Python Walrus operator

Good examples of the Python walrus operator.

TODO: check out presentation from PyCon 2019 introducing it.

From: https://www.reddit.com/r/Python/comments/1rw18i5/comment/oaxdl2e/ 

# perfect for re.match
if has_whatever := re.match(stuff)
    a, b = has_whatever.groups()

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only call other_function if res is some value
if (result := some_fun()) in ("a", "b"):
   other_function(res)

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# conditional based on the result of res
if func(res := gfunc()):
    # use res safely


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while line := fp.readline():
    ...

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if bad := [r for r in records if r.bad()]:
    raise ValueError('Bad records: ' + ', '.join(b.name for b in bad))