Python: issue when using vars() dictionary

Question

I have the following snippet:

a, b = 1, 2
params = ['a', 'b']
res = {p: vars()[p] for p in params}

Which gives me KeyError: 'a' whereas the following code works fine:

a, b = 1, 2
params = ['a', 'b']
res = {}
for p in params:
    res[p] = vars()[p] 

What's the difference here?

Answer 1

vars() without any argument acts like locals() and since a dictionary comprehension has its own scope it has no variable named a or b.

You can use eval() here. Without any argument it will execute in LEGB manner, or specify globals() dict explicitly to eval:

>>> res = {p: eval(p) for p in params}
>>> res
{'a': 1, 'b': 2}

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But then again the correct way will be to create a dictionary from the start if you want to access variables using their names.