To calculate the tenth Fibonacci number, you should really only need to calculate the preceding Fibonacci numbers, but this implementation somehow needs a whopping 177 calculations. It gets worse quickly: 21891 calculations are needed for fibonacci(20) and almost 2.7 million calculations for the 30th number. This is because the code keeps recalculating Fibonacci numbers that are already known.
After 2.4a2 was released, in response to community reaction, Guido stated that he'd re-examine a community proposal, if the community could come up with a community consensus, a decent proposal, and an implementation. After an amazing number of posts, collecting a vast number of alternatives in the Python wiki , a community consensus emerged (below). Guido subsequently rejected  this alternate form, but added:
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