The marshal module

This module is used to serialize data; that is, convert data to and from character strings, so that they can be stored on file or sent over a network.

Marshal uses a simple self-describing data format. For each data item, the marshalled string contains a type code, followed by one or more type specific fields. Integers are stored in little-endian order, strings as a length field followed by the string’s contents (which can include null bytes), tuples as a length field followed by the objects that make up the tuple, etc.

Example: Using the marshal module to serialize data

# File: marshal-example-1.py

import marshal

value = (
    "this is a string",
    [1, 2, 3, 4],
    ("more tuples", 1.0, 2.3, 4.5),
    "this is yet another string"
    )

data = marshal.dumps(value)

# intermediate format
print type(data), len(data)

print "-"*50
print repr(data)
print "-"*50

print marshal.loads(data)

<type 'string'> 118
--------------------------------------------------
'(\004\000\000\000s\020\000\000\000this is a string
[\004\000\000\000i\001\000\000\000i\002\000\000\000
i\003\000\000\000i\004\000\000\000(\004\000\000\000
s\013\000\000\000more tuplesf\0031.0f\0032.3f\0034.
5s\032\000\000\000this is yet another string'
--------------------------------------------------
('this is a string', [1, 2, 3, 4], ('more tuples',
1.0, 2.3, 4.5), 'this is yet another string')

The marshal module can also handle code objects (it’s used to store precompiled Python modules).

Example: Using the marshal module to serialize code

# File: marshal-example-2.py

import marshal

script = """
print 'hello'
"""

code = compile(script, "<script>", "exec")

data = marshal.dumps(code)

# intermediate format
print type(data), len(data)

print "-"*50
print repr(data)
print "-"*50

exec marshal.loads(data)

<type 'string'> 81
--------------------------------------------------
'c\000\000\000\000\001\000\000\000s\017\000\000\00
0\177\000\000\177\002\000d\000\000GHd\001\000S(\00
2\000\000\000s\005\000\000\000helloN(\000\000\000\
000(\000\000\000\000s\010\000\000\000<script>s\001
\000\000\000?\002\000s\000\000\000\000'
--------------------------------------------------
hello

Note that the byte code is portable between platforms, but not necessarily between different Python releases. The imp.get_magic() function returns a 4-byte string that identifies the current byte code version. If you’re planning to store byte code in some persistent storage, it’s a good idea to include the “magic” value, and check that before executing the code.