ENH: Support Level2 files without a volume header (Fixes Unidata#1470)

This, for instance, makes it possible to use Level2File with the
realtime chunks available on AWS.

src/metpy/io/_tools.py

@@ -174,14 +174,18 @@ class IOBuffer:
     def __init__(self, source):
         """Initialize the IOBuffer with the source data."""
         self._data = bytearray(source)
-        self._offset = 0
-        self.clear_marks()
+        self.reset()
 
     @classmethod
     def fromfile(cls, fobj):
         """Initialize the IOBuffer with the contents of the file object."""
         return cls(fobj.read())
 
+    def reset(self):
+        """Reset buffer back to initial state."""
+        self._offset = 0
+        self.clear_marks()
+
     def set_mark(self):
         """Mark the current location and return its id so that the buffer can return later."""
         self._bookmarks.append(self._offset)

src/metpy/io/nexrad.py

@@ -160,7 +160,7 @@ class Level2File:
     MISSING = float('nan')
     RANGE_FOLD = float('nan')  # TODO: Need to separate from missing
 
-    def __init__(self, filename):
+    def __init__(self, filename, *, has_volume_header=True):
         r"""Create instance of `Level2File`.
 
         Parameters
@@ -177,10 +177,19 @@ def __init__(self, filename):
         with contextlib.closing(fobj):
             self._buffer = IOBuffer.fromfile(fobj)
 
-        self._read_volume_header()
-        start = self._buffer.set_mark()
+        # Try to read the volume header. If this fails, or we're told we don't have one
+        # then we fall back and try to just read messages, assuming we have e.g. one of
+        # the real-time chunks.
+        try:
+            if has_volume_header:
+                self._read_volume_header()
+        except ValueError:
+            log.error('testing')
+            log.warning('Unable to read volume header. Attempting to read messages.')
+            self._buffer.reset()
 
         # See if we need to apply bz2 decompression
+        start = self._buffer.set_mark()
         try:
             self._buffer = IOBuffer(self._buffer.read_func(bzip_blocks_decompress_all))
         except ValueError:

src/metpy/static-data-manifest.txt

@@ -8,6 +8,7 @@ KTLX20130520_201643_V06.gz 772e01b154a5c966982a6d0aa2fc78bc64f08a9b77165b74dc02d
 KTLX20150530_000802_V06.bz2 d78689afc525c853dec8ccab4a4eccc2daacef5c7df198a35a3a791016e993b0
 Level2_FOP1_20191223_003655.ar2v 4a086f6190c0d324612922e9fcb46262c3a1d825810caa8a11aca3823db17ae3
 Level2_KFTG_20150430_1419.ar2v 77c3355c8a503561eb3cddc3854337e640d983a4acdfc27bdfbab60c0b18cfc1
+Level2_KLBB_single_chunk 0fc5598a83ff7ab1f5751d43a70160eb3d155d1c8d44d8a7c990dd2815c5f9aa
 Level3_Composite_dhr_1km_20180309_2225.gini 19fcc0179c9d3e87c462262ea817e87f52f60db4830314b8f936baa3b9817a44
 NAM_test.nc 12338ad06d5bd223e99e2872b20a9c80d58af0c546731e4b00a6619adc247cd0
 NHEM-MULTICOMP_1km_IR_20151208_2100.gini c144b29284aa915e6fd1b8f01939c656f2c72c3d7a9e0af5397f93067fe0d952

tests/io/test_nexrad.py

@@ -69,6 +69,28 @@ def test_conditional_radconst(fname, has_v2):
     assert hasattr(f.sweeps[0][0][3], 'calib_dbz0_v') == has_v2
 
 
+def test_msg15():
+    """Check proper decoding of message type 15."""
+    f = Level2File(get_test_data('KTLX20130520_201643_V06.gz', as_file_obj=False))
+    data = f.clutter_filter_map['data']
+    assert isinstance(data[0][0], list)
+    assert f.clutter_filter_map['datetime'] == datetime(2013, 5, 19, 0, 0, 0, 315000)
+
+
+def test_single_chunk(caplog):
+    """Check that Level2File copes with reading a file containing a single chunk."""
+    # Need to override the test level set above
+    caplog.set_level(logging.WARNING, 'metpy.io.nexrad')
+    f = Level2File(get_test_data('Level2_KLBB_single_chunk'))
+    assert len(f.sweeps) == 1
+    assert 'Unable to read volume header' in caplog.text
+
+    # Make sure the warning is not present if we pass the right kwarg.
+    caplog.clear()
+    Level2File(get_test_data('Level2_KLBB_single_chunk'), has_volume_header=False)
+    assert 'Unable to read volume header' not in caplog.text
+
+
 #
 # NIDS/Level 3 Tests
 #
@@ -188,14 +210,6 @@ def test31_clear_air():
     assert not is_precip_mode(31), 'VCP 31 is not precip'
 
 
-def test_msg15():
-    """Check proper decoding of message type 15."""
-    f = Level2File(get_test_data('KTLX20130520_201643_V06.gz', as_file_obj=False))
-    data = f.clutter_filter_map['data']
-    assert isinstance(data[0][0], list)
-    assert f.clutter_filter_map['datetime'] == datetime(2013, 5, 19, 0, 0, 0, 315000)
-
-
 def test_tracks():
     """Check that tracks are properly decoded."""
     f = Level3File(get_test_data('nids/KOUN_SDUS34_NSTTLX_201305202016'))