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