Barry noticed:

Anyway, did you know that you can use functions as keys to a dictionary, but that you can mutate them to "lose" the element?

-------------------- snip snip -------------------- Python 2.0 (#13, Jan 10 2001, 13:06:39) [GCC egcs-2.91.66 19990314/Linux (egcs-1.1.2 release)] on linux2 Type "copyright", "credits" or "license" for more information.

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d = {} def foo(): pass ... def bar(): pass ... d[foo] = 1 d[foo] 1 foocode = foo.func_code foo.func_code = bar.func_code d[foo] Traceback (most recent call last): File "<stdin>", line 1, in ? KeyError: <function foo at 0x81ef474> d[bar] = 2 d[bar] 2 d[foo] 2 foo.func_code = foocode d[foo] 1 -------------------- snip snip --------------------

It's because a function's func_code attribute is used in its hash calculation, but func_code is writable!

Clearly, something changed. I'm pretty sure it's the function attributes. Either the function attributes shouldn't be used in comparing function objects, or hash() on functions should be unimplemented, or comparison on functions should use simple pointer compares. What's the right solution? Do people use functions as dict keys? If not, we can remove the hash() implementation. But I suspect they *are* used as dict keys. Not using the __dict__ on comparisons appears ugly, so probably the best solution is to change function comparisons to use simple pointer compares. That removes the possibility to see whether two different functions implement the same code -- but does anybody really use that? --Guido van Rossum (home page: http://www.python.org/~guido/)