Use OrderedDict to store Expression coefficients

diff --git a/pypol/linexprs.py b/pypol/linexprs.py
index 9ab5c86..d8b020d 100644 (file)
--- a/pypol/linexprs.py
+++ b/pypol/linexprs.py
@@ -3,6 +3,7 @@ import functools
 import numbers
 import re
 
+from collections import OrderedDict
 from fractions import Fraction, gcd
 
 
@@ -67,13 +68,14 @@ class Expression:
                 raise TypeError('coefficients must be rational numbers '
                     'or Constant instances')
             self._coefficients[symbol] = coefficient
+        self._coefficients = OrderedDict(sorted(self._coefficients.items()))
         if isinstance(constant, Constant):
             constant = constant.constant
         if not isinstance(constant, numbers.Rational):
             raise TypeError('constant must be a rational number '
                 'or a Constant instance')
         self._constant = constant
-        self._symbols = tuple(sorted(self._coefficients))
+        self._symbols = tuple(self._coefficients)
         self._dimension = len(self._symbols)
         return self
 
@@ -90,8 +92,7 @@ class Expression:
     __getitem__ = coefficient
 
     def coefficients(self):
-        for symbol in self.symbols:
-            yield symbol, self.coefficient(symbol)
+        yield from self._coefficients.items()
 
     @property
     def constant(self):
@@ -220,7 +221,7 @@ class Expression:
         return Gt(self, other)
 
     def __hash__(self):
-        return hash((tuple(sorted(self._coefficients.items())), self._constant))
+        return hash((tuple(self.coefficients()), self._constant))
 
     def _toint(self):
         lcm = functools.reduce(lambda a, b: a*b // gcd(a, b),