2 :mod:`marshal` --- Internal Python object serialization
3 =======================================================
6 :synopsis: Convert Python objects to streams of bytes and back (with different
10 This module contains functions that can read and write Python values in a binary
11 format. The format is specific to Python, but independent of machine
12 architecture issues (e.g., you can write a Python value to a file on a PC,
13 transport the file to a Sun, and read it back there). Details of the format are
14 undocumented on purpose; it may change between Python versions (although it
22 This is not a general "persistence" module. For general persistence and
23 transfer 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
25 writing the "pseudo-compiled" code for Python modules of :file:`.pyc` files.
26 Therefore, the Python maintainers reserve the right to modify the marshal format
27 in backward incompatible ways should the need arise. If you're serializing and
28 de-serializing Python objects, use the :mod:`pickle` module instead -- the
29 performance is comparable, version independence is guaranteed, and pickle
30 supports a substantially wider range of objects than marshal.
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.
38 Not all Python object types are supported; in general, only objects whose value
39 is independent from a particular invocation of Python can be written and read by
40 this module. The following types are supported: booleans, integers, long
41 integers, floating point numbers, complex numbers, strings, Unicode objects,
42 tuples, lists, sets, frozensets, dictionaries, and code objects, where it should
43 be understood that tuples, lists, sets, frozensets and dictionaries are only
44 supported as long as the values contained therein are themselves supported; and
45 recursive lists, sets and dictionaries should not be written (they will cause
46 infinite loops). The singletons :const:`None`, :const:`Ellipsis` and
47 :exc:`StopIteration` can also be marshalled and unmarshalled.
51 On machines where C's ``long int`` type has more than 32 bits (such as the
52 DEC Alpha), it is possible to create plain Python integers that are longer
53 than 32 bits. If such an integer is marshaled and read back in on a machine
54 where C's ``long int`` type has only 32 bits, a Python long integer object
55 is returned instead. While of a different type, the numeric value is the
56 same. (This behavior is new in Python 2.2. In earlier versions, all but the
57 least-significant 32 bits of the value were lost, and a warning message was
60 There are functions that read/write files as well as functions operating on
63 The module defines these functions:
66 .. function:: dump(value, file[, version])
68 Write the value on the open file. The value must be a supported type. The
69 file must be an open file object such as ``sys.stdout`` or returned by
70 :func:`open` or :func:`os.popen`. It must be opened in binary mode (``'wb'``
73 If the value has (or contains an object that has) an unsupported type, a
74 :exc:`ValueError` exception is raised --- but garbage data will also be written
75 to the file. The object will not be properly read back by :func:`load`.
78 The *version* argument indicates the data format that ``dump`` should use
82 .. function:: load(file)
84 Read one value from the open file and return it. If no valid value is read
85 (e.g. because the data has a different Python version's incompatible marshal
86 format), raise :exc:`EOFError`, :exc:`ValueError` or :exc:`TypeError`. The
87 file must be an open file object opened in binary mode (``'rb'`` or
92 If an object containing an unsupported type was marshalled with :func:`dump`,
93 :func:`load` will substitute ``None`` for the unmarshallable type.
96 .. function:: dumps(value[, version])
98 Return the string that would be written to a file by ``dump(value, file)``. The
99 value must be a supported type. Raise a :exc:`ValueError` exception if value
100 has (or contains an object that has) an unsupported type.
102 .. versionadded:: 2.4
103 The *version* argument indicates the data format that ``dumps`` should use
107 .. function:: loads(string)
109 Convert the string to a value. If no valid value is found, raise
110 :exc:`EOFError`, :exc:`ValueError` or :exc:`TypeError`. Extra characters in the
114 In addition, the following constants are defined:
118 Indicates the format that the module uses. Version 0 is the historical format,
119 version 1 (added in Python 2.4) shares interned strings and version 2 (added in
120 Python 2.5) uses a binary format for floating point numbers. The current version
123 .. versionadded:: 2.4
126 .. rubric:: Footnotes
128 .. [#] The name of this module stems from a bit of terminology used by the designers of
129 Modula-3 (amongst others), who use the term "marshalling" for shipping of data
130 around in a self-contained form. Strictly speaking, "to marshal" means to
131 convert some data from internal to external form (in an RPC buffer for instance)
132 and "unmarshalling" for the reverse process.