Handling nodata values in raster in Python

Question

I would like to set value of raster -999.0 as NaN in crop_image function, while reading/cropping file:

for band_path in range_bands:
    # open raster as numpy array by rater.io in content manager(open/close file)
    with rio.open(band_path) as band_src:
        if "band4" in band_path:
            print("band4")
            ## use crop.image() to read and crop
            band4, band4_meta = es.crop_image(band_src, bound)
            print(band4_meta)
            # Update with NaN data values
            band4_meta.update({'nodata': -999.0})
            print(band4_meta)

How is possible to do?