12.4. "marshal" --- Internal Python object serialization
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This module contains functions that can read and write Python values
in a binary format.  The format is specific to Python, but independent
of machine architecture issues (e.g., you can write a Python value to
a file on a PC, transport the file to a Sun, and read it back there).
Details of the format are undocumented on purpose; it may change
between Python versions (although it rarely does). [1]

This is not a general "persistence" module.  For general persistence
and transfer of Python objects through RPC calls, see the modules
"pickle" and "shelve".  The "marshal" module exists mainly to support
reading and writing the "pseudo-compiled" code for Python modules of
".pyc" files. Therefore, the Python maintainers reserve the right to
modify the marshal format in backward incompatible ways should the
need arise.  If you're serializing and de-serializing Python objects,
use the "pickle" module instead -- the performance is comparable,
version independence is guaranteed, and pickle supports a
substantially wider range of objects than marshal.

警告: The "marshal" module is not intended to be secure against
  erroneous or maliciously constructed data.  Never unmarshal data
  received from an untrusted or unauthenticated source.

Not all Python object types are supported; in general, only objects
whose value is independent from a particular invocation of Python can
be written and read by this module.  The following types are
supported: booleans, integers, floating point numbers, complex
numbers, strings, bytes, bytearrays, tuples, lists, sets, frozensets,
dictionaries, and code objects, where it should be understood that
tuples, lists, sets, frozensets and dictionaries are only supported as
long as the values contained therein are themselves supported.  The
singletons "None", "Ellipsis" and "StopIteration" can also be
marshalled and unmarshalled. For format *version* lower than 3,
recursive lists, sets and dictionaries cannot be written (see below).

There are functions that read/write files as well as functions
operating on bytes-like objects.

The module defines these functions:

marshal.dump(value, file[, version])

   Write the value on the open file.  The value must be a supported
   type.  The file must be a writeable *binary file*.

   If the value has (or contains an object that has) an unsupported
   type, a "ValueError" exception is raised --- but garbage data will
   also be written to the file.  The object will not be properly read
   back by "load()".

   The *version* argument indicates the data format that "dump" should
   use (see below).

marshal.load(file)

   Read one value from the open file and return it.  If no valid value
   is read (e.g. because the data has a different Python version's
   incompatible marshal format), raise "EOFError", "ValueError" or
   "TypeError".  The file must be a readable *binary file*.

   注解: If an object containing an unsupported type was marshalled
     with "dump()", "load()" will substitute "None" for the
     unmarshallable type.

marshal.dumps(value[, version])

   Return the bytes object that would be written to a file by
   "dump(value, file)".  The value must be a supported type.  Raise a
   "ValueError" exception if value has (or contains an object that
   has) an unsupported type.

   The *version* argument indicates the data format that "dumps"
   should use (see below).

marshal.loads(bytes)

   Convert the *bytes-like object* to a value.  If no valid value is
   found, raise "EOFError", "ValueError" or "TypeError".  Extra bytes
   in the input are ignored.

In addition, the following constants are defined:

marshal.version

   Indicates the format that the module uses. Version 0 is the
   historical format, version 1 shares interned strings and version 2
   uses a binary format for floating point numbers. Version 3 adds
   support for object instancing and recursion. The current version is
   4.

-[ 脚注 ]-

[1] The name of this module stems from a bit of terminology used
    by the designers of Modula-3 (amongst others), who use the term
    "marshalling" for shipping of data around in a self-contained
    form. Strictly speaking, "to marshal" means to convert some data
    from internal to external form (in an RPC buffer for instance) and
    "unmarshalling" for the reverse process.
