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Add documentation for the help() built-in; contributed by Ka-Ping Yee. This is part of SF patch #494622.
author Fred Drake <fdrake@acm.org>
date Tue, 18 Dec 2001 16:31:08 +0000
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children 687b0c8254fa
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1 \section{\module{copy} ---
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2 Shallow and deep copy operations}
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3
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4 \declaremodule{standard}{copy}
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5 \modulesynopsis{Shallow and deep copy operations.}
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6
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7
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8 This module provides generic (shallow and deep) copying operations.
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9 \withsubitem{(in copy)}{\ttindex{copy()}\ttindex{deepcopy()}}
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10
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11 Interface summary:
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12
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13 \begin{verbatim}
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14 import copy
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15
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16 x = copy.copy(y) # make a shallow copy of y
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17 x = copy.deepcopy(y) # make a deep copy of y
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18 \end{verbatim}
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19 %
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20 For module specific errors, \exception{copy.error} is raised.
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21
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22 The difference between shallow and deep copying is only relevant for
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23 compound objects (objects that contain other objects, like lists or
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24 class instances):
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25
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26 \begin{itemize}
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27
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28 \item
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29 A \emph{shallow copy} constructs a new compound object and then (to the
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30 extent possible) inserts \emph{references} into it to the objects found
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31 in the original.
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32
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33 \item
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34 A \emph{deep copy} constructs a new compound object and then,
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35 recursively, inserts \emph{copies} into it of the objects found in the
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36 original.
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37
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38 \end{itemize}
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39
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40 Two problems often exist with deep copy operations that don't exist
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41 with shallow copy operations:
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42
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43 \begin{itemize}
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44
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45 \item
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46 Recursive objects (compound objects that, directly or indirectly,
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47 contain a reference to themselves) may cause a recursive loop.
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48
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49 \item
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50 Because deep copy copies \emph{everything} it may copy too much,
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51 e.g., administrative data structures that should be shared even
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52 between copies.
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53
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54 \end{itemize}
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55
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56 The \function{deepcopy()} function avoids these problems by:
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57
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58 \begin{itemize}
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59
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60 \item
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61 keeping a ``memo'' dictionary of objects already copied during the current
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62 copying pass; and
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63
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64 \item
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65 letting user-defined classes override the copying operation or the
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66 set of components copied.
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67
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68 \end{itemize}
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69
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70 This version does not copy types like module, class, function, method,
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71 stack trace, stack frame, file, socket, window, array, or any similar
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72 types.
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73
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74 Classes can use the same interfaces to control copying that they use
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75 to control pickling: they can define methods called
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76 \method{__getinitargs__()}, \method{__getstate__()} and
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77 \method{__setstate__()}. See the description of module
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78 \refmodule{pickle}\refstmodindex{pickle} for information on these
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79 methods. The \module{copy} module does not use the
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80 \refmodule[copyreg]{copy_reg} registration module.
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81 \withsubitem{(copy protocol)}{\ttindex{__getinitargs__()}
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82 \ttindex{__getstate__()}\ttindex{__setstate__()}}
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83
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84 In order for a class to define its own copy implementation, it can
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85 define special methods \method{__copy__()} and
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86 \method{__deepcopy__()}. The former is called to implement the
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87 shallow copy operation; no additional arguments are passed. The
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88 latter is called to implement the deep copy operation; it is passed
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89 one argument, the memo dictionary. If the \method{__deepcopy__()}
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90 implementation needs to make a deep copy of a component, it should
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91 call the \function{deepcopy()} function with the component as first
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92 argument and the memo dictionary as second argument.
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93 \withsubitem{(copy protocol)}{\ttindex{__copy__()}\ttindex{__deepcopy__()}}
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94
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95 \begin{seealso}
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96 \seemodule{pickle}{Discussion of the special methods used to
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97 support object state retrieval and restoration.}
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98 \end{seealso}