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(python2.1-lib.info)copy

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Shallow and deep copy operations
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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 version does not copy types like module, class, function, method,
stack trace, stack frame, file, socket, window, array, or any similar
types.

Classes can use the same interfaces to control copying that they use to
control pickling: they can define methods called `__getinitargs__()',
`__getstate__()' and `__setstate__()'.  See the description of module
`pickle'  for information on these methods.  The `copy' module does not
use the `copy_reg' 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:
     Note: pickle Discussion of the special methods used to support
     object state retrieval and restoration.