Small implementation improvement in linexprs.py

[linpy.git] / pypol / linexprs.py

diff --git a/pypol/linexprs.py b/pypol/linexprs.py
index b74628b..c5f4336 100644 (file)
--- a/pypol/linexprs.py
+++ b/pypol/linexprs.py
@@ -40,26 +40,26 @@ class Expression:
             return Rational(constant)
         if isinstance(coefficients, Mapping):
             coefficients = coefficients.items()
+        coefficients = list(coefficients)
         for symbol, coefficient in coefficients:
             if not isinstance(symbol, Symbol):
                 raise TypeError('symbols must be Symbol instances')
             if not isinstance(coefficient, numbers.Rational):
                 raise TypeError('coefficients must be rational numbers')
-        coefficients = [(symbol, Fraction(coefficient))
-            for symbol, coefficient in coefficients if coefficient != 0]
         if not isinstance(constant, numbers.Rational):
             raise TypeError('constant must be a rational number')
-        constant = Fraction(constant)
         if len(coefficients) == 0:
             return Rational(constant)
         if len(coefficients) == 1 and constant == 0:
             symbol, coefficient = coefficients[0]
             if coefficient == 1:
                 return symbol
+        coefficients = [(symbol, Fraction(coefficient))
+            for symbol, coefficient in coefficients if coefficient != 0]
+        coefficients.sort(key=lambda item: item[0].sortkey())
         self = object().__new__(cls)
-        self._coefficients = OrderedDict(sorted(coefficients,
-            key=lambda item: item[0].sortkey()))
-        self._constant = constant
+        self._coefficients = OrderedDict(coefficients)
+        self._constant = Fraction(constant)
         self._symbols = tuple(self._coefficients)
         self._dimension = len(self._symbols)
         return self
@@ -67,10 +67,7 @@ class Expression:
     def coefficient(self, symbol):
         if not isinstance(symbol, Symbol):
             raise TypeError('symbol must be a Symbol instance')
-        try:
-            return Rational(self._coefficients[symbol])
-        except KeyError:
-            return Rational(0)
+        return Rational(self._coefficients.get(symbol, 0))
 
     __getitem__ = coefficient
 
@@ -131,14 +128,14 @@ class Expression:
         constant = self._constant - other._constant
         return Expression(coefficients, constant)
 
+    @_polymorphic
     def __rsub__(self, other):
-        return -(self - other)
+        return other - self
 
     def __mul__(self, other):
         if isinstance(other, numbers.Rational):
-            coefficients = dict(self._coefficients)
-            for symbol in coefficients:
-                coefficients[symbol] *= other
+            coefficients = ((symbol, coefficient * other)
+                for symbol, coefficient in self._coefficients.items())
             constant = self._constant * other
             return Expression(coefficients, constant)
         return NotImplemented
@@ -147,38 +144,32 @@ class Expression:
 
     def __truediv__(self, other):
         if isinstance(other, numbers.Rational):
-            coefficients = dict(self._coefficients)
-            for symbol in coefficients:
-                coefficients[symbol] /= other
+            coefficients = ((symbol, coefficient / other)
+                for symbol, coefficient in self._coefficients.items())
             constant = self._constant / other
-            # import pdb; pdb.set_trace()
             return Expression(coefficients, constant)
         return NotImplemented
 
     @_polymorphic
     def __eq__(self, other):
-        # "normal" equality
+        # returns a boolean, not a constraint
         # see http://docs.sympy.org/dev/tutorial/gotchas.html#equals-signs
         return isinstance(other, Expression) and \
             self._coefficients == other._coefficients and \
             self._constant == other._constant
 
-    @_polymorphic
     def __le__(self, other):
         from .polyhedra import Le
         return Le(self, other)
 
-    @_polymorphic
     def __lt__(self, other):
         from .polyhedra import Lt
         return Lt(self, other)
 
-    @_polymorphic
     def __ge__(self, other):
         from .polyhedra import Ge
         return Ge(self, other)
 
-    @_polymorphic
     def __gt__(self, other):
         from .polyhedra import Gt
         return Gt(self, other)
@@ -349,8 +340,7 @@ class Symbol(Expression):
         return True
 
     def __eq__(self, other):
-        return not isinstance(other, Dummy) and isinstance(other, Symbol) \
-            and self.name == other.name
+        return self.sortkey() == other.sortkey()
 
     def asdummy(self):
         return Dummy(self.name)
@@ -374,8 +364,10 @@ class Symbol(Expression):
     @classmethod
     def fromsympy(cls, expr):
         import sympy
-        if isinstance(expr, sympy.Symbol):
-            return cls(expr.name)
+        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')
 
@@ -387,6 +379,8 @@ class Dummy(Symbol):
     def __new__(cls, name=None):
         if name is None:
             name = 'Dummy_{}'.format(Dummy._count)
+        elif not isinstance(name, str):
+            raise TypeError('name must be a string')
         self = object().__new__(cls)
         self._index = Dummy._count
         self._name = name.strip()
@@ -403,9 +397,6 @@ class Dummy(Symbol):
     def sortkey(self):
         return self._name, self._index
 
-    def __eq__(self, other):
-        return isinstance(other, Dummy) and self._index == other._index
-
     def __repr__(self):
         return '_{}'.format(self.name)
 
@@ -442,12 +433,6 @@ class Rational(Expression, Fraction):
     def __bool__(self):
         return Fraction.__bool__(self)
 
-    @classmethod
-    def fromstring(cls, string):
-        if not isinstance(string, str):
-            raise TypeError('string must be a string instance')
-        return Rational(string)
-
     def __repr__(self):
         if self.denominator == 1:
             return '{!r}'.format(self.numerator)