Help with a reverse dictionary lookup

[Lonnie Princehouse]

Here's the illegible gibberish version of your function.  Once you
understand completely the following line of code, you will be well on
your way to Python nirvana:

getNodes = lambda Foundry=None,Process=None: [node for node,foundries
in dict.iteritems() if ((Foundry is None) and ((Process is None) or
(len([foundry for foundry,processes in foundries.iteritems() if Process
in processes]) > 0))) or ((Foundry in foundries) and ((Process is None)
or (Process in foundries[Foundry])))]

# ex:
getNodes(Foundry="umc", Process="2p6m_1.8-3.3_sal_ms")

# note: returns an empty list rather than None if no matching nodes
exist
# hint: You'll probably want to do rearrange your data structure or do
some caching if you have a large dataset.  The dict-and-list solution
doesn't make this search very efficient.

And:  I just noticed that len() won't work with a generator expression
as its argument.  What's up with that?