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119 lines
4.8 KiB
ReStructuredText
119 lines
4.8 KiB
ReStructuredText
:mod:`marshal` --- Internal Python object serialization
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=======================================================
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.. module:: marshal
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:synopsis: Convert Python objects to streams of bytes and back (with different
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constraints).
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This module contains functions that can read and write Python values in a binary
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format. The format is specific to Python, but independent of machine
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architecture issues (e.g., you can write a Python value to a file on a PC,
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transport the file to a Sun, and read it back there). Details of the format are
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undocumented on purpose; it may change between Python versions (although it
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rarely does). [#]_
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.. index::
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module: pickle
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module: shelve
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object: code
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This is not a general "persistence" module. For general persistence and
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transfer of Python objects through RPC calls, see the modules :mod:`pickle` and
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:mod:`shelve`. The :mod:`marshal` module exists mainly to support reading and
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writing the "pseudo-compiled" code for Python modules of :file:`.pyc` files.
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Therefore, the Python maintainers reserve the right to modify the marshal format
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in backward incompatible ways should the need arise. If you're serializing and
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de-serializing Python objects, use the :mod:`pickle` module instead -- the
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performance is comparable, version independence is guaranteed, and pickle
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supports a substantially wider range of objects than marshal.
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.. warning::
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The :mod:`marshal` module is not intended to be secure against erroneous or
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maliciously constructed data. Never unmarshal data received from an
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untrusted or unauthenticated source.
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Not all Python object types are supported; in general, only objects whose value
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is independent from a particular invocation of Python can be written and read by
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this module. The following types are supported: booleans, integers, floating
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point numbers, complex numbers, strings, bytes, bytearrays, tuples, lists, sets,
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frozensets, dictionaries, and code objects, where it should be understood that
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tuples, lists, sets, frozensets and dictionaries are only supported as long as
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the values contained therein are themselves supported.
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singletons :const:`None`, :const:`Ellipsis` and :exc:`StopIteration` can also be
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marshalled and unmarshalled.
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For format *version* lower than 3, recursive lists, sets and dictionaries cannot
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be written (see below).
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There are functions that read/write files as well as functions operating on
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strings.
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The module defines these functions:
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.. function:: dump(value, file[, version])
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Write the value on the open file. The value must be a supported type. The
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file must be an open file object such as ``sys.stdout`` or returned by
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:func:`open` or :func:`os.popen`. It must be opened in binary mode (``'wb'``
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or ``'w+b'``).
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If the value has (or contains an object that has) an unsupported type, a
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:exc:`ValueError` exception is raised --- but garbage data will also be written
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to the file. The object will not be properly read back by :func:`load`.
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The *version* argument indicates the data format that ``dump`` should use
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(see below).
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.. function:: load(file)
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Read one value from the open file and return it. If no valid value is read
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(e.g. because the data has a different Python version's incompatible marshal
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format), raise :exc:`EOFError`, :exc:`ValueError` or :exc:`TypeError`. The
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file must be an open file object opened in binary mode (``'rb'`` or
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``'r+b'``).
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.. note::
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If an object containing an unsupported type was marshalled with :func:`dump`,
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:func:`load` will substitute ``None`` for the unmarshallable type.
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.. function:: dumps(value[, version])
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Return the string that would be written to a file by ``dump(value, file)``. The
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value must be a supported type. Raise a :exc:`ValueError` exception if value
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has (or contains an object that has) an unsupported type.
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The *version* argument indicates the data format that ``dumps`` should use
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(see below).
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.. function:: loads(string)
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Convert the string to a value. If no valid value is found, raise
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:exc:`EOFError`, :exc:`ValueError` or :exc:`TypeError`. Extra characters in the
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string are ignored.
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In addition, the following constants are defined:
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.. data:: version
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Indicates the format that the module uses. Version 0 is the historical
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format, version 1 shares interned strings and version 2 uses a binary format
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for floating point numbers.
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Version 3 adds support for object instancing and recursion.
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The current version is 3.
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.. rubric:: Footnotes
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.. [#] The name of this module stems from a bit of terminology used by the designers of
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Modula-3 (amongst others), who use the term "marshalling" for shipping of data
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around in a self-contained form. Strictly speaking, "to marshal" means to
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convert some data from internal to external form (in an RPC buffer for instance)
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and "unmarshalling" for the reverse process.
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