Fix compatibility issues with Sympy 1.2. Closes #203.

SymPy 1.2 brought changes to LambdaPrinter that broke kwant.continuum
for inputs containing functions like "sin", "cos", or constant "pi".
By this fix we assert that "kwant.continuum.sympify" represents in
its output all functions as AppliedUndef in favour to built-in SymPy
functions. We also put a hard constaint that all functions and constants
in the input expression (with except to pi) must be provided through params.

kwant/continuum/_common.py

@@ -190,6 +190,15 @@ def sympify(expr, locals=None):
             converter[list] = stored_value
         else:
             del converter[list]
+
+    # We assume that all present functions, like "sin", "cos", will be
+    # provided by user during the final evaluation through "params".
+    # Therefore we make sure they are define as AppliedUndef, not built-in
+    # sympy types.
+    subs = {r: sympy.Symbol(str(r.func))(*r.args)
+            for r in hamiltonian.atoms(sympy.Function)}
+
+    hamiltonian = hamiltonian.subs(subs)
     return hamiltonian
 
 

kwant/continuum/discretizer.py

@@ -457,6 +457,11 @@ def _discretize_expression(expression, coords):
 
 class _NumericPrinter(LambdaPrinter):
 
+    def __init__(self):
+        LambdaPrinter.__init__(self)
+        self.known_functions = {}
+        self.known_constants = {'pi': 'pi', 'Pi': 'pi'}
+
     def _print_ImaginaryUnit(self, expr):
         # prevent sympy from printing 'I' for imaginary unit
         return "1j"