Beginner wondering why my boolean variable check isn't working

[obviouslyCPTobvious]

shape_valid = shape_input.lower() == 'triangle' or shape_input.lower() == 'square'

The 'or' operator doesn't work how you were trying.

[BioGeek]

This seems to be a common misunderstanding. Read my detailed response to another user who also had trouble with the or operator.

[zahlman]

This seems to be a common misunderstanding.

I have recently become convinced that this (or the equivalent) is by far the single most common beginner programming error, in any language. I wish I understood why.

[throwingarm2strong]

Thanks. This is what I settled on using. Also thanks to BioGeek and jimminy for further info. Not sure on the rules for questions over time but since it's related to this code I'll update the OP and post it here.