Raise error if a Python dict comprehension overwrites a key

Question

Is there a way to get a dict comprehension to raise an exception if it would override a key?

For example, I would like the following to error because there are two values for the key 'a':

>>> {k:v for k, v in ('a1', 'a2', 'b3')}
{'a': '2', 'b': '3'}

I realise this can be done with a for loop. Is there a way to do it while keeping the comprehension syntax?

Answer 1

You can use a generator with a helper function:

class DuplicateKeyError(ValueError): pass

def dict_no_dupl(it):
    d = {}
    for k, v in it:
        if k in d: raise DuplicateKeyError(k)
        d[k] = v
    return d

dict_no_dupl((k, v) for k, v in ('a1', 'a2', 'b3'))

This does add a helper function, but keeps the comprehension syntax (reasonably) intact.

Answer 2

If you don't care about which key caused a collision:

Check that the generated dict has the appropriate size with len().