Unit checking for unfilled default argument

[zplizzi]

I'd like to use the unit checking functionality in the following situation:

@ureg.check('[length]')
def test(a = 10 * ureg.meters):
    print(a)

test()

However, this fails with the error 'NoneType' object has no attribute 'dimensionality'.

Is there a way to handle such a situation elegantly? Ie, check the parameter if it is supplied, otherwise use the default argument.

[hgrecco]

I agree. It would be nice to have support for optional values.

[onealj]

03332fa doesn't check default arguments of the wrapped function. It's a good idea to check this, but the most likely time this error would come up would be when the decorator type does not agree with the default argument in the function defined on the following line. For simple code, this error would be quick to spot. For complex code, this check would be critical.

@ureg.check('[time]')
def dfunc(a=10 * ureg.meters):
    return a

dfunc(10*ureg.seconds) should pass
dfunc(10*ureg.meters) should fail
dfunc() should fail

See onealj@c8cac87

I don't know if it will be possible to process the default parameters.
The decorator will need to inspect the wrapped function for the default arguments and figure out which are missing from the supplied invocation of the decorator-wrapped function. The order of default arguments is undefined, so unless ureg.check had matching positional and keyword arguments to check the dimension, it wouldn't be possible to implement this.

Consider def dfunc(a, b, c, d=1*ureg.kilograms, e=2*ureg.meters, f=3*ureg.seconds, *arg, **kwargs).
In CPython 2.7.6, dfunc.__defaults__ = (1*ureg.kilograms, 2*ureg.meters, 3*ureg.seconds)
In CPython 3.4.3, dfunc.__kwdefaults__ = {'d': 1*ureg.kilograms, 'f': 3*ureg.seconds, 'e': 2*ureg.meters}

[hgrecco]

Won't this work: https://docs.python.org/3.6/library/inspect.html#inspect.Signature.bind ?

If not, at least we should raise an error on decoration like 'Default values are not supported'

[bilderbuchi]

Maybe some clues could be found in the way functools.partial or functools.wraps work/are implemented? (Just a hunch)