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98 lines
3.4 KiB
ReStructuredText
98 lines
3.4 KiB
ReStructuredText
:mod:`copy` --- Shallow and deep copy operations
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================================================
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.. module:: copy
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:synopsis: Shallow and deep copy operations.
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**Source code:** :source:`Lib/copy.py`
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--------------
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Assignment statements in Python do not copy objects, they create bindings
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between a target and an object. For collections that are mutable or contain
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mutable items, a copy is sometimes needed so one can change one copy without
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changing the other. This module provides generic shallow and deep copy
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operations (explained below).
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Interface summary:
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.. function:: copy(x)
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Return a shallow copy of *x*.
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.. function:: deepcopy(x[, memo])
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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
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objects (objects that contain other objects, like lists or class instances):
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* A *shallow copy* constructs a new compound object and then (to the extent
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possible) inserts *references* into it to the objects found in the original.
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* A *deep copy* constructs a new compound object and then, recursively, inserts
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*copies* into it of the objects found in the original.
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Two problems often exist with deep copy operations that don't exist with shallow
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copy operations:
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* Recursive objects (compound objects that, directly or indirectly, contain a
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reference to themselves) may cause a recursive loop.
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* Because deep copy copies everything it may copy too much, such as data
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which is intended to be shared between copies.
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The :func:`deepcopy` function avoids these problems by:
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* keeping a ``memo`` dictionary of objects already copied during the current
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copying pass; and
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* letting user-defined classes override the copying operation or the set of
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components copied.
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This module does not copy types like module, method, stack trace, stack frame,
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file, socket, window, array, or any similar types. It does "copy" functions and
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classes (shallow and deeply), by returning the original object unchanged; this
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is compatible with the way these are treated by the :mod:`pickle` module.
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Shallow copies of dictionaries can be made using :meth:`dict.copy`, and
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of lists by assigning a slice of the entire list, for example,
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``copied_list = original_list[:]``.
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.. index:: module: pickle
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Classes can use the same interfaces to control copying that they use to control
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pickling. See the description of module :mod:`pickle` for information on these
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methods. In fact, the :mod:`copy` module uses the registered
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pickle functions from the :mod:`copyreg` module.
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.. index::
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single: __copy__() (copy protocol)
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single: __deepcopy__() (copy protocol)
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In order for a class to define its own copy implementation, it can define
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special methods :meth:`__copy__` and :meth:`__deepcopy__`. The former is called
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to implement the shallow copy operation; no additional arguments are passed.
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The latter is called to implement the deep copy operation; it is passed one
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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
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with the component as first argument and the memo dictionary as second argument.
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The memo dictionary should be treated as an opaque object.
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.. seealso::
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Module :mod:`pickle`
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Discussion of the special methods used to support object state retrieval and
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restoration.
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