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author Guido van Rossum <guido@python.org>
date Thu, 17 Jul 1997 16:29:42 +0000
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1 \section{Standard Module \sectcode{copy}}
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2 \stmodindex{copy}
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3 \renewcommand{\indexsubitem}{(copy function)}
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4 \ttindex{copy}
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5 \ttindex{deepcopy}
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6
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7 This module provides generic (shallow and deep) copying operations.
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8
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9 Interface summary:
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10
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11 \begin{verbatim}
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12 import copy
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13
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14 x = copy.copy(y) # make a shallow copy of y
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15 x = copy.deepcopy(y) # make a deep copy of y
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16 \end{verbatim}
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17
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18 For module specific errors, \code{copy.error} is raised.
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19
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20 The difference between shallow and deep copying is only relevant for
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21 compound objects (objects that contain other objects, like lists or
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22 class instances):
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23
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24 \begin{itemize}
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26 \item
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27 A {\em shallow copy} constructs a new compound object and then (to the
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28 extent possible) inserts {\em references} into it to the objects found
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29 in the original.
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30
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31 \item
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32 A {\em deep copy} constructs a new compound object and then,
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33 recursively, inserts {\em copies} into it of the objects found in the
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34 original.
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35
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36 \end{itemize}
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37
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38 Two problems often exist with deep copy operations that don't exist
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39 with shallow copy operations:
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40
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41 \begin{itemize}
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42
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43 \item
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44 Recursive objects (compound objects that, directly or indirectly,
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45 contain a reference to themselves) may cause a recursive loop.
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47 \item
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48 Because deep copy copies {\em everything} it may copy too much, e.g.\
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49 administrative data structures that should be shared even between
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50 copies.
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51
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52 \end{itemize}
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53
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54 Python's \code{deepcopy()} operation avoids these problems by:
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55
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56 \begin{itemize}
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57
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58 \item
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59 keeping a table of objects already copied during the current
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60 copying pass; and
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61
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62 \item
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63 letting user-defined classes override the copying operation or the
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64 set of components copied.
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65
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66 \end{itemize}
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67
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68 This version does not copy types like module, class, function, method,
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69 nor stack trace, stack frame, nor file, socket, window, nor array, nor
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70 any similar types.
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71
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72 Classes can use the same interfaces to control copying that they use
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73 to control pickling: they can define methods called
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74 \code{__getinitargs__()}, \code{__getstate__()} and
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75 \code{__setstate__()}. See the description of module \code{pickle}
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76 for information on these methods.
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77 \stmodindex{pickle}
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78 \renewcommand{\indexsubitem}{(copy protocol)}
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79 \ttindex{__getinitargs__}
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80 \ttindex{__getstate__}
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81 \ttindex{__setstate__}