
``copy`` --- Shallow and deep copy operations
*********************************************

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

Interface summary:

   import copy

   x = copy.copy(y)        # make a shallow copy of y
   x = copy.deepcopy(y)    # make a deep copy of y

For module specific errors, ``copy.error`` is raised.

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, e.g.,
  administrative data structures that should be shared even between
  copies.

The ``deepcopy()`` function avoids these problems by:

* keeping a "memo" dictionary of objects already copied during the
  current 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 ``pickle`` module.

Shallow copies of dictionaries can be made using ``dict.copy()``, and
of lists by assigning a slice of the entire list, for example,
``copied_list = original_list[:]``.

Classes can use the same interfaces to control copying that they use
to control pickling.  See the description of module ``pickle`` for
information on these methods.  The ``copy`` module does not use the
``copyreg`` registration module.

In order for a class to define its own copy implementation, it can
define special methods ``__copy__()`` and ``__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 ``__deepcopy__()`` implementation needs to make a deep copy of
a component, it should call the ``deepcopy()`` function with the
component as first argument and the memo dictionary as second
argument.

See also:

   Module ``pickle``
      Discussion of the special methods used to support object state
      retrieval and restoration.
