Forgive me for playing stupid here, but I want to understand English
better. I would fully understand the confusion had the sentence been

"dict.has_key(key) is equivalent to key in d, but it is deprecated."

Terry's and Martin' example sentences are transferable to that. However,
the actual sentence was

"dict.has_key(key) is equivalent to key in d, but deprecated."

Let me try to construct a similar sentence:

"Guido was once a colleague of Joe, but much smarter."

Can the "but" clause really be taken as referring to Joe? Or is it
simply not an English sentence? ;)