```
def invert_dict(d):
    inv = dict()
    for key in d:
        val = d[key]
        if val not in inv:
            inv[val] = [key]
        else:
            inv[val].append(key)
return inv

```

This is an example from Think Python book, a function for inverting(swapping) keys and values in a dictionary. New values (former keys) are stored as lists, so if there was multiple dictionary values (bound to a different keys) that were equal before inverting, then this function simply appends them to the list of former keys.

Example:

```
somedict = {'one': 1, 'two': 2, 'doubletwo': 2, 'three': 3}
invert_dict(somedict) ---> {1: ['one'], 2: ['doubletwo', 'two'], 3: ['three']}

```

My question is, can the same be done with dictionary comprehensions? This function creates an empty dict `inv = dict()`, which is then checked later in the function with `if/else` for the presence of values. Dict comprehension, in this case, should check itself. Is that possible, and how the syntax should look like?

General dict comprehension syntax for swapping values is:

```
{value:key for key, value in somedict.items()}

```

but if I want to add an 'if' clausule, what it should look like? `if value not in (what)`?

Thanks.