Python netCDF: I accidentally wrote a unwanted variable in a netcdf file. Now a simple ds. var doesn't work anymore. What should I do?

Question

I open the netCDF file by xarray. ds ＝ xr.open_dataset(filename) But there's an extra variable 「u」exsited already which I think I accidentally wrote in the file.I can't remember how I did it. When I run newname ＝ ds. var in the file, it doesn't work. But when I run the same code in the window, newname does occur in the variable window. What should I do to make it work in the file so that I can process thousands of files at one time？