New method Expression.subs

[linpy.git] / pypol / linexprs.py

diff --git a/pypol/linexprs.py b/pypol/linexprs.py
index 9449219..b330045 100644 (file)
--- a/pypol/linexprs.py
+++ b/pypol/linexprs.py
@@ -9,7 +9,7 @@ from fractions import Fraction, gcd
 
 __all__ = [
     'Expression',
-    'Symbol', 'symbols',
+    'Symbol', 'symbols', 'symbolname', 'symbolnames',
     'Constant',
 ]
 
@@ -59,10 +59,7 @@ class Expression:
         self = object().__new__(cls)
         self._coefficients = {}
         for symbol, coefficient in coefficients:
-            if isinstance(symbol, Symbol):
-                symbol = symbol.name
-            elif not isinstance(symbol, str):
-                raise TypeError('symbols must be strings or Symbol instances')
+            symbol = symbolname(symbol)
             if isinstance(coefficient, Constant):
                 coefficient = coefficient.constant
             if not isinstance(coefficient, numbers.Rational):
@@ -82,10 +79,7 @@ class Expression:
         return self
 
     def coefficient(self, symbol):
-        if isinstance(symbol, Symbol):
-            symbol = symbol.name
-        elif not isinstance(symbol, str):
-            raise TypeError('symbol must be a string or a Symbol instance')
+        symbol = symbolname(symbol)
         try:
             return self._coefficients[symbol]
         except KeyError:
@@ -255,6 +249,29 @@ class Expression:
                 return left / right
         raise SyntaxError('invalid syntax')
 
+    def subs(self, symbol, expression=None):
+        if expression is None:
+            if isinstance(symbol, dict):
+                symbol = symbol.items()
+            substitutions = symbol
+        else:
+            substitutions = [(symbol, expression)]
+        result = self
+        for symbol, expression in substitutions:
+            symbol = symbolname(symbol)
+            result = result._subs(symbol, expression)
+        return result
+
+    def _subs(self, symbol, expression):
+        coefficients = {name: coefficient
+            for name, coefficient in self.coefficients()
+            if name != symbol}
+        constant = self.constant
+        coefficient = self.coefficient(symbol)
+        result = Expression(coefficients, self.constant)
+        result += coefficient * expression
+        return result
+
     _RE_NUM_VAR = re.compile(r'(\d+|\))\s*([^\W\d_]\w*|\()')
 
     @classmethod
@@ -345,11 +362,7 @@ class Symbol(Expression):
     )
 
     def __new__(cls, name):
-        if isinstance(name, Symbol):
-            name = name.name
-        elif not isinstance(name, str):
-            raise TypeError('name must be a string or a Symbol instance')
-        name = name.strip()
+        name = symbolname(name)
         self = object().__new__(cls)
         self._name = name
         self._hash = hash(self._name)
@@ -363,10 +376,7 @@ class Symbol(Expression):
         return self._hash
 
     def coefficient(self, symbol):
-        if isinstance(symbol, Symbol):
-            symbol = symbol.name
-        elif not isinstance(symbol, str):
-            raise TypeError('symbol must be a string or a Symbol instance')
+        symbol = symbolname(symbol)
         if symbol == self.name:
             return 1
         else:
@@ -420,6 +430,19 @@ def symbols(names):
         names = names.replace(',', ' ').split()
     return (Symbol(name) for name in names)
 
+def symbolname(symbol):
+    if isinstance(symbol, str):
+        return symbol.strip()
+    elif isinstance(symbol, Symbol):
+        return symbol.name
+    else:
+        raise TypeError('symbol must be a string or a Symbol instance')
+
+def symbolnames(symbols):
+    if isinstance(symbols, str):
+        return symbols.replace(',', ' ').split()
+    return (symbolname(symbol) for symbol in symbols)
+
 
 class Constant(Expression):
 
@@ -441,10 +464,7 @@ class Constant(Expression):
         return self._hash
 
     def coefficient(self, symbol):
-        if isinstance(symbol, Symbol):
-            symbol = symbol.name
-        elif not isinstance(symbol, str):
-            raise TypeError('symbol must be a string or a Symbol instance')
+        symbol = symbolname(symbol)
         return 0
 
     def coefficients(self):