Doc/lib/libcopy.tex

   1 \section{\module{copy} ---
   2          Shallow and deep copy operations.}
   3 \declaremodule{standard}{copy}
   4
   5 \modulesynopsis{Shallow and deep copy operations.}
   6
   7 \setindexsubitem{(copy function)}
   8 \ttindex{copy}
   9 \ttindex{deepcopy}
  10
  11 This module provides generic (shallow and deep) copying operations.
  12
  13 Interface summary:
  14
  15 \begin{verbatim}
  16 import copy
  17
  18 x = copy.copy(y)        # make a shallow copy of y
  19 x = copy.deepcopy(y)    # make a deep copy of y
  20 \end{verbatim}
  21 %
  22 For module specific errors, \code{copy.error} is raised.
  23
  24 The difference between shallow and deep copying is only relevant for
  25 compound objects (objects that contain other objects, like lists or
  26 class instances):
  27
  28 \begin{itemize}
  29
  30 \item
  31 A \emph{shallow copy} constructs a new compound object and then (to the
  32 extent possible) inserts \emph{references} into it to the objects found
  33 in the original.
  34
  35 \item
  36 A \emph{deep copy} constructs a new compound object and then,
  37 recursively, inserts \emph{copies} into it of the objects found in the
  38 original.
  39
  40 \end{itemize}
  41
  42 Two problems often exist with deep copy operations that don't exist
  43 with shallow copy operations:
  44
  45 \begin{itemize}
  46
  47 \item
  48 Recursive objects (compound objects that, directly or indirectly,
  49 contain a reference to themselves) may cause a recursive loop.
  50
  51 \item
  52 Because deep copy copies \emph{everything} it may copy too much, e.g.\
  53 administrative data structures that should be shared even between
  54 copies.
  55
  56 \end{itemize}
  57
  58 Python's \code{deepcopy()} operation avoids these problems by:
  59
  60 \begin{itemize}
  61
  62 \item
  63 keeping a ``memo'' dictionary of objects already copied during the current
  64 copying pass; and
  65
  66 \item
  67 letting user-defined classes override the copying operation or the
  68 set of components copied.
  69
  70 \end{itemize}
  71
  72 This version does not copy types like module, class, function, method,
  73 nor stack trace, stack frame, nor file, socket, window, nor array, nor
  74 any similar types.
  75
  76 Classes can use the same interfaces to control copying that they use
  77 to control pickling: they can define methods called
  78 \code{__getinitargs__()}, \code{__getstate__()} and
  79 \code{__setstate__()}.  See the description of module \code{pickle}
  80 for information on these methods.
  81 The copy module does not use the \module{copy_reg} registration
  82 module.
  83 \refstmodindex{pickle}
  84 \setindexsubitem{(copy protocol)}
  85 \ttindex{__getinitargs__}
  86 \ttindex{__getstate__}
  87 \ttindex{__setstate__}
  88
  89 In order for a class to define its own copy implementation, it can
  90 define special methods \method{__copy__()}\ttindex{__copy__} and
  91 \method{__deepcopy__()}\ttindex{__deepcopy__}.  The former is called to
  92 implement the shallow copy operation; no additional arguments are
  93 passed.  The latter is called to implement the deep copy operation; it
  94 is passed one argument, the memo dictionary.  If the
  95 \method{__deepcopy__()} implementation needs to make a deep copy of a
  96 component, it should call the \function{deepcopy()} function with the
  97 component as first argument and the memo dictionary as second
  98 argument.