def try_convert(target_type, obj, default, ignored=(TypeError,))
           return target_type(obj)
       except ignored:
           return default

The problem with a general convert function is that to make it work, you would need to account for several variations and the signature gets rather clunky.  Personally, I think that the try format:

        return float('some text')
    except ValueError:
        return 42

is more readable than

    try_convert('some text', float, 42, (ValueError,))

because it is clear what it does. The second form is shorter, but not as descriptive. However,

    float('some text', default=42)

follows the existing syntax quite nicely, and is more readable than either of the other options.

A generalised try_convert method would be useful, but I think I would rather see a one-line version of the try statements, perhaps something like this:

    x = try float('some text') else 42 if ValueError