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(python2.1-lib.info)marshal


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Alternate Python object serialization
=====================================

Convert Python objects to streams of bytes and back (with different
constraints).

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.

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:
`None', integers, long integers, floating point numbers, strings,
Unicode objects, tuples, lists, dictionaries, and code objects, where
it should be understood that tuples, lists and dictionaries are only
supported as long as the values contained therein are themselves
supported; and recursive lists and dictionaries should not be written
(they will cause infinite loops).

*Caveat:* 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.  Since the current `marshal' module uses
32 bits to transfer plain Python integers, such values are silently
truncated.  This particularly affects the use of very long integer
literals in Python modules -- these will be accepted by the parser on
such machines, but will be silently be truncated when the module is read
from the `.pyc' instead.(2)

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

The module defines these functions:

`dump(value, file)'
     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 `posix.popen()'.  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()'.

`load(file)'
     Read one value from the open file and return it.  If no valid value
     is read, raise `EOFError', `ValueError' or `TypeError'.  The file
     must be an open file object opened in binary mode (`'rb'' or
     `'r+b'').

     *Warning:* If an object containing an unsupported type was
     marshalled with `dump()', `load()' will substitute `None' for the
     unmarshallable type.

`dumps(value)'
     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.

`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.

---------- 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.

(2)  A solution would be to refuse such literals in the parser, since
they are inherently non-portable.  Another solution would be to let the
`marshal' module raise an exception when an integer value would be
truncated.  At least one of these solutions will be implemented in a
future version.


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