mirror of https://github.com/python/cpython
158 lines
6.3 KiB
ReStructuredText
158 lines
6.3 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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--------------
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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 Mac, 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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pair: module; pickle
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pair: module; shelve
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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.
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The format of code objects is not compatible between Python versions,
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even if the version of the format is the same.
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De-serializing a code object in the incorrect Python version has undefined behavior.
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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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.. index:: object; code, code object
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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-point
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numbers, complex numbers, strings, bytes, bytearrays, tuples, lists, sets,
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frozensets, dictionaries, and code objects (if *allow_code* is true),
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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. The
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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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bytes-like objects.
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The module defines these functions:
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.. function:: dump(value, file, version=version, /, *, allow_code=True)
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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 a writeable :term:`binary file`.
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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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:ref:`Code objects <code-objects>` are only supported if *allow_code* is true.
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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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.. audit-event:: marshal.dumps value,version marshal.dump
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.. versionchanged:: 3.13
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Added the *allow_code* parameter.
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.. function:: load(file, /, *, allow_code=True)
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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`.
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:ref:`Code objects <code-objects>` are only supported if *allow_code* is true.
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The file must be a readable :term:`binary file`.
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.. audit-event:: marshal.load "" marshal.load
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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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.. versionchanged:: 3.10
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This call used to raise a ``code.__new__`` audit event for each code object. Now
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it raises a single ``marshal.load`` event for the entire load operation.
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.. versionchanged:: 3.13
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Added the *allow_code* parameter.
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.. function:: dumps(value, version=version, /, *, allow_code=True)
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Return the bytes object 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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:ref:`Code objects <code-objects>` are only supported if *allow_code* is true.
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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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.. audit-event:: marshal.dumps value,version marshal.dump
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.. versionchanged:: 3.13
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Added the *allow_code* parameter.
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.. function:: loads(bytes, /, *, allow_code=True)
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Convert the :term:`bytes-like object` to a value. If no valid value is found, raise
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:exc:`EOFError`, :exc:`ValueError` or :exc:`TypeError`.
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:ref:`Code objects <code-objects>` are only supported if *allow_code* is true.
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Extra bytes in the input are ignored.
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.. audit-event:: marshal.loads bytes marshal.load
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.. versionchanged:: 3.10
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This call used to raise a ``code.__new__`` audit event for each code object. Now
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it raises a single ``marshal.loads`` event for the entire load operation.
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.. versionchanged:: 3.13
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Added the *allow_code* parameter.
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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 4.
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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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