cpython/Doc/library/copy.rst

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:mod:`copy` --- Shallow and deep copy operations
================================================
.. module:: copy
:synopsis: Shallow and deep copy operations.
**Source code:** :source:`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).
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Interface summary:
.. function:: copy(x)
Return a shallow copy of *x*.
.. function:: deepcopy(x[, memo])
Return a deep copy of *x*.
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.. exception:: error
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Raised for module specific errors.
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.. _shallow_vs_deep_copy:
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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.
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The :func:`deepcopy` function avoids these problems by:
* keeping a ``memo`` dictionary of objects already copied during the current
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copying pass; and
* letting user-defined classes override the copying operation or the set of
components copied.
This module does not copy types like module, method, stack trace, stack frame,
file, socket, window, array, or any similar types. It does "copy" functions and
classes (shallow and deeply), by returning the original object unchanged; this
is compatible with the way these are treated by the :mod:`pickle` module.
Merged revisions 57778-58052 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r57820 | georg.brandl | 2007-08-31 08:59:27 +0200 (Fri, 31 Aug 2007) | 2 lines Document new shorthand notation for index entries. ........ r57827 | georg.brandl | 2007-08-31 10:47:51 +0200 (Fri, 31 Aug 2007) | 2 lines Fix subitem markup. ........ r57833 | martin.v.loewis | 2007-08-31 12:01:07 +0200 (Fri, 31 Aug 2007) | 1 line Mark registry components as 64-bit on Win64. ........ r57854 | bill.janssen | 2007-08-31 21:02:23 +0200 (Fri, 31 Aug 2007) | 1 line deprecate use of FakeSocket ........ r57855 | bill.janssen | 2007-08-31 21:02:46 +0200 (Fri, 31 Aug 2007) | 1 line remove mentions of socket.ssl in comments ........ r57856 | bill.janssen | 2007-08-31 21:03:31 +0200 (Fri, 31 Aug 2007) | 1 line remove use of non-existent SSLFakeSocket in apparently untested code ........ r57859 | martin.v.loewis | 2007-09-01 08:36:03 +0200 (Sat, 01 Sep 2007) | 3 lines Bug #1737210: Change Manufacturer of Windows installer to PSF. Will backport to 2.5. ........ r57865 | georg.brandl | 2007-09-01 09:51:24 +0200 (Sat, 01 Sep 2007) | 2 lines Fix RST link (backport from Py3k). ........ r57876 | georg.brandl | 2007-09-01 17:49:49 +0200 (Sat, 01 Sep 2007) | 2 lines Document sets' ">" and "<" operations (backport from py3k). ........ r57878 | skip.montanaro | 2007-09-01 19:40:03 +0200 (Sat, 01 Sep 2007) | 4 lines Added a note and examples to explain that re.split does not split on an empty pattern match. (issue 852532). ........ r57879 | walter.doerwald | 2007-09-01 20:18:09 +0200 (Sat, 01 Sep 2007) | 2 lines Fix wrong function names. ........ r57880 | walter.doerwald | 2007-09-01 20:34:05 +0200 (Sat, 01 Sep 2007) | 2 lines Fix typo. ........ r57889 | andrew.kuchling | 2007-09-01 22:31:59 +0200 (Sat, 01 Sep 2007) | 1 line Markup fix ........ r57892 | andrew.kuchling | 2007-09-01 22:43:36 +0200 (Sat, 01 Sep 2007) | 1 line Add various items ........ r57895 | andrew.kuchling | 2007-09-01 23:17:58 +0200 (Sat, 01 Sep 2007) | 1 line Wording change ........ r57896 | andrew.kuchling | 2007-09-01 23:18:31 +0200 (Sat, 01 Sep 2007) | 1 line Add more items ........ r57904 | ronald.oussoren | 2007-09-02 11:46:07 +0200 (Sun, 02 Sep 2007) | 3 lines Macosx: this patch ensures that the value of MACOSX_DEPLOYMENT_TARGET used by the Makefile is also used at configure-time. ........ r57925 | georg.brandl | 2007-09-03 09:16:46 +0200 (Mon, 03 Sep 2007) | 2 lines Fix #883466: don't allow Unicode as arguments to quopri and uu codecs. ........ r57936 | matthias.klose | 2007-09-04 01:33:04 +0200 (Tue, 04 Sep 2007) | 2 lines - Added support for linking the bsddb module against BerkeleyDB 4.6.x. ........ r57954 | mark.summerfield | 2007-09-04 10:16:15 +0200 (Tue, 04 Sep 2007) | 3 lines Added cross-references plus a note about dict & list shallow copying. ........ r57958 | martin.v.loewis | 2007-09-04 11:51:57 +0200 (Tue, 04 Sep 2007) | 3 lines Document that we rely on the OS to release the crypto context. Fixes #1626801. ........ r57960 | martin.v.loewis | 2007-09-04 15:13:14 +0200 (Tue, 04 Sep 2007) | 3 lines Patch #1388440: Add set_completion_display_matches_hook and get_completion_type to readline. ........ r57961 | martin.v.loewis | 2007-09-04 16:19:28 +0200 (Tue, 04 Sep 2007) | 3 lines Patch #1031213: Decode source line in SyntaxErrors back to its original source encoding. Will backport to 2.5. ........ r57972 | matthias.klose | 2007-09-04 20:17:36 +0200 (Tue, 04 Sep 2007) | 3 lines - Makefile.pre.in(buildbottest): Run an optional script pybuildbot.identify to include some information about the build environment. ........ r57973 | matthias.klose | 2007-09-04 21:05:38 +0200 (Tue, 04 Sep 2007) | 2 lines - Makefile.pre.in(buildbottest): Remove whitespace at eol. ........ r57975 | matthias.klose | 2007-09-04 22:46:02 +0200 (Tue, 04 Sep 2007) | 2 lines - Fix libffi configure for hppa*-*-linux* | parisc*-*-linux*. ........ r57980 | bill.janssen | 2007-09-05 02:46:27 +0200 (Wed, 05 Sep 2007) | 1 line SSL certificate distinguished names should be represented by tuples ........ r57985 | martin.v.loewis | 2007-09-05 08:39:17 +0200 (Wed, 05 Sep 2007) | 3 lines Patch #1105: Explain that one needs to build the solution to get dependencies right. ........ r57987 | armin.rigo | 2007-09-05 09:51:21 +0200 (Wed, 05 Sep 2007) | 4 lines PyDict_GetItem() returns a borrowed reference. There are probably a number of places that are open to attacks such as the following one, in bltinmodule.c:min_max(). ........ r57991 | martin.v.loewis | 2007-09-05 13:47:34 +0200 (Wed, 05 Sep 2007) | 3 lines Patch #786737: Allow building in a tree of symlinks pointing to a readonly source. ........ r57993 | georg.brandl | 2007-09-05 15:36:44 +0200 (Wed, 05 Sep 2007) | 2 lines Backport from Py3k: Bug #1684991: explain lookup semantics for __special__ methods (new-style classes only). ........ r58004 | armin.rigo | 2007-09-06 10:30:51 +0200 (Thu, 06 Sep 2007) | 4 lines Patch #1733973 by peaker: ptrace_enter_call() assumes no exception is currently set. This assumption is broken when throwing into a generator. ........ r58006 | armin.rigo | 2007-09-06 11:30:38 +0200 (Thu, 06 Sep 2007) | 4 lines PyDict_GetItem() returns a borrowed reference. This attack is against ceval.c:IMPORT_NAME, which calls an object (__builtin__.__import__) without holding a reference to it. ........ r58013 | georg.brandl | 2007-09-06 16:49:56 +0200 (Thu, 06 Sep 2007) | 2 lines Backport from 3k: #1116: fix reference to old filename. ........ r58021 | thomas.heller | 2007-09-06 22:26:20 +0200 (Thu, 06 Sep 2007) | 1 line Fix typo: c_float represents to C float type. ........ r58022 | skip.montanaro | 2007-09-07 00:29:06 +0200 (Fri, 07 Sep 2007) | 3 lines If this is correct for py3k branch and it's already in the release25-maint branch, seems like it ought to be on the trunk as well. ........ r58023 | gregory.p.smith | 2007-09-07 00:59:59 +0200 (Fri, 07 Sep 2007) | 4 lines Apply the fix from Issue1112 to make this test more robust and keep windows happy. ........ r58031 | brett.cannon | 2007-09-07 05:17:50 +0200 (Fri, 07 Sep 2007) | 4 lines Make uuid1 and uuid4 tests conditional on whether ctypes can be imported; implementation of either function depends on ctypes but uuid as a whole does not. ........ r58032 | brett.cannon | 2007-09-07 06:18:30 +0200 (Fri, 07 Sep 2007) | 6 lines Fix a crasher where Python code managed to infinitely recurse in C code without ever going back out to Python code in PyObject_Call(). Required introducing a static RuntimeError instance so that normalizing an exception there is no reliance on a recursive call that would put the exception system over the recursion check itself. ........ r58034 | thomas.heller | 2007-09-07 08:32:17 +0200 (Fri, 07 Sep 2007) | 1 line Add a 'c_longdouble' type to the ctypes module. ........ r58035 | thomas.heller | 2007-09-07 11:30:40 +0200 (Fri, 07 Sep 2007) | 1 line Remove unneeded #include. ........ r58036 | thomas.heller | 2007-09-07 11:33:24 +0200 (Fri, 07 Sep 2007) | 6 lines Backport from py3k branch: Add a workaround for a strange bug on win64, when _ctypes is compiled with the SDK compiler. This should fix the failing Lib\ctypes\test\test_as_parameter.py test. ........ r58037 | georg.brandl | 2007-09-07 16:14:40 +0200 (Fri, 07 Sep 2007) | 2 lines Fix a wrong indentation for sublists. ........ r58043 | georg.brandl | 2007-09-07 22:10:49 +0200 (Fri, 07 Sep 2007) | 2 lines #1095: ln -f doesn't work portably, fix in Makefile. ........ r58049 | skip.montanaro | 2007-09-08 02:34:17 +0200 (Sat, 08 Sep 2007) | 1 line be explicit about the actual location of the missing file ........
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Shallow copies of dictionaries can be made using :meth:`dict.copy`, and
of lists by assigning a slice of the entire list, for example,
``copied_list = original_list[:]``.
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.. index:: module: pickle
Classes can use the same interfaces to control copying that they use to control
pickling. See the description of module :mod:`pickle` for information on these
methods. In fact, the :mod:`copy` module uses the registered
pickle functions from the :mod:`copyreg` module.
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.. index::
single: __copy__() (copy protocol)
single: __deepcopy__() (copy protocol)
In order for a class to define its own copy implementation, it can define
special methods :meth:`__copy__` and :meth:`__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 :meth:`__deepcopy__` implementation needs
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to make a deep copy of a component, it should call the :func:`deepcopy` function
with the component as first argument and the memo dictionary as second argument.
The memo dictionary should be treated as an opaque object.
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.. seealso::
Module :mod:`pickle`
Discussion of the special methods used to support object state retrieval and
restoration.