[Python] Why does extracting the only key from a dictionary inside a lambda used in map()...

I ran into a strange behavior while trying to extract the only key from a dictionary using a lambda inside a map() function. The dictionaries are guaranteed to have exactly one key-value pair, and I’m using Python 3.13.5.

Here’s the setup:

data = [{"alpha": 1}, {"beta": 2}, {"gamma": 3}]

# Attempt to extract the only key from each dictionary
keys = list(map(lambda d: next(iter(d)), data))
print(keys)


This works fine in most cases, but when I run this inside a larger multiprocessing pipeline or wrap it in a generator expression, I sometimes get:

RuntimeError: dictionary changed size during iteration Or even worse, inconsistent results like missing keys or duplicated ones. I tried replacing next(iter(d)) with a for-loop or converting to a list first, but the performance drops significantly, and the issue still occasionally appears when the dictionaries are dynamically generated.

I suspect this has something to do with internal optimizations or memory handling in newer Python versions, but I can't pinpoint the root cause.

Questions:

1. Why would next(iter(d)) behave inconsistently inside map() or generator expressions?
2. Is there a safer and equally performant way to extract the only key from a dictionary in such contexts?
3. Could this be related to dictionary resizing or garbage collection in Python 3.13+?

Any insights or workarounds would be greatly appreciated. This is part of a real-time data pipeline, so performance and reliability are both critical.

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