[Python] How to check if a column exists in Pandas

How do I check if a column exists in a Pandas DataFrame df?

A B C
0 3 40 100
1 6 30 200


How would I check if the column "A" exists in the above DataFrame so that I can compute:

df['sum'] = df['A'] + df['C']


And if "A" doesn't exist:

df['sum'] = df['B'] + df['C']

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