fix(converter): Fix Py2.7 error when can't find unit-type

ph_units/converter.py

@@ -4,7 +4,7 @@
 """Functions for converting Values from one unit to another (ie: meter->foot)."""
 
 try:
-    from typing import Union, Optional, Tuple, Dict
+    from typing import Union, Optional, Tuple, Dict, List
 except ImportError:
     pass  # IronPython 2.7
 
@@ -17,8 +17,29 @@
 
 class UnitTypeNameNotFound(Exception):
     def __init__(self, message):
+        # type: (str) -> None
         self.message = message
-        super().__init__(self.message)
+        super(UnitTypeNameNotFound, self).__init__(self.message)
+
+
+def _find_valid_unit_names_matching_first_letter(_unit_type_alias_dict, _input_string):
+    # type: (Dict[str, str], str) -> List[str]
+    """Find all valid unit-type names that start with a given letter."""
+    matches = []
+
+    try:
+        first_letter = str(_input_string).strip()[0].upper()
+    except IndexError:
+        return matches
+
+    for k in sorted(_unit_type_alias_dict.keys()):
+        try:
+            if k[0] == first_letter:
+                matches.append(k)
+        except IndexError:
+            pass
+
+    return matches
 
 def _standardize_unit_name(_input, _unit_type_alias_dict):
     # type: (str, Dict[str, str]) -> str
@@ -40,9 +61,12 @@ def _standardize_unit_name(_input, _unit_type_alias_dict):
         if _input_string == "%":
             input_unit = "%"
         else:
+            suggested_matches = _find_valid_unit_names_matching_first_letter(_unit_type_alias_dict, _input_string)
             raise UnitTypeNameNotFound(
-                "\n\tI do not know how to understand the input unit: '{}'?\nValid formats include: {}".format(
-                    _input_string, sorted(_unit_type_alias_dict.keys())
+                "\nI do not understand the unit: '{}'? "\
+                "\nPerhaps you meant on of these: '{}'?"
+                "\n\nValid formats include only: {}".format(
+                    _input_string, suggested_matches, sorted(_unit_type_alias_dict.keys())
                 )
             )