marshal
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).
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.
There are functions that read/write files as well as functions operating on strings.
The module defines these functions:
sys.stdout
or returned by open()
or
posix.popen()
.
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()
.
EOFError
, ValueError
or
TypeError
. The file must be an open file object.
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.
EOFError
, ValueError
or TypeError
. Extra
characters in the string are ignored.