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97 lines
4.4 KiB
TeX
97 lines
4.4 KiB
TeX
\section{\module{marshal} ---
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Alternate Python object serialization}
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\declaremodule{builtin}{marshal}
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\modulesynopsis{Convert Python objects to streams of bytes and back
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(with different constraints).}
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This module contains functions that can read and write Python
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values in a binary format. The format is specific to Python, but
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independent of machine architecture issues (e.g., you can write a
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Python value to a file on a PC, transport the file to a Sun, and read
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it back there). Details of the format are undocumented on purpose;
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it may change between Python versions (although it rarely
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does).\footnote{The name of this module stems from a bit of
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terminology used by the designers of Modula-3 (amongst others), who
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use the term ``marshalling'' for shipping of data around in a
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self-contained form. Strictly speaking, ``to marshal'' means to
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convert some data from internal to external form (in an RPC buffer for
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instance) and ``unmarshalling'' for the reverse process.}
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This is not a general ``persistency'' module. For general persistency
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and transfer of Python objects through RPC calls, see the modules
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\refmodule{pickle} and \refmodule{shelve}. The \module{marshal} module exists
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mainly to support reading and writing the ``pseudo-compiled'' code for
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Python modules of \file{.pyc} files.
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\refstmodindex{pickle}
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\refstmodindex{shelve}
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\obindex{code}
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Not all Python object types are supported; in general, only objects
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whose value is independent from a particular invocation of Python can
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be written and read by this module. The following types are supported:
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\code{None}, integers, long integers, floating point numbers,
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strings, Unicode objects, tuples, lists, dictionaries, and code
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objects, where it should be understood that tuples, lists and
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dictionaries are only supported as long as the values contained
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therein are themselves supported; and recursive lists and dictionaries
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should not be written (they will cause infinite loops).
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\strong{Caveat:} On machines where C's \code{long int} type has more than
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32 bits (such as the DEC Alpha), it
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is possible to create plain Python integers that are longer than 32
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bits. Since the current \module{marshal} module uses 32 bits to
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transfer plain Python integers, such values are silently truncated.
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This particularly affects the use of very long integer literals in
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Python modules --- these will be accepted by the parser on such
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machines, but will be silently be truncated when the module is read
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from the \file{.pyc} instead.\footnote{
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A solution would be to refuse such literals in the parser,
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since they are inherently non-portable. Another solution would be to
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let the \module{marshal} module raise an exception when an integer
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value would be truncated. At least one of these solutions will be
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implemented in a future version.}
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There are functions that read/write files as well as functions
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operating on strings.
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The module defines these functions:
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\begin{funcdesc}{dump}{value, file}
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Write the value on the open file. The value must be a supported
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type. The file must be an open file object such as
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\code{sys.stdout} or returned by \function{open()} or
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\function{posix.popen()}. It must be opened in binary mode
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(\code{'wb'} or \code{'w+b'}).
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If the value has (or contains an object that has) an unsupported type,
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a \exception{ValueError} exception is raised --- but garbage data
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will also be written to the file. The object will not be properly
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read back by \function{load()}.
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\end{funcdesc}
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\begin{funcdesc}{load}{file}
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Read one value from the open file and return it. If no valid value
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is read, raise \exception{EOFError}, \exception{ValueError} or
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\exception{TypeError}. The file must be an open file object opened
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in binary mode (\code{'rb'} or \code{'r+b'}).
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\strong{Warning:} If an object containing an unsupported type was
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marshalled with \function{dump()}, \function{load()} will substitute
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\code{None} for the unmarshallable type.
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\end{funcdesc}
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\begin{funcdesc}{dumps}{value}
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Return the string that would be written to a file by
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\code{dump(\var{value}, \var{file})}. The value must be a supported
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type. Raise a \exception{ValueError} exception if value has (or
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contains an object that has) an unsupported type.
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\end{funcdesc}
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\begin{funcdesc}{loads}{string}
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Convert the string to a value. If no valid value is found, raise
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\exception{EOFError}, \exception{ValueError} or
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\exception{TypeError}. Extra characters in the string are ignored.
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\end{funcdesc}
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