E731 do not assign a lambda expression, use a def

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The recommendation in PEP-8 you are running into is:

Always use a def statement instead of an assignment statement that
binds a lambda expression directly to a name.

Yes:

def f(x): return 2*x

No:

f = lambda x: 2*x

The first form means that the name of the resulting
function object is specifically ‘f’ instead of the generic ‘<lambda>’.
This is more useful for tracebacks and string representations in
general. The use of the assignment statement eliminates the sole
benefit a lambda expression can offer over an explicit def statement
(i.e. that it can be embedded inside a larger expression)

Assigning lambdas to names basically just duplicates the functionality of def – and in general, it’s best to do something a single way to avoid confusion and increase clarity.

The legitimate use case for lambda is where you want to use a function without assigning it, e.g:

sorted(players, key=lambda player: player.rank)

In general, the main argument against doing this is that def statements will result in more lines of code. My main response to that would be: yes, and that is fine. Unless you are code golfing, minimising the number of lines isn’t something you should be doing: go for clear over short.

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