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 

    13 This module provides generic (shallow and deep) copying operations.

    14 

    15 Interface 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 

    22 For module specific errors, :exc:`copy.error` is raised.

    23 

    24 The difference between shallow and deep copying is only relevant for compound

    25 objects (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 

    33 Two problems often exist with deep copy operations that don't exist with shallow

    34 copy 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 

    42 The :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 

    50 This module does not copy types like module, method, stack trace, stack frame,

    51 file, socket, window, array, or any similar types.  It does "copy" functions and

    52 classes (shallow and deeply), by returning the original object unchanged; this

    53 is compatible with the way these are treated by the :mod:`pickle` module.

    54 

    55 Shallow copies of dictionaries can be made using :meth:`dict.copy`, and

    56 of 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 

    64 Classes can use the same interfaces to control copying that they use to control

    65 pickling.  See the description of module :mod:`pickle` for information on these

    66 methods.  The :mod:`copy` module does not use the :mod:`copy_reg` registration

    67 module.

    68 

    69 .. index::

    70    single: __copy__() (copy protocol)

    71    single: __deepcopy__() (copy protocol)

    72 

    73 In order for a class to define its own copy implementation, it can define

    74 special methods :meth:`__copy__` and :meth:`__deepcopy__`.  The former is called

    75 to implement the shallow copy operation; no additional arguments are passed.

    76 The latter is called to implement the deep copy operation; it is passed one

    77 argument, the memo dictionary.  If the :meth:`__deepcopy__` implementation needs

    78 to make a deep copy of a component, it should call the :func:`deepcopy` function

    79 with 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