objects already copied during the current
45  copying pass; and
46
47* letting user-defined classes override the copying operation or the set of
48  components copied.
49
50This module does not copy types like module, method, stack trace, stack frame,
51file, socket, window, array, or any similar types.  It does "copy" functions and
52classes (shallow and deeply), by returning the original object unchanged; this
53is compatible with the way these are treated by the :mod:`pickle` module.
54
55Shallow copies of dictionaries can be made using :meth:`dict.copy`, and
56of lists by assigning a slice of the entire list, for example,
57``copied_list = original_list[:]``.
58
59.. versionchanged:: 2.5
60   Added copying functions.
61
62.. index:: module: pickle
63
64Classes can use the same interfaces to control copying that they use to control
65pickling.  See the description of module :mod:`pickle` for information on these
66methods.  The :mod:`copy` module does not use the :mod:`copy_reg` registration
67module.
68
69.. index::
70   single: __copy__() (copy protocol)
71   single: __deepcopy__() (copy protocol)
72
73In order for a class to define its own copy implementation, it can define
74special methods :meth:`__copy__` and :meth:`__deepcopy__`.  The former is called
75to implement the shallow copy operation; no additional arguments are passed.
76The latter is called to implement the deep copy operation; it is passed one
77argument, the memo dictionary.  If the :meth:`__deepcopy__` implementation needs
78to make a deep copy of a component, it should call the :func:`deepcopy` function
79with the component as first argument and the memo dictionary as second argument.
80
81
82.. seealso::
83
84   Module :mod:`pickle`
85      Discussion of the special methods used to support object state retrieval and
86      restoration.
87