CHANGELOG.md

@@ -10,6 +10,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Added citation instructions in CITING.md
 - Added a tutorial with downloadable Python files and Jupyter notebooks
 
+### Fixed
+- Bug when multiplying a model by a sympy expression (undefined variable)
+- Bug when multiplying BlochCoeff with sympy expression (sympy multiplication was
+  being used, when BlochCoeff multiplication should have been used)
+
 ### Removed
 - Removed the notebooks from the source repository
 

qsymm/hamiltonian_generator.py

@@ -288,7 +288,9 @@ def hamiltonian_from_family(family, coeffs=None, nsimplify=True, tosympy=True):
         coeffs = list(sympy.symbols('c0:%d'%len(family), real=True))
     else:
         assert len(coeffs) == len(family), 'Length of family and coeffs do not match.'
-    ham = sum(c * term for c, term in zip(coeffs, family))
+    # The order of multiplication is important here, so that __mul__ of 'term'
+    # gets used. 'c' is a sympy symbol, which multiplies 'term' incorrectly.
+    ham = sum(term * c for c, term in zip(coeffs, family))
     if tosympy:
         return ham.tosympy(nsimplify=nsimplify)
     else:

qsymm/model.py

@@ -360,6 +360,7 @@ class Model(UserDict):
             result = self.zeros_like()
             result.data = {key: val * other for key, val in self.items()}
         elif isinstance(other, Basic):
+            keep = self.keep
             result = sum((type(self)({key * other: copy(val)},
                                      keep=keep,
                                      momenta=self.momenta)
@@ -396,11 +397,15 @@ class Model(UserDict):
         if isinstance(other, Number):
             result = self.__mul__(other)
         elif isinstance(other, Basic):
-            result = sum((type(self)({other * key: copy(val)},
-                                     keep=self.keep,
+            keep = self.keep
+            # The order 'key * other' is important: we want to force
+            # the implementation of __mul__ of 'key' to be used. This
+            # is correct as long as the symbols in 'key' and 'other' commute.
+            result = sum((type(self)({key * other: copy(val)},
+                                     keep=keep,
                                      momenta=self.momenta)
                          for key, val in self.items()
-                         if (key * other in self.keep or not self.keep)),
+                         if (key * other in keep or not keep)),
                          self.zeros_like())
         else:
             # Otherwise try to multiply every value with other