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are enough subtleties to pickling that we don't want misunderstanding to spread because we don't provide all the information twice. The reference to the pickle module for information will have to suffice; at least only one portion of the docs will be out of date. ;-(
95 lines
2.9 KiB
TeX
95 lines
2.9 KiB
TeX
\section{\module{copy} ---
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Shallow and deep copy operations}
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\declaremodule{standard}{copy}
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\modulesynopsis{Shallow and deep copy operations.}
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This module provides generic (shallow and deep) copying operations.
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\withsubitem{(in copy)}{\ttindex{copy()}\ttindex{deepcopy()}}
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Interface summary:
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\begin{verbatim}
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import copy
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x = copy.copy(y) # make a shallow copy of y
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x = copy.deepcopy(y) # make a deep copy of y
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\end{verbatim}
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%
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For module specific errors, \exception{copy.error} is raised.
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The difference between shallow and deep copying is only relevant for
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compound objects (objects that contain other objects, like lists or
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class instances):
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\begin{itemize}
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\item
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A \emph{shallow copy} constructs a new compound object and then (to the
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extent possible) inserts \emph{references} into it to the objects found
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in the original.
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\item
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A \emph{deep copy} constructs a new compound object and then,
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recursively, inserts \emph{copies} into it of the objects found in the
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original.
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\end{itemize}
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Two problems often exist with deep copy operations that don't exist
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with shallow copy operations:
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\begin{itemize}
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\item
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Recursive objects (compound objects that, directly or indirectly,
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contain a reference to themselves) may cause a recursive loop.
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\item
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Because deep copy copies \emph{everything} it may copy too much,
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e.g., administrative data structures that should be shared even
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between copies.
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\end{itemize}
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The \function{deepcopy()} function avoids these problems by:
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\begin{itemize}
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\item
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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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\item
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letting user-defined classes override the copying operation or the
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set of components copied.
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\end{itemize}
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This version does not copy types like module, class, function, method,
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stack trace, stack frame, file, socket, window, array, or any similar
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types.
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Classes can use the same interfaces to control copying that they use
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to control pickling. See the description of module
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\refmodule{pickle}\refstmodindex{pickle} for information on these
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methods. The \module{copy} module does not use the
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\refmodule[copyreg]{copy_reg} registration module.
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In order for a class to define its own copy implementation, it can
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define special methods \method{__copy__()} and
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\method{__deepcopy__()}. The former is called to implement the
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shallow copy operation; no additional arguments are passed. The
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latter is called to implement the deep copy operation; it is passed
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one argument, the memo dictionary. If the \method{__deepcopy__()}
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implementation needs to make a deep copy of a component, it should
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call the \function{deepcopy()} function with the component as first
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argument and the memo dictionary as second argument.
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\withsubitem{(copy protocol)}{\ttindex{__copy__()}\ttindex{__deepcopy__()}}
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\begin{seealso}
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\seemodule{pickle}{Discussion of the special methods used to
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support object state retrieval and restoration.}
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\end{seealso}
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