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Author vstinner
Recipients vstinner
Date 2019-07-15.15:05:11
SpamBayes Score -1.0
Marked as misclassified Yes
Message-id <>
See bpo-29708 meta issue and for reproducible builds.

pyc files are not fully reproducible yet: frozenset items are not serialized in a deterministic order

One solution would be to modify marshal to sort frozenset items before serializing them. The issue is how to handle items which cannot be compared. Example:

>>> l=[float("nan"), b'bytes', 'unicode']
>>> l.sort()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: '<' not supported between instances of 'bytes' and 'float'

One workaround for types which cannot be compared is to use the type name in the key used to compare items:

>>> l.sort(key=lambda x: (type(x).__name__, x))
>>> l
[b'bytes', nan, 'unicode']

Note: comparison between bytes and str raises a BytesWarning exception when using python3 -bb.

Second problem: how to handle exceptions when comparison raises an error anyway?

Another solution would be to use the PYTHONHASHSEED environment variable. For example, if SOURCE_DATE_EPOCH is set, PYTHONHASHSEED would be set to 0. This option is not my favorite because it disables a security fix against denial of service on dict and set:


Previous discussions on reproducible frozenset:


See also bpo-34093: "Reproducible pyc: FLAG_REF is not stable" and PEP 552 "Deterministic pycs".
Date User Action Args
2019-07-15 15:05:11vstinnersetrecipients: + vstinner
2019-07-15 15:05:11vstinnersetmessageid: <>
2019-07-15 15:05:11vstinnerlinkissue37596 messages
2019-07-15 15:05:11vstinnercreate