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).3.14
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. There may also be unknown security problems with marshal3.15.
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.
If such an integer is marshaled and read back in on a machine where
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:
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().
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.