[URGENT] I am encountering an unexpected and utterly confounding exception when attempting to execute a nested dictionary comprehension that includes conditional logic within the value assignment — specifically, `d = {k: {'v': v if condition else Non

Opening thread commentary.

You didn't actually paste source_data. You pasted your confusion instead.

The KeyError isn't coming from the comprehension — comprehensions don't raise KeyErrors on value assignment unless you have a side effect in there, and if you do, that's your problem not

Ah, I see you're wrestling with what appears to be a nested comprehension that is behaving in a non-intuitive fashion — let me try to unpack this systematically because there are several distinct failure modes here and we should isolate which one applies to your specific execution context before drawing any premature conclusions about the Python interpreter.

First, we need to establish exactly where that KeyError is originating. A dictionary comprehension with conditional value assignment (v if condition else None) cannot itself raise a KeyError during the iteration phase because there's no key access happening — it's merely constructing a new dict from an existing one. So the error must be coming from either source_data.items() (which would fail before we even enter), or more likely, from inside the conditional expression itself if you have any side effects hidden within what you're calling 'condition'. If condition is actually something like some_lookup[k] and that lookup fails on 98% of your keys, then the error isn't in the comprehension structure but in an access operation being performed as part of a predicate.

Second — this is important for debugging purposes — you should consider whether there's any chance another thread or callback is mutating source_data while this runs; Python's dict iterators are not thread-safe and will raise RuntimeError (not KeyError) on concurrent modification, so that's probably not it but worth ruling out if your code lives in a multi-threaded context.

Third, let me ask about the 'condition' itself — you don't specify what it is here. If condition involves any attribute access like v.metadata['type'] and 98% of your v objects lack that key, KeyError would fire right there during evaluation before the comprehension can proceed to assign. This is a very common pitfall where developers assume 'condition' is a pure boolean predicate when it actually contains deep-access patterns through nested structures.

Fourth — this addresses

That sounds backwards — if it's conditional logic you should be getting None, not a KeyError