Asking "is hashable" about a Python value

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I am interested in taking an arbitrary dict and copying it into a new dict, mutating it along the way.

One mutation I would like to do is swap keys and value. Unfortunately, some values are dicts in their own right. However, this generates a "unhashable type: 'dict'" error. I don't really mind just stringifying the value and giving it the key. But, I'd like to be able to do something like this:

for key in olddict:
  if hashable(olddict[key]):
    newdict[olddict[key]] = key
  else
    newdict[str(olddict[key])] = key

Is there a clean way to do this that doesn't involve trapping an exception and parsing the message string for "unhashable type" ?

hash python
5 Answers

I guess the best solution is to use collections.Hashable:

import collections
if isinstance(olddict[key], collections.Hashable):
    newdict[olddict[key]] = key
else:
    newdict[str(olddict[key])] = key
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