Ternary Expression In Dictionary Comprehension

I'm trying to invert a dictionary. In the case of many keys having the same value, the new key (old value) should associate with a set of the new values (old keys). I solved the pr

Solution 1:

Close!

The following code

d = {'a': 'x', 'b': 'y', 'c': 'x'}

defhas_unique_value(k):
    return d.values().count(d[k]) == 1defkeys_with_same_value_as_key(k):
    returnset([key for key in d.keys() if d[key] == d[k]])

print( {d[k]:k if has_unique_value(k) else keys_with_same_value_as_key(k) for k in d.keys()} )

Produces

{'y': 'b', 'x': set(['a', 'c'])}

The only difference is the second d[k]: is removed.

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In general, the ternary expression looks like

a = val_if_true if test else val_if_false

not something closer to what you had:

a = val_if_true if test else a = val_if_false

You specify where the value from the ternary expression should go once, at the beginning.

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RE: Question in comments

This is still definitely a dictionary comprehension.

It's basically doing the following:

m = {}
for k in d.keys():
    orig_key = k
    orig_val = d[k]

    if has_unique_value(k):
        m[orig_val] = orig_key
    else:
        m[orig_val] = keys_with_same_value_as_key(orig_key)

print m

The only difference is that, in the dictionary comprehension, the m dictionary is not kept in the namespace (and the variables orig_key and orig_val I used to clarify the code never exist).

Solution 2:

The ternary expression can only be applied to one value and not to the equivalent of a dictionary assignment. Try this instead:

{d[k]: k if has_unique_value(k) else keys_with_same_value_as_key(k) for k in d.keys()}

Your first approach would be similar to the following (which does not work):

d[k] = k if k else d[k] = None# doesn't work