/Doc/library/marshal.rst

  1
  2:mod:`marshal` --- Internal Python object serialization
  3=======================================================
  4
  5.. module:: marshal
  6   :synopsis: Convert Python objects to streams of bytes and back (with different
  7              constraints).
  8
  9
 10This module contains functions that can read and write Python values in a binary
 11format.  The format is specific to Python, but independent of machine
 12architecture issues (e.g., you can write a Python value to a file on a PC,
 13transport the file to a Sun, and read it back there).  Details of the format are
 14undocumented on purpose; it may change between Python versions (although it
 15rarely does). [#]_
 16
 17.. index::
 18   module: pickle
 19   module: shelve
 20   object: code
 21
 22This is not a general "persistence" module.  For general persistence and
 23transfer of Python objects through RPC calls, see the modules :mod:`pickle` and
 24:mod:`shelve`.  The :mod:`marshal` module exists mainly to support reading and
 25writing the "pseudo-compiled" code for Python modules of :file:`.pyc` files.
 26Therefore, the Python maintainers reserve the right to modify the marshal format
 27in backward incompatible ways should the need arise.  If you're serializing and
 28de-serializing Python objects, use the :mod:`pickle` module instead -- the
 29performance is comparable, version independence is guaranteed, and pickle
 30supports a substantially wider range of objects than marshal.
 31
 32.. warning::
 33
 34   The :mod:`marshal` module is not intended to be secure against erroneous or
 35   maliciously constructed data.  Never unmarshal data received from an
 36   untrusted or unauthenticated source.
 37
 38Not all Python object types are supported; in general, only objects whose value
 39is independent from a particular invocation of Python can be written and read by
 40this module.  The following types are supported: ``None``, integers, long
 41integers, floating point numbers, strings, Unicode objects, tuples, lists, sets,
 42dictionaries, and code objects, where it should be understood that tuples, lists
 43and dictionaries are only supported as long as the values contained therein are
 44themselves supported; and recursive lists and dictionaries should not be written
 45(they will cause infinite loops).
 46
 47.. warning::
 48
 49   On machines where C's ``long int`` type has more than 32 bits (such as the
 50   DEC Alpha), it is possible to create plain Python integers that are longer
 51   than 32 bits. If such an integer is marshaled and read back in on a machine
 52   where C's ``long int`` type has only 32 bits, a Python long integer object
 53   is returned instead.  While of a different type, the numeric value is the
 54   same.  (This behavior is new in Python 2.2.  In earlier versions, all but the
 55   least-significant 32 bits of the value were lost, and a warning message was
 56   printed.)
 57
 58There are functions that read/write files as well as functions operating on
 59strings.
 60
 61The module defines these functions:
 62
 63
 64.. function:: dump(value, file[, version])
 65
 66   Write the value on the open file.  The value must be a supported type.  The
 67   file must be an open file object such as ``sys.stdout`` or returned by
 68   :func:`open` or :func:`os.popen`.  It must be opened in binary mode (``'wb'``
 69   or ``'w+b'``).
 70
 71   If the value has (or contains an object that has) an unsupported type, a
 72   :exc:`ValueError` exception is raised --- but garbage data will also be written
 73   to the file.  The object will not be properly read back by :func:`load`.
 74
 75   .. versionadded:: 2.4
 76      The *version* argument indicates the data format that ``dump`` should use
 77      (see below).
 78
 79
 80.. function:: load(file)
 81
 82   Read one value from the open file and return it.  If no valid value is read
 83   (e.g. because the data has a different Python version's incompatible marshal
 84   format), raise :exc:`EOFError`, :exc:`ValueError` or :exc:`TypeError`.  The
 85   file must be an open file object opened in binary mode (``'rb'`` or
 86   ``'r+b'``).
 87
 88   .. note::
 89
 90      If an object containing an unsupported type was marshalled with :func:`dump`,
 91      :func:`load` will substitute ``None`` for the unmarshallable type.
 92
 93
 94.. function:: dumps(value[, version])
 95
 96   Return the string that would be written to a file by ``dump(value, file)``.  The
 97   value must be a supported type.  Raise a :exc:`ValueError` exception if value
 98   has (or contains an object that has) an unsupported type.
 99
100   .. versionadded:: 2.4
101      The *version* argument indicates the data format that ``dumps`` should use
102      (see below).
103
104
105.. function:: loads(string)
106
107   Convert the string to a value.  If no valid value is found, raise
108   :exc:`EOFError`, :exc:`ValueError` or :exc:`TypeError`.  Extra characters in the
109   string are ignored.
110
111
112In addition, the following constants are defined:
113
114.. data:: version
115
116   Indicates the format that the module uses. Version 0 is the historical format,
117   version 1 (added in Python 2.4) shares interned strings and version 2 (added in
118   Python 2.5) uses a binary format for floating point numbers. The current version
119   is 2.
120
121   .. versionadded:: 2.4
122
123
124.. rubric:: Footnotes
125
126.. [#] The name of this module stems from a bit of terminology used by the designers of
127   Modula-3 (amongst others), who use the term "marshalling" for shipping of data
128   around in a self-contained form. Strictly speaking, "to marshal" means to
129   convert some data from internal to external form (in an RPC buffer for instance)
130   and "unmarshalling" for the reverse process.
131