Temporary variables in list comprehensions

Steven D'Aprano steve+comp.lang.python at pearwood.info
Sun Jan 8 22:53:26 EST 2017

Suppose you have an expensive calculation that gets used two or more times in a 
loop. The obvious way to avoid calculating it twice in an ordinary loop is with 
a temporary variable:

result = []
for x in data:
    tmp = expensive_calculation(x)
    result.append((tmp, tmp+1))

But what if you are using a list comprehension? Alas, list comps don't let you 
have temporary variables, so you have to write this:

[(expensive_calculation(x), expensive_calculation(x) + 1) for x in data]

Or do you? ... no, you don't!

[(tmp, tmp + 1) for x in data for tmp in [expensive_calculation(x)]]

I can't decide whether that's an awesome trick or a horrible hack...

"Ever since I learned about confirmation bias, I've been seeing 
it everywhere." - Jon Ronson

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