Symplify class verification in LinExpr.fromsympy()

[linpy.git] / linpy / linexprs.py

diff --git a/linpy/linexprs.py b/linpy/linexprs.py
index fce77a6..38fa7a1 100644 (file)
--- a/linpy/linexprs.py
+++ b/linpy/linexprs.py
@@ -411,7 +411,7 @@ 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 +421,18 @@ 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):
         """
@@ -507,16 +513,6 @@ 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 +628,3 @@ 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')