How to reuse an expression in a comprehension expression?

Question

Imagine a theoretical snippet:

# just for this example: `bad_structure` contains a list of dicts with different keys
# for the same semantic
bad_structure =          


        

Answer 1

You can use an intermediate comprehension to bind to a name:

result = {
    p: some_func(p)
    # bind intermediate result to p
    for p in (  # nested comprehension to produce intermediate result
        next(k for k in ('path', 'subdir') if k in e)
        for e in bad_structure
    )
 }

Instead of mapping directly to two separate expressions, it first maps to a common expression which then is mapped to two separate expressions.

You can pass along and rename an arbitrary number of values. Create a tuple in the inner comprehension, and unpack it to multiple names in the outer comprehension.

result = {
    p: some_func(e, p)
    for e, p in (
        (e, next(iter(e)))
        for e in bad_structure
    )
 }