How best to write this try/except block?

[Fredrik Lundh]

Rich Harkins wrote:

> Because of the is comparison even if "MYBLANKVALUE" is in d then this code
> will still work.  Just don't intern whatever "MYBLANKVALUE" is (I frequently
> use undefined="UNDEFINED").

bad idea.  Python automatically interns string literals that happen
to look like identifiers (see the "Intern selected string constants"
section in Python/compile.c), so this only works if you can guarantee
that your "fn" function is never called with a string literal.

undefined=["UNDEFINED"] is a bit safer, but you really should use
has_key or try/except for things like this.

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