Improve test coverage.

doc/user.rst

@@ -910,10 +910,30 @@ naturally want different precisions:
     ...     frequencies.append((Temperature(temp, 'C'),  Frequency(freq, 'Hz')))
 
     >>> for temp, freq in frequencies:
-    ...     print(temp, freq)
-    -25C 999.988kHz
-    25C 1.00021MHz
-    75C 1.00178MHz
+    ...     print(f'{temp:>4s}  {freq}')
+    -25C  999.988kHz
+     25C  1.00021MHz
+     75C  1.00178MHz
+
+In this example, a subclass is created that is intended to report in 
+concentrations.
+
+.. code-block:: python
+
+    >>> class Concentration(Quantity):
+    ...     pass
+    >>> Concentration.set_prefs(
+    ...     map_sf = dict(u=' PPM', n= ' PPB', p=' PPT'),
+    ...     show_label = True,
+    ... )
+
+    >>> pollutants = dict(CO=5, SO2=20, NO2=0.10)
+    >>> concentrations = [Concentration(v, scale=1e-6, name=k) for k, v in pollutants.items()]
+    >>> for each in concentrations:
+    ...     print(each)
+    CO = 5 PPM
+    SO2 = 20 PPM
+    NO2 = 100 PPB
 
 When a subclass is created, the preferences active in the main class are copied 
 into the subclass. Subsequent changes to the preferences in the main class do 

quantiphy.py

@@ -714,7 +714,7 @@ def _initialize_recognizers(cls):
 
         # identify desired scale factors {{{3
         known_sf = ''.join(MAPPINGS)
-        if cls.get_pref('input_sf') is None:
+        if cls.get_pref('input_sf') is None: # pragma: no cover
             input_sf = known_sf
         else:
             input_sf = cls.get_pref('input_sf')

test_format.py

@@ -77,6 +77,7 @@ def test_scaled_format():
         assert '{:n°F}'.format(q) == 'Tboil'
         assert '{:d°F}'.format(q) == 'boiling point of water'
         assert '{:X°F}'.format(q) == '100 °C'
+        assert '{!r}'.format(q) == "Quantity('100 °C')"
 
 def test_number_fmt():
     Quantity.set_prefs(spacer=None, show_label=None, label_fmt=None, label_fmt_full=None, show_desc=False)

test_misc.py

@@ -277,6 +277,17 @@ class Foo(Quantity):
         assert Quantity('10e-6 m').render() == '10 μm'
         assert Quantity('10e-6 m').render(show_si=False) == '10×10⁻⁶ m'
         Quantity.set_prefs(map_sf=None)
+    sf_map = {
+        'u': ' PPM',
+        'n': ' PPB',
+        'p': ' PPT',
+        'f': ' PPQ',
+    }
+    with Quantity.prefs(map_sf=sf_map):
+        assert Quantity('10e-6').render() == '10 PPM'
+        assert Quantity('1e-7').render() == '100 PPB'
+        assert Quantity('1e-12').render() == '1 PPT'
+        assert Quantity('1e-13').render() == '100 PPQ'
 
     # test set_prefs error handling
     with pytest.raises(KeyError):