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

Question

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.

@Marcin's answer covers it all, but just in case someone wants to see an actual example, I add two below:

Let's say you have the following dictionary of sets

d = {'key1': {'a', 'b', 'c'}, 'key2': {'foo', 'bar'}, 'key3': {'so', 'sad'}}

and you want to create a new dictionary whose keys indicate whether the string 'a' is contained in the values or not, you can use

dout = {"a_in_values_of_{}".format(k) if 'a' in v else "a_not_in_values_of_{}".format(k): v for k, v in d.items()}

which yields

{'a_in_values_of_key1': {'a', 'b', 'c'},
 'a_not_in_values_of_key2': {'bar', 'foo'},
 'a_not_in_values_of_key3': {'sad', 'so'}}

Now let's suppose you have two dictionaries like this

d1 = {'bad_key1': {'a', 'b', 'c'}, 'bad_key2': {'foo', 'bar'}, 'bad_key3': {'so', 'sad'}}
d2 = {'good_key1': {'foo', 'bar', 'xyz'}, 'good_key2': {'a', 'b', 'c'}}

and you want to replace the keys in d1 by the keys of d2 if there respective values are identical, you could do

# here we assume that the values in d2 are unique
# Python 2
dout2 = {d2.keys()[d2.values().index(v1)] if v1 in d2.values() else k1: v1 for k1, v1 in d1.items()}

# Python 3
dout2 = {list(d2.keys())[list(d2.values()).index(v1)] if v1 in d2.values() else k1: v1 for k1, v1 in d1.items()}

which gives

{'bad_key2': {'bar', 'foo'},
 'bad_key3': {'sad', 'so'},
 'good_key2': {'a', 'b', 'c'}}

In case you have different conditions to evaluate for keys and values, @Marcin's answer is the way to go.

If you have the same condition for keys and values, you're better off with building (key, value)-tuples in a generator-expression feeding into dict():

dict((modify_k(k), modify_v(v)) if condition else (k, v) for k, v in dct.items())

It's easier to read and the condition is only evaluated once per key, value.

Example with borrowing @Cleb's dictionary of sets:

d = {'key1': {'a', 'b', 'c'}, 'key2': {'foo', 'bar'}, 'key3': {'so', 'sad'}}

Assume you want to suffix only keys with a in its value and you want the value replaced with the length of the set in such a case. Otherwise, the key-value pair should stay unchanged.

dict((f"{k}_a", len(v)) if "a" in v else (k, v) for k, v in d.items())
# {'key1_a': 3, 'key2': {'bar', 'foo'}, 'key3': {'sad', 'so'}}

It also might be worth mentioning that If only statements put the if at the end:

{_ for _ in iterable if True}