ENH: Support packing NamedStructs

This paves the way towards writing instances of NamedStructs.

src/metpy/io/_tools.py

@@ -97,6 +97,11 @@ def unpack_file(self, fobj):
         """Unpack the next bytes from a file object."""
         return self.unpack(fobj.read(self.size))
 
+    def pack(self, **kwargs):
+        """Pack the arguments into bytes using the structure."""
+        t = self.make_tuple(**kwargs)
+        return super().pack(*t)
+
 
 # This works around times when we have more than 255 items and can't use
 # NamedStruct. This is a CPython limit for arguments.

tests/io/test_tools.py

@@ -0,0 +1,23 @@
+# Copyright (c) 2020 MetPy Developers.
+# Distributed under the terms of the BSD 3-Clause License.
+# SPDX-License-Identifier: BSD-3-Clause
+"""Test the `_tools` module."""
+
+from metpy.io._tools import NamedStruct
+
+
+def test_unpack():
+    """Test unpacking a NamedStruct from bytes."""
+    struct = NamedStruct([('field1', 'i'), ('field2', 'h')], '>')
+
+    s = struct.unpack(b'\x00\x01\x00\x01\x00\x02')
+    assert s.field1 == 65537
+    assert s.field2 == 2
+
+
+def test_pack():
+    """Test packing a NamedStruct into bytes."""
+    struct = NamedStruct([('field1', 'i'), ('field2', 'h')], '>')
+
+    b = struct.pack(field1=8, field2=3)
+    assert b == b'\x00\x00\x00\x08\x00\x03'