Serialisation notes

Constraints and requirements

• Concerned with Python <-> Python. Javascript treatment of integers outside float64 is a non-issue.
• Require compatibility with all supported Python versions (2.4 - 2.7, >= 3.6)
• If using code outside stdlib, then must be able to bootstrap mitogen.core.Importer before relying on it.

Formats

Marshal

Stdlib module used to dump/load .pyc files. Docs warn "Not a general persistence module", "Details of the format are undocumented on purpose", "Python maintainers reserve the right to modify the marshal format in backward incompatible ways", "Warning The marshal module is not intended to be secure against erroneous or maliciously constructed data", and so on.

Version	Python	Changes
0
1	>= 2.4	Interned strings
2	>= 2.5	Binary float format
3	>= 3.4	Object instancing and recursion
4	>= 3.4	Effcient short strings
5	>= 3.14	Slice objects support

Pickle

stdlib module, currently used by Mitogen (Jan 2026).

Protocol	Python	PEPs	Changes
0			ASCII
1			Binary
2	>= 2.3	PEP-307	New-style classes
3	>= 3.0		Explicit bytes support
4	>= 3.4	PEP-3154	Strings > 2**32 , sets, framing
5	>= 3.8	PEP-574	Out of band data

See also

• https://github.com/mitogen-hq/mitogen/issues/485
• https://github.com/moreati/pickle-fuzz