11.5. "marshal" — Internal Python object serialization
******************************************************

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.

Warning:

  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, long integers, floating point numbers,
complex numbers, strings, Unicode objects, 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; and recursive lists, sets and dictionaries should not be
written (they will cause infinite loops).  The singletons "None",
"Ellipsis" and "StopIteration" can also be marshalled and
unmarshalled.

Warning:

  On machines where C’s "long int" type has more than 32 bits (such as
  the DEC Alpha), it is possible to create plain Python integers that
  are longer than 32 bits. If such an integer is marshaled and read
  back in on a machine where C’s "long int" type has only 32 bits, a
  Python long integer object is returned instead.  While of a
  different type, the numeric value is the same.  (This behavior is
  new in Python 2.2.  In earlier versions, all but the least-
  significant 32 bits of the value were lost, and a warning message
  was printed.)

There are functions that read/write files as well as functions
operating on strings.

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 an open file object such as "sys.stdout" or
   returned by "open()" or "os.popen()".  It may not be a wrapper such
   as TemporaryFile on Windows. It must be opened in binary mode
   ("'wb'" or "'w+b'").

   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()".

   New in version 2.4: 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 an open file object opened in binary
   mode ("'rb'" or "'r+b'").

   Note:

     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 string 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.

   New in version 2.4: The *version* argument indicates the data
   format that "dumps" should use (see below).

marshal.loads(string)

   Convert the string to a value.  If no valid value is found, raise
   "EOFError", "ValueError" or "TypeError".  Extra characters in the
   string 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 (added in Python 2.4) shares interned
   strings and version 2 (added in Python 2.5) uses a binary format
   for floating point numbers. The current version is 2.

   New in version 2.4.

-[ Footnotes ]-

[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.
