Doc/lib/libmarshal.tex

   1 \section{\module{marshal} ---
   2          Alternate Python object serialization}
   3
   4 \declaremodule{builtin}{marshal}
   5 \modulesynopsis{Convert Python objects to streams of bytes and back
   6                 (with different constraints).}
   7
   8
   9 This module contains functions that can read and write Python
  10 values in a binary format.  The format is specific to Python, but
  11 independent of machine architecture issues (e.g., you can write a
  12 Python value to a file on a PC, transport the file to a Sun, and read
  13 it back there).  Details of the format are undocumented on purpose;
  14 it may change between Python versions (although it rarely
  15 does).\footnote{The name of this module stems from a bit of
  16   terminology used by the designers of Modula-3 (amongst others), who
  17   use the term ``marshalling'' for shipping of data around in a
  18   self-contained form. Strictly speaking, ``to marshal'' means to
  19   convert some data from internal to external form (in an RPC buffer for
  20   instance) and ``unmarshalling'' for the reverse process.}
  21
  22 This is not a general ``persistence'' module.  For general persistence
  23 and transfer of Python objects through RPC calls, see the modules
  24 \refmodule{pickle} and \refmodule{shelve}.  The \module{marshal} module exists
  25 mainly to support reading and writing the ``pseudo-compiled'' code for
  26 Python modules of \file{.pyc} files.
  27 \refstmodindex{pickle}
  28 \refstmodindex{shelve}
  29 \obindex{code}
  30
  31 Not all Python object types are supported; in general, only objects
  32 whose value is independent from a particular invocation of Python can
  33 be written and read by this module.  The following types are supported:
  34 \code{None}, integers, long integers, floating point numbers,
  35 strings, Unicode objects, tuples, lists, dictionaries, and code
  36 objects, where it should be understood that tuples, lists and
  37 dictionaries are only supported as long as the values contained
  38 therein are themselves supported; and recursive lists and dictionaries
  39 should not be written (they will cause infinite loops).
  40
  41 \strong{Caveat:} On machines where C's \code{long int} type has more than
  42 32 bits (such as the DEC Alpha), it
  43 is possible to create plain Python integers that are longer than 32
  44 bits.  Since the current \module{marshal} module uses 32 bits to
  45 transfer plain Python integers, such values are silently truncated.
  46 This particularly affects the use of very long integer literals in
  47 Python modules --- these will be accepted by the parser on such
  48 machines, but will be silently be truncated when the module is read
  49 from the \file{.pyc} instead.\footnote{
  50   A solution would be to refuse such literals in the parser,
  51   since they are inherently non-portable.  Another solution would be to
  52   let the \module{marshal} module raise an exception when an integer
  53   value would be truncated.  At least one of these solutions will be
  54   implemented in a future version.}
  55
  56 There are functions that read/write files as well as functions
  57 operating on strings.
  58
  59 The module defines these functions:
  60
  61 \begin{funcdesc}{dump}{value, file}
  62   Write the value on the open file.  The value must be a supported
  63   type.  The file must be an open file object such as
  64   \code{sys.stdout} or returned by \function{open()} or
  65   \function{posix.popen()}.  It must be opened in binary mode
  66   (\code{'wb'} or \code{'w+b'}).
  67
  68   If the value has (or contains an object that has) an unsupported type,
  69   a \exception{ValueError} exception is raised --- but garbage data
  70   will also be written to the file.  The object will not be properly
  71   read back by \function{load()}.
  72 \end{funcdesc}
  73
  74 \begin{funcdesc}{load}{file}
  75   Read one value from the open file and return it.  If no valid value
  76   is read, raise \exception{EOFError}, \exception{ValueError} or
  77   \exception{TypeError}.  The file must be an open file object opened
  78   in binary mode (\code{'rb'} or \code{'r+b'}).
  79
  80   \strong{Warning:} If an object containing an unsupported type was
  81   marshalled with \function{dump()}, \function{load()} will substitute
  82   \code{None} for the unmarshallable type.
  83 \end{funcdesc}
  84
  85 \begin{funcdesc}{dumps}{value}
  86   Return the string that would be written to a file by
  87   \code{dump(\var{value}, \var{file})}.  The value must be a supported
  88   type.  Raise a \exception{ValueError} exception if value has (or
  89   contains an object that has) an unsupported type.
  90 \end{funcdesc}
  91
  92 \begin{funcdesc}{loads}{string}
  93   Convert the string to a value.  If no valid value is found, raise
  94   \exception{EOFError}, \exception{ValueError} or
  95   \exception{TypeError}.  Extra characters in the string are ignored.
  96 \end{funcdesc}