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EDIT: its answered, I did not understand what a ternary operator is. For people from the future that have the similar question: https://book.pythontips.com/en/latest/ternary_operators.html

I'm studying 'assert' statements in python and I don't understand the following sentence.

assert .. if ... else ... and ...

So if I understand correctly you have to use the above if you want to test an 'if else ' statement. You have to insert it right after the "if" statement the following: assert (P1 if E else P2) and E

For example

assert (y == builtins.max(x, y) if x < y else x == builtins.max(x, y)) and x < y

If understand assert y == builtins.max(x,y) It just checks if the condition is true or not and when it is not true it returns an assertion error. However in the case of : assert (y == builtins.max(x, y) if x < y else x == builtins.max(x, y)) and x < y

I have no clue what is happening. It apparently always returns true as well. But I cannot even guess what is exactly happening. I looked up what an assert statement does and the only thing it does is: assert <condition>,<error message> so check the condition and possibly return an error message. However I don't understand how ... if ... else ... and ... is a condition. I understand the and but how exactly do you interpret the if else part in that condition?

I don't really understand what I'm not understanding. It's probably very trivial. Hopefully someone can help me. Sorry for my spelling mistakes.

EDIT: its answered, I did not understand what a ternary operator is. For people from the future that have the similar question: https://book.pythontips.com/en/latest/ternary_operators.html

bananenheld
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1 Answers1

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What is going on here is the assertion of a ternary.

This:

(y == builtins.max(x, y) if x < y else x == builtins.max(x, y)) and x < y

is conceptually:

A if cond else B # and cond2, but we'll come to that

Which evaluates first to A or B. We then assert that, so

assert A if cond else B

is the same as

x = A if cond else B
assert x

This particular ternary is not straightforward. It's equivalent to this:

if x < y:
    res = y == builtins.max(x, y)
else:
    res  = x == builtins.max(x,y)

assert res and x < y

IMHO it's rather unclear. It's funny anyhow, as in most contexts you can just do max() rather than builtins.max(). Perhaps it belongs in a test, or a context where max() has (unwisely) been overidden? [Thinking it through this is probably a test for builtins.max isn't it?]

References

For more information on python's ternary construction see e.g. this question.

2e0byo
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