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98 lines
3.1 KiB
TeX
98 lines
3.1 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: they can define methods called
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\method{__getinitargs__()}, \method{__getstate__()} and
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\method{__setstate__()}. 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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\withsubitem{(copy protocol)}{\ttindex{__getinitargs__()}
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\ttindex{__getstate__()}\ttindex{__setstate__()}}
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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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