Current implementation of decorators loses function signature

[sphuber]

The current implementation of decorators in aiida-core cause the functions that are wrapped to lose their function signature. This will cause inspect to fail miserably. The same problem occurs with it losing its __name__ and __doc__, which is sometimes fixed by using functools.wraps, however, that does not properly restore the function signature. Example:

import inspect
from aiida.cmdline.utils import decorators

def unwrapped(a, b):
    pass

inspect.getargspec(wrapped)
ArgSpec(args=['a', 'b'], varargs=None, keywords=None, defaults=None)

Now when we wrap it, for example with with_dbenv decorator, we get:

@decorators.with_dbenv()
def wrapped(a, b):
    pass

inspect.getargspec(wrapped)
ArgSpec(args=[], varargs='args', keywords='kwargs', defaults=None)

Now this is currently not causing any problems, that we know of, but it might in the future. In addition, I use the inspect functionality in launch scripts that I have to wrap with with_dbenv and those now stopped working. There are various solutions, of which the wrapt library is the best as in most correct, but also slowest. Let's discuss if we would be willing to put this in our dependencies and fix our decorators.