Avoid precision loss when converting between units. Fixes #330

1 files changed, 10 insertions, 9 deletions

diff --git a/scss/cssdefs.py b/scss/cssdefs.py
index 5d9b7eb..79be7b4 100644
--- a/scss/cssdefs.py
+++ b/scss/cssdefs.py
@@ -1,6 +1,7 @@
 """Constants and functions defined by the CSS specification, not specific to
 Sass.
 """
+from fractions import Fraction
 from math import pi
 import re
 
@@ -166,15 +167,15 @@ BASE_UNIT_CONVERSIONS = {
     # Lengths
     'mm': (1, 'mm'),
     'cm': (10, 'mm'),
-    'in': (25.4, 'mm'),
-    'px': (25.4 / 96, 'mm'),
-    'pt': (25.4 / 72, 'mm'),
-    'pc': (25.4 / 6, 'mm'),
+    'in': (Fraction(254, 10), 'mm'),
+    'px': (Fraction(254, 960), 'mm'),
+    'pt': (Fraction(254, 720), 'mm'),
+    'pc': (Fraction(254, 60), 'mm'),
 
     # Angles
-    'deg': (1 / 360, 'turn'),
-    'grad': (1 / 400, 'turn'),
-    'rad': (pi / 2, 'turn'),
+    'deg': (Fraction(1, 360), 'turn'),
+    'grad': (Fraction(1, 400), 'turn'),
+    'rad': (Fraction.from_float(pi / 2), 'turn'),
     'turn': (1, 'turn'),
 
     # Times
@@ -187,7 +188,7 @@ BASE_UNIT_CONVERSIONS = {
 
     # Resolutions
     'dpi': (1, 'dpi'),
-    'dpcm': (2.54, 'dpi'),
+    'dpcm': (Fraction(254 / 100), 'dpi'),
     'dppx': (96, 'dpi'),
 }
 
@@ -252,7 +253,7 @@ def cancel_base_units(units, to_remove):
     # Copy the dict since we're about to mutate it
     to_remove = to_remove.copy()
     remaining_units = []
-    total_factor = 1
+    total_factor = Fraction(1)
 
     for unit in units:
         factor, base_unit = get_conversion_factor(unit)