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).3.1
This is not a general ``persistency'' module. For general persistency 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, 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.3.2
There are functions that read/write files as well as functions operating on strings.
The module defines these functions:
sys.stdoutor returned by open() or posix.popen(). It must be opened in binary mode (
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().
Warning: If an object containing an unsupported type was
marshalled with dump(), load() will substitute
None for the unmarshallable type.
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.