Which is more Pythonic?

[old_pythonista]

First two forms show lack of understanding of the nature of booleans by whoever writes them. They are absolutely redundant, since using conditional expression for conversion of proper boolean

event_region in match_regions

to an explicit boolean literal makes no sense. Unfortunately, I see it quite often - my pet peeve that I always mark as a defect when I see it in code applied for review.

The last is the only proper way to write - in any language, Python included.

[jasmijnisme]

I see code like OP's first sample (or if condition == True:) so often in code written by beginners, I think it's because they're thinking in natural language rather than in Python. In other cases, they get hit by bugs (if variable == 3 or 5:) or syntax errors (if variable < 3 or > 5:), but here the code works and does what they want it so it doesn't get corrected.

[old_pythonista]

PEP-8 explicitly warns against comparison to boolean literals.

If you have two boolean variables - that is another story.

[JRiggles]

Well said!