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92 lines
4.1 KiB
TeX
92 lines
4.1 KiB
TeX
\section{Built-in Module \sectcode{marshal}}
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\label{module-marshal}
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\bimodindex{marshal}
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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 does).%
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\footnote{The name of this module stems from a bit of terminology used
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by the designers of Modula-3 (amongst others), who use the term
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``marshalling'' for shipping of data around in a self-contained form.
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Strictly speaking, ``to marshal'' means to convert some data from
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internal to external form (in an RPC buffer for instance) and
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``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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\code{pickle} and \code{shelve}. The \code{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 \samp{.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, tuples, lists, dictionaries, and code objects, where it
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should be understood that tuples, lists and dictionaries are only
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supported as long as the values contained therein are themselves
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supported; and recursive lists and dictionaries should not be written
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(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 \code{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 \code{.pyc} instead.%
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\footnote{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 \code{marshal} module raise an exception when an integer value
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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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\renewcommand{\indexsubitem}{(in module marshal)}
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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 \code{open()} or
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\code{posix.popen()}.
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If the value has (or contains an object that has) an unsupported type,
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a \code{ValueError} exception is raised -- but garbage data will also
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be written to the file. The object will not be properly read back by
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\code{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 \code{EOFError}, \code{ValueError} or
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\code{TypeError}. The file must be an open file object.
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Warning: If an object containing an unsupported type was marshalled
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with \code{dump()}, \code{load()} will substitute \code{None} for the
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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(value, file)}. The value must be a supported type.
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Raise a \code{ValueError} exception if value has (or contains an
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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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\code{EOFError}, \code{ValueError} or \code{TypeError}. Extra
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characters in the string are ignored.
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\end{funcdesc}
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