From 51de8ceb35663c53bae36d9047f945f48838281b Mon Sep 17 00:00:00 2001
From: Jon Thielen <github@jont.cc>
Date: Thu, 9 Jan 2020 10:13:00 -0600
Subject: [PATCH] Add percent sign and UDUNITS exponent syntax support via
 Pint's new preprocessor option

---
 src/metpy/units.py        | 13 ++++++++++++-
 tests/units/test_units.py | 15 +++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/src/metpy/units.py b/src/metpy/units.py
index bbbb3895490..1059ef42a62 100644
--- a/src/metpy/units.py
+++ b/src/metpy/units.py
@@ -16,6 +16,7 @@
 import functools
 from inspect import Parameter, signature
 import logging
+import re
 import warnings
 
 import numpy as np
@@ -27,7 +28,17 @@
 UndefinedUnitError = pint.UndefinedUnitError
 DimensionalityError = pint.DimensionalityError
 
-units = pint.UnitRegistry(autoconvert_offset_to_baseunit=True)
+# Create registry, with preprocessors for UDUNITS-style powers (m2 s-2) and percent signs
+try:
+    units = pint.UnitRegistry(autoconvert_offset_to_baseunit=True,
+                              preprocessors=[functools.partial(
+                                             re.sub,
+                                             (r'(?<=[A-Za-z])(?![A-Za-z])(?<![0-9\-][eE])'
+                                              r'(?<![0-9\-])(?=[0-9\-])'),
+                                             '**'),
+                                             lambda string: string.replace('%', 'percent')])
+except TypeError:
+    units = pint.UnitRegistry(autoconvert_offset_to_baseunit=True)
 
 # Capture v0.10 NEP 18 warning on first creation
 with warnings.catch_warnings():
diff --git a/tests/units/test_units.py b/tests/units/test_units.py
index 6efe150a510..90ea0d0f5f1 100644
--- a/tests/units/test_units.py
+++ b/tests/units/test_units.py
@@ -3,6 +3,7 @@
 # SPDX-License-Identifier: BSD-3-Clause
 r"""Tests the operation of MetPy's unit support code."""
 
+from distutils.version import LooseVersion
 import sys
 
 import matplotlib.pyplot as plt
@@ -217,3 +218,17 @@ def test_assert_nan_checks_units():
     """Test that assert_nan properly checks units."""
     with pytest.raises(AssertionError):
         assert_nan(np.nan * units.m, units.second)
+
+
+@pytest.mark.skipif(LooseVersion(pint.__version__) < LooseVersion('0.10'),
+                    reason='Custom preprocessors only available in Pint 0.10')
+def test_percent_units():
+    """Test that percent sign units are properly parsed and interpreted."""
+    assert str(units('%').units) == 'percent'
+
+
+@pytest.mark.skipif(LooseVersion(pint.__version__) < LooseVersion('0.10'),
+                    reason='Custom preprocessors only available in Pint 0.10')
+def test_udunits_power_syntax():
+    """Test that UDUNITS style powers are properly parsed and interpreted."""
+    assert units('m2 s-2').units == units.m ** 2 / units.s ** 2