How can I use if/else in a dictionary comprehension?

python dictionary dictionary-comprehension
diegueus9 picture diegueus9 · Feb 25, 2012 · Viewed 106.5k times · Source

Does there exist a way in Python 2.7+ to make something like the following?

{ something_if_true if condition else something_if_false for key, value in dict_.items() }

I know you can make anything with just 'if':

{ something_if_true for key, value in dict_.items() if condition}

Answer

Marcin picture Marcin · Feb 25, 2012

You've already got it: A if test else B is a valid Python expression. The only problem with your dict comprehension as shown is that the place for an expression in a dict comprehension must have two expressions, separated by a colon:

{ (some_key if condition else default_key):(something_if_true if condition
          else something_if_false) for key, value in dict_.items() }

The final if clause acts as a filter, which is different from having the conditional expression.

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