Doc/lib/libcopy.tex

   1 \section{\module{copy} ---
   2          Shallow and deep copy operations}
   3
   4 \declaremodule{standard}{copy}
   5 \modulesynopsis{Shallow and deep copy operations.}
   6
   7
   8 This module provides generic (shallow and deep) copying operations.
   9 \withsubitem{(in copy)}{\ttindex{copy()}\ttindex{deepcopy()}}
  10
  11 Interface summary:
  12
  13 \begin{verbatim}
  14 import copy
  15
  16 x = copy.copy(y)        # make a shallow copy of y
  17 x = copy.deepcopy(y)    # make a deep copy of y
  18 \end{verbatim}
  19 %
  20 For module specific errors, \exception{copy.error} is raised.
  21
  22 The difference between shallow and deep copying is only relevant for
  23 compound objects (objects that contain other objects, like lists or
  24 class instances):
  25
  26 \begin{itemize}
  27
  28 \item
  29 A \emph{shallow copy} constructs a new compound object and then (to the
  30 extent possible) inserts \emph{references} into it to the objects found
  31 in the original.
  32
  33 \item
  34 A \emph{deep copy} constructs a new compound object and then,
  35 recursively, inserts \emph{copies} into it of the objects found in the
  36 original.
  37
  38 \end{itemize}
  39
  40 Two problems often exist with deep copy operations that don't exist
  41 with shallow copy operations:
  42
  43 \begin{itemize}
  44
  45 \item
  46 Recursive objects (compound objects that, directly or indirectly,
  47 contain a reference to themselves) may cause a recursive loop.
  48
  49 \item
  50 Because deep copy copies \emph{everything} it may copy too much,
  51 e.g., administrative data structures that should be shared even
  52 between copies.
  53
  54 \end{itemize}
  55
  56 The \function{deepcopy()} function avoids these problems by:
  57
  58 \begin{itemize}
  59
  60 \item
  61 keeping a ``memo'' dictionary of objects already copied during the current
  62 copying pass; and
  63
  64 \item
  65 letting user-defined classes override the copying operation or the
  66 set of components copied.
  67
  68 \end{itemize}
  69
  70 This version does not copy types like module, class, function, method,
  71 stack trace, stack frame, file, socket, window, array, or any similar
  72 types.
  73
  74 Classes can use the same interfaces to control copying that they use
  75 to control pickling: they can define methods called
  76 \method{__getinitargs__()}, \method{__getstate__()} and
  77 \method{__setstate__()}.  See the description of module
  78 \refmodule{pickle}\refstmodindex{pickle} for information on these
  79 methods.  The \module{copy} module does not use the
  80 \refmodule[copyreg]{copy_reg} registration module.
  81 \withsubitem{(copy protocol)}{\ttindex{__getinitargs__()}
  82   \ttindex{__getstate__()}\ttindex{__setstate__()}}
  83
  84 In order for a class to define its own copy implementation, it can
  85 define special methods \method{__copy__()} and
  86 \method{__deepcopy__()}.  The former is called to implement the
  87 shallow copy operation; no additional arguments are passed.  The
  88 latter is called to implement the deep copy operation; it is passed
  89 one argument, the memo dictionary.  If the \method{__deepcopy__()}
  90 implementation needs to make a deep copy of a component, it should
  91 call the \function{deepcopy()} function with the component as first
  92 argument and the memo dictionary as second argument.
  93 \withsubitem{(copy protocol)}{\ttindex{__copy__()}\ttindex{__deepcopy__()}}
  94
  95 \begin{seealso}
  96 \seemodule{pickle}{Discussion of the special disciplines used to
  97 support object state retrieval and restoration.}
  98 \end{seealso}