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 21This is not a general "persistence" module. For general persistence and 22transfer of Python objects through RPC calls, see the modules :mod:`pickle` and 23:mod:`shelve`. The :mod:`marshal` module exists mainly to support reading and 24writing the "pseudo-compiled" code for Python modules of :file:`.pyc` files. 25Therefore, the Python maintainers reserve the right to modify the marshal format 26in backward incompatible ways should the need arise. If you're serializing and 27de-serializing Python objects, use the :mod:`pickle` module instead -- the 28performance is comparable, version independence is guaranteed, and pickle 29supports a substantially wider range of objects than marshal. 30 31.. warning:: 32 33 The :mod:`marshal` module is not intended to be secure against erroneous or 34 maliciously constructed data. Never unmarshal data received from an 35 untrusted or unauthenticated source. 36 37.. index:: object; code, code object 38 39Not all Python object types are supported; in general, only objects whose value 40is independent from a particular invocation of Python can be written and read by 41this module. The following types are supported: booleans, integers, long 42integers, floating point numbers, complex numbers, strings, Unicode objects, 43tuples, lists, sets, frozensets, dictionaries, and code objects, where it should 44be understood that tuples, lists, sets, frozensets and dictionaries are only 45supported as long as the values contained therein are themselves supported; and 46recursive lists, sets and dictionaries should not be written (they will cause 47infinite loops). The singletons :const:`None`, :const:`Ellipsis` and 48:exc:`StopIteration` can also be marshalled and unmarshalled. 49 50.. warning:: 51 52 On machines where C's ``long int`` type has more than 32 bits (such as the 53 DEC Alpha), it is possible to create plain Python integers that are longer 54 than 32 bits. If such an integer is marshaled and read back in on a machine 55 where C's ``long int`` type has only 32 bits, a Python long integer object 56 is returned instead. While of a different type, the numeric value is the 57 same. (This behavior is new in Python 2.2. In earlier versions, all but the 58 least-significant 32 bits of the value were lost, and a warning message was 59 printed.) 60 61There are functions that read/write files as well as functions operating on 62strings. 63 64The module defines these functions: 65 66 67.. function:: dump(value, file[, version]) 68 69 Write the value on the open file. The value must be a supported type. The 70 file must be an open file object such as ``sys.stdout`` or returned by 71 :func:`open` or :func:`os.popen`. It may not be a wrapper such as 72 TemporaryFile on Windows. It must be opened in binary mode (``'wb'`` 73 or ``'w+b'``). 74 75 If the value has (or contains an object that has) an unsupported type, a 76 :exc:`ValueError` exception is raised --- but garbage data will also be written 77 to the file. The object will not be properly read back by :func:`load`. 78 79 .. versionadded:: 2.4 80 The *version* argument indicates the data format that ``dump`` should use 81 (see below). 82 83 84.. function:: load(file) 85 86 Read one value from the open file and return it. If no valid value is read 87 (e.g. because the data has a different Python version's incompatible marshal 88 format), raise :exc:`EOFError`, :exc:`ValueError` or :exc:`TypeError`. The 89 file must be an open file object opened in binary mode (``'rb'`` or 90 ``'r+b'``). 91 92 .. note:: 93 94 If an object containing an unsupported type was marshalled with :func:`dump`, 95 :func:`load` will substitute ``None`` for the unmarshallable type. 96 97 98.. function:: dumps(value[, version]) 99 100 Return the string that would be written to a file by ``dump(value, file)``. The 101 value must be a supported type. Raise a :exc:`ValueError` exception if value 102 has (or contains an object that has) an unsupported type. 103 104 .. versionadded:: 2.4 105 The *version* argument indicates the data format that ``dumps`` should use 106 (see below). 107 108 109.. function:: loads(string) 110 111 Convert the string to a value. If no valid value is found, raise 112 :exc:`EOFError`, :exc:`ValueError` or :exc:`TypeError`. Extra characters in the 113 string are ignored. 114 115 116In addition, the following constants are defined: 117 118.. data:: version 119 120 Indicates the format that the module uses. Version 0 is the historical format, 121 version 1 (added in Python 2.4) shares interned strings and version 2 (added in 122 Python 2.5) uses a binary format for floating point numbers. The current version 123 is 2. 124 125 .. versionadded:: 2.4 126 127 128.. rubric:: Footnotes 129 130.. [#] The name of this module stems from a bit of terminology used by the designers of 131 Modula-3 (amongst others), who use the term "marshalling" for shipping of data 132 around in a self-contained form. Strictly speaking, "to marshal" means to 133 convert some data from internal to external form (in an RPC buffer for instance) 134 and "unmarshalling" for the reverse process. 135 136