Construct set with list comprehension via function call only if not None

Question

Having the call

set(self.oid_to_path(oid, entries_parents) for oid in modified_by_one_parent_oids)

this works, but also adds None to the list if the function returns None

So modifying it to

set(self.oid_to_path(oid, entries_parents) for oid in modified_by_one_parent_oids if None != self.oid_to_path(oid, entries_parents))

also works, without the Nones.

However this has the drawback of calling the function twice. How to call it only once, and use some memoization instead?

Oleh Prypin · Accepted Answer · 2013-01-01 19:11:56Z

3

Use a set comprehension on a generator expression...

g = (self.oid_to_path(oid, entries_parents) for oid in modified_by_one_parent_oids)
result = {el for el in g if el is not None}

Please note that you don't actually have a list comprehension in your code. You're calling the set constructor with a generator expression, which can be done simpler – with a set comprehension, as shown above.

answered Jan 1, 2013 at 19:11

Oleh Prypin

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