Usage of parse_grid_arguments

[winash12]

I am trying to get a minimum working example of parse_grid_arguments and I have encountered an error using it

Here is my toy example -

import numpy as np

from metpy.calc.tools import parse_grid_arguments


@parse_grid_arguments
def test_decorator(a,b):
    c = uwnd+a+b
    print(c)

def main():
    uwnd = 5
    a = 4
    b = 6
    test_decorator(a,b,uwnd)
main()

and I get this error -

File "/home/winash12/testpython/testmetpydec.py", line 16, in <module>
    main()
  File "/home/winash12/testpython/testmetpydec.py", line 15, in main
    test_decorator(a,b,uwnd)
  File "/usr/local/lib/python3.9/dist-packages/metpy/calc/tools.py", line 1032, in wrapper
    bound_args = newsig.bind(*args, **kwargs)
  File "/usr/lib/python3.9/inspect.py", line 3062, in bind
    return self._bind(args, kwargs)
  File "/usr/lib/python3.9/inspect.py", line 2988, in _bind
    raise TypeError(
TypeError: too many positional arguments

Where am I going wrong ?

[dopplershift]

What are you trying to accomplish? The purpose of parse_grid_arguments is to handle arguments needed for calculations on grids:

• dx, dy, dz
• latitude, longitude, crs
• x_dim, y_dim, vertical_dim
• parallel_scale, meridional_scale

So fundamentally what's going wrong is that you created a function that only takes a and b, decorated it, which in this case added latitude, longitude, and crs keyword-only arguments, and are calling it with 3 positional arguments, which doesn't work.

If you're in a notebook, the IPython kernel will show the signature if you just type the function name (test_decorator):

<function __main__.test_decorator(a, b, *, latitude=None, longitude=None, crs=None)>

[winash12]

I was looking for a kwargs alternative i.e. trying to dynamically alter the signature of a function as shown in this link - https://tommasoamici.com/blog/dynamically-change-signature-function-python/ and maybe I mis understood the comment in the code

" Dynamically add new parameters for lat, lon, and crs to the function signature
# which is used to handle arguments inside the wrapper--but only if they're not in the
# original signature"

I guess it cannot add anything else other than lat, lon and crs.

[dopplershift]

That's correct, those are the only parameters MetPy's decorator adds. You're welcome to make your own version of the decorator for other parameter modifications based on what we do:

MetPy/src/metpy/calc/tools.py

Lines 1021 to 1027 in edf2cbd

	sig = signature(func)
	orig_func_uses = {param: param in sig.parameters
	for param in ('latitude', 'longitude', 'crs')}
	newsig = sig.replace(parameters=[*sig.parameters.values(),
	*(Parameter(name, Parameter.KEYWORD_ONLY, default=None)
	for name, needed in orig_func_uses.items()
	if not needed)])