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The observation has been made that there are some idioms related to key functions passed to various methods and functions that aren't particularly easy to discover.
One suggestion is to create a "key function" glossary entry that provides examples of standard library operations that exist primarily for use as key functions (i.e. the three functions mentioned in the issue title), as well as mentioning some of the APIs that accept key functions (e.g. sorted, list.sort, min, max). The documentation of these various could then cross link to the "key function" glossary entry to make this idioms more discoverable without needing to repeat ourselves in the documentation of every method that accepts a key function.
As per discussion on python-ideas, including the glossary entry suggestion from Terry Reedy:
http://mail.python.org/pipermail/python-ideas/2010-September/008093.html