cast units to str before applying any other preprocessor (#498)

* add a preprocessor that casts everything to `str`

* check that integer units are properly converted to `str`

* remove the xfail, this has been fixed 2-3 years ago

* Update cf_xarray/units.py

---------

Co-authored-by: Deepak Cherian <dcherian@users.noreply.github.com>

Author: keewis

cf_xarray/tests/test_units.py

@@ -51,7 +51,13 @@ def test_percent_units():
     assert str(ureg("%").units) == "percent"
 
 
-@pytest.mark.xfail(reason="not supported by pint, yet: hgrecco/pint#1295")
+def test_integer_units():
+    """Test that integer 1 units is equal to dimensionless"""
+    # need to explicitly use parse_units to bypass the runtime type checking
+    # in the quantity constructor
+    assert str(ureg.parse_units(1)) == "dimensionless"
+
+
 def test_udunits_power_syntax():
     """Test that UDUNITS style powers are properly parsed and interpreted."""
     assert ureg("m2 s-2").units == ureg.m**2 / ureg.s**2

cf_xarray/units.py

@@ -77,7 +77,13 @@ def repl(m):
     ],
     force_ndarray_like=True,
 )
+# ----- end block copied from metpy
+
+# need to insert to make sure this is the first preprocessor
+# This ensures we convert integer `1` to string `"1"`, as needed by pint.
+units.preprocessors.insert(0, str)
 
+# -----
 units.define("percent = 0.01 = %")
 
 # Define commonly encountered units (both CF and non-CF) not defined by pint