Fix Symbol == LinExpr comparisons

[linpy.git] / linpy / linexprs.py

diff --git a/linpy/linexprs.py b/linpy/linexprs.py
index fce77a6..e4ed1cc 100644 (file)
--- a/linpy/linexprs.py
+++ b/linpy/linexprs.py
@@ -122,7 +122,7 @@ class LinExpr:
         """
         if not isinstance(symbol, Symbol):
             raise TypeError('symbol must be a Symbol instance')
-        return Rational(self._coefficients.get(symbol, 0))
+        return self._coefficients.get(symbol, Fraction(0))
 
     __getitem__ = coefficient
 
@@ -131,15 +131,14 @@ class LinExpr:
         Iterate over the pairs (symbol, value) of linear terms in the
         expression. The constant term is ignored.
         """
-        for symbol, coefficient in self._coefficients.items():
-            yield symbol, Rational(coefficient)
+        yield from self._coefficients.items()
 
     @property
     def constant(self):
         """
         The constant term of the expression.
         """
-        return Rational(self._constant)
+        return self._constant
 
     @property
     def symbols(self):
@@ -179,9 +178,8 @@ class LinExpr:
         Iterate over the coefficient values in the expression, and the constant
         term.
         """
-        for coefficient in self._coefficients.values():
-            yield Rational(coefficient)
-        yield Rational(self._constant)
+        yield from self._coefficients.values()
+        yield self._constant
 
     def __bool__(self):
         return True
@@ -249,9 +247,10 @@ class LinExpr:
         """
         Test whether two linear expressions are equal.
         """
-        return isinstance(other, LinExpr) and \
-            self._coefficients == other._coefficients and \
-            self._constant == other._constant
+        if isinstance(other, LinExpr):
+            return self._coefficients == other._coefficients and \
+                self._constant == other._constant
+        return NotImplemented
 
     def __le__(self, other):
         from .polyhedra import Le
@@ -411,7 +410,7 @@ class LinExpr:
     @classmethod
     def fromsympy(cls, expr):
         """
-        Create a linear expression from a sympy expression. Raise ValueError is
+        Create a linear expression from a sympy expression. Raise TypeError is
         the sympy expression is not linear.
         """
         import sympy
@@ -421,12 +420,18 @@ class LinExpr:
             coefficient = Fraction(coefficient.p, coefficient.q)
             if symbol == sympy.S.One:
                 constant = coefficient
+            elif isinstance(symbol, sympy.Dummy):
+                # we cannot properly convert dummy symbols
+                raise TypeError('cannot convert dummy symbols')
             elif isinstance(symbol, sympy.Symbol):
                 symbol = Symbol(symbol.name)
                 coefficients.append((symbol, coefficient))
             else:
-                raise ValueError('non-linear expression: {!r}'.format(expr))
-        return LinExpr(coefficients, constant)
+                raise TypeError('non-linear expression: {!r}'.format(expr))
+        expr = LinExpr(coefficients, constant)
+        if not isinstance(expr, cls):
+            raise TypeError('cannot convert to a {} instance'.format(cls.__name__))
+        return expr
 
     def tosympy(self):
         """
@@ -493,7 +498,9 @@ class Symbol(LinExpr):
         return True
 
     def __eq__(self, other):
-        return self.sortkey() == other.sortkey()
+        if isinstance(other, Symbol):
+            return self.sortkey() == other.sortkey()
+        return NotImplemented
 
     def asdummy(self):
         """
@@ -507,16 +514,6 @@ class Symbol(LinExpr):
     def _repr_latex_(self):
         return '$${}$$'.format(self.name)
 
-    @classmethod
-    def fromsympy(cls, expr):
-        import sympy
-        if isinstance(expr, sympy.Dummy):
-            return Dummy(expr.name)
-        elif isinstance(expr, sympy.Symbol):
-            return Symbol(expr.name)
-        else:
-            raise TypeError('expr must be a sympy.Symbol instance')
-
 
 def symbols(names):
     """
@@ -632,13 +629,3 @@ class Rational(LinExpr, Fraction):
         else:
             return '$$\\frac{{{}}}{{{}}}$$'.format(self.numerator,
                 self.denominator)
-
-    @classmethod
-    def fromsympy(cls, expr):
-        import sympy
-        if isinstance(expr, sympy.Rational):
-            return Rational(expr.p, expr.q)
-        elif isinstance(expr, numbers.Rational):
-            return Rational(expr)
-        else:
-            raise TypeError('expr must be a sympy.Rational instance')