MetPy Level2File error - local variable 'offset' referenced before assignment

Question

Unidata/MetPy has an example on Plotting AWS-hosted NEXRAD Level 2 Data. It starts off with the following:

import boto3
import botocore
from botocore.client import Config
import matplotlib.pyplot as plt
from metpy.io import Level2File
from metpy.plots import add_timestamp, ctables
from mpl_toolkits.axes_grid1 import make_axes_locatable
import numpy as np

s3 = boto3.resource('s3', config=Config(signature_version=botocore.UNSIGNED,
                                        user_agent_extra='Resource'))
bucket = s3.Bucket('noaa-nexrad-level2')
for obj in bucket.objects.filter(Prefix='2019/06/26/KVWX/KVWX20190626_221105_V06'):
    print(obj.key)

    # Use MetPy to read the file
    f = Level2File(obj.get()['Body'])

This works just fine for the station and time they chose. If I change the date and time and rerun the last few lines:

for obj in bucket.objects.filter(Prefix='2005/06/26/KVWX/KVWX20050626_221551'):
    print(obj.key)

    # Use MetPy to read the file
    f = Level2File(obj.get()['Body'])

it prints off a valid key:

2005/06/26/KVWX/KVWX20050626_221551.gz

but Level2File gives the following error message:

Message 15 left data -- Used: 0 Avail: 16271

---------------------------------------------------------------------------
UnboundLocalError                         Traceback (most recent call last)
<ipython-input-4-415bcd3f289e> in <module>
      3 
      4     # Use MetPy to read the file
----> 5     f = Level2File(obj.get()['Body'])

/opt/conda/lib/python3.8/site-packages/metpy/io/nexrad.py in __init__(self, filename, has_volume_header)
    196 
    197         # Now we're all initialized, we can proceed with reading in data
--> 198         self._read_data()
    199 
    200     vol_hdr_fmt = NamedStruct([('version', '9s'), ('vol_num', '3s'),

/opt/conda/lib/python3.8/site-packages/metpy/io/nexrad.py in _read_data(self)
    258                 decoder = f'_decode_msg{msg_hdr.msg_type:d}'
    259                 if hasattr(self, decoder):
--> 260                     getattr(self, decoder)(msg_hdr)
    261                 else:
    262                     log.warning('Unknown message: %d', msg_hdr.msg_type)

/opt/conda/lib/python3.8/site-packages/metpy/io/nexrad.py in _decode_msg13(self, msg_hdr)
    475                 self.clutter_filter_bypass_map['data'].append(az_data)
    476 
--> 477             if offset != len(data):
    478                 log.warning('Message 13 left data -- Used: %d Avail: %d', offset, len(data))
    479 

UnboundLocalError: local variable 'offset' referenced before assignment

I suspect this is being caused by differences in formatting for different date ranges. Is there any way to use Level2File to read the second scan?

Thanks in advance for any assistance!

Answer 1

The error is due to a combination of a minor bug in MetPy and some oddities in this particular file (or maybe files from this time range) that I had not seen before. Reading this data (and hopefully all its brothers and sisters) is fixed by this pull request. The fix will be included in MetPy 1.0.1, which is releasing shortly.