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Diffstat (limited to 'Doc/lib/libpickle.tex')
| -rw-r--r-- | Doc/lib/libpickle.tex | 74 |
1 files changed, 49 insertions, 25 deletions
diff --git a/Doc/lib/libpickle.tex b/Doc/lib/libpickle.tex index cb054a7..110a074 100644 --- a/Doc/lib/libpickle.tex +++ b/Doc/lib/libpickle.tex @@ -27,6 +27,13 @@ to send them across a network or store them in a database. The module objects on ``dbm''-style database files. \stmodindex{shelve} +\strong{Note:} The \code{pickle} module is rather slow. A +reimplementation of the same algorithm in C, which is up to 1000 times +faster, is available as the \code{cPickle} module. This has the same +interface except that \code{Pickler} and \code{Unpickler} are factory +functions, not classes (so they cannot be used as a base class for +inheritance). + Unlike the built-in module \code{marshal}, \code{pickle} handles the following correctly: \stmodindex{marshal} @@ -47,20 +54,19 @@ standards such as CORBA (which probably can't represent pointer sharing or recursive objects); however it means that non-Python programs may not be able to reconstruct pickled Python objects. -The \code{pickle} data format uses a printable \ASCII{} representation. -This is slightly more voluminous than a binary representation. -However, small integers actually take {\em less} space when -represented as minimal-size decimal strings than when represented as -32-bit binary numbers, and strings are only much longer if they -contain many control characters or 8-bit characters. The big -advantage of using printable \ASCII{} (and of some other characteristics -of \code{pickle}'s representation) is that for debugging or recovery -purposes it is possible for a human to read the pickled file with a -standard text editor. (I could have gone a step further and used a -notation like S-expressions, but the parser -(currently written in Python) would have been -considerably more complicated and slower, and the files would probably -have become much larger.) +By default, the \code{pickle} data format uses a printable \ASCII{} +representation. This is slightly more voluminous than a binary +representation. The big advantage of using printable \ASCII{} (and of +some other characteristics of \code{pickle}'s representation) is that +for debugging or recovery purposes it is possible for a human to read +the pickled file with a standard text editor. + +A binary format, which is slightly more efficient, can be chosen by +specifying a nonzero (true) value for the \var{bin} argument to the +\code{Pickler} constructor or the \code{dump()} and \code{dumps()} +functions. The binary format is not the default because of backwards +compatibility with the Python 1.4 pickle module. In a future version, +the default may change to binary. The \code{pickle} module doesn't handle code objects, which the \code{marshal} module does. I suppose \code{pickle} could, and maybe @@ -83,16 +89,21 @@ returns either \code{None} or the persistent ID of the object. There are some restrictions on the pickling of class instances. First of all, the class must be defined at the top level in a module. +Furthermore, all its instance variables must be picklable. \renewcommand{\indexsubitem}{(pickle protocol)} -Next, it must normally be possible to create class instances by -calling the class without arguments. Usually, this is best -accomplished by providing default values for all arguments to its -\code{__init__} method (if it has one). If this is undesirable, the -class can define a method \code{__getinitargs__()}, which should -return a {\em tuple} containing the arguments to be passed to the -class constructor (\code{__init__()}). +When a pickled class instance is unpickled, its \code{__init__} method +is normally \emph{not} invoked. \strong{Note:} This is a deviation +from previous versions of this module; the change was introduced in +Python 1.5b2. The reason for the change is that in many cases it is +desirable to have a constructor that requires arguments; it is a +(minor) nuisance to have to provide a \code{__getinitargs__} method. + +If it is desirable that the \code{__init__} method be called on +unpickling, a class can define a method \code{__getinitargs__()}, +which should return a {\em tuple} containing the arguments to be +passed to the class constructor (\code{__init__()}). \ttindex{__getinitargs__} \ttindex{__init__} @@ -166,6 +177,13 @@ objects here, as long as they have the right methods. \ttindex{Unpickler} \ttindex{Pickler} +The constructor for the \code{Pickler} class has an optional second +argument, \var{bin}. If this is present and nonzero, the binary +pickle format is used; if it is zero or absent, the (less efficient, +but backwards compatible) text pickle format is used. The +\code{Unpickler} class does not have an argument to distinguish +between binary and text pickle formats; it accepts either format. + The following types can be pickled: \begin{itemize} @@ -206,9 +224,13 @@ Collection may also become a problem here.) Apart from the \code{Pickler} and \code{Unpickler} classes, the module defines the following functions, and an exception: -\begin{funcdesc}{dump}{object\, file} +\begin{funcdesc}{dump}{object\, file\optional{, bin}} Write a pickled representation of \var{obect} to the open file object -\var{file}. This is equivalent to \code{Pickler(file).dump(object)}. +\var{file}. This is equivalent to +\code{Pickler(\var{file}, \var{bin}).dump(\var{object})}. +If the optional \var{bin} argument is present and nonzero, the binary +pickle format is used; if it is zero or absent, the (less efficient) +text pickle format is used. \end{funcdesc} \begin{funcdesc}{load}{file} @@ -216,9 +238,11 @@ Read a pickled object from the open file object \var{file}. This is equivalent to \code{Unpickler(file).load()}. \end{funcdesc} -\begin{funcdesc}{dumps}{object} +\begin{funcdesc}{dumps}{object\optional{, bin}} Return the pickled representation of the object as a string, instead -of writing it to a file. +of writing it to a file. If the optional \var{bin} argument is +present and nonzero, the binary pickle format is used; if it is zero +or absent, the (less efficient) text pickle format is used. \end{funcdesc} \begin{funcdesc}{loads}{string} |