I missed this.  It still defaults to True in Generator.  It should default to False in the new policies (but True in compat32).

I created the patch, please review it.

Sorry, my message wasn't clear.  The current default needs to remain the same.  What needs to be added is email.policy.Policy.mange_from, which should be True in the compat32 policy and False in EmailPolicy. Then it needs to be hooked up the Generator, so that an explicit specificaion in the __init__ overrides the policy, but specifying one of EmailPolicy dervived policies will override the default value of the __init__ argument if the argument is not speicifed explicitly in the Generator constructor call.  (Backward compatibility is a pain.)

(@r.david.murray) I just implemented what you suggested:
- Policy has a mangle_from_ property, overridden in EMailPolicy
- Generator and DecodedGenerator default to policy.mangle_from_ or True if policy is None.

Would be glad if it helps :)

Updates patch to include necessary changes to test_policy.py.

Added documentation for mangle_from. since the name "mangle_from_" is not supported by the markup, changed it to "mangle_from" everywhere, which looks better anyway (I understand why it was the other way so). If it was a bad idea, I can undo it but am unable to do the doc.

Rollback to mangle_from_, sorry for the noise.

This is an easy issue?

Updated the last patch according to the review comments at https://bugs.python.org/review/20098/.

New changeset df81f9b67736 by R David Murray in branch 'default':
#20098: add mangle_from_ policy option.
https://hg.python.org/cpython/rev/df81f9b67736

Thanks, Milan.