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As a follow-up, I was just looking through their docs--I don't suppose you have a cert file set in your environment, do you? I wonder if requests or urllib is pulling that as a fallback and since that value now isn't initialising as None, it leads to the cert policy being 'use the one from the environment'? I think it would be REQUESTS_CA_BUNDLE or CURL_CA_BUNDLE, or perhaps something similar? It would presumably have a value that pointed to a local cert file in your system dir somewhere.