Do not access '_name' attribute of object with no name (#328)

When collecting a common subexpression from a long expression in the
AST, a temporary variable is created whose name is obtained from the
`_name` attribute of the first SymPy object in a list. In certain
unfortunate cases such an object does not have the `_name` attribute and
an `AttributeError` is raised.

The problem is solved by cycling through each element in the
aforementioned list, and extracting `_name` from the first object with
such an attribute. This is done via the new `get_name` function. Fixes
#327.

psydac/api/ast/parser.py

@@ -73,7 +73,7 @@
 
 from .nodes import Zeros, ZerosLike, Array
 from .fem import expand, expand_hdiv_hcurl
-from psydac.api.ast.utilities import variables, math_atoms_as_str
+from psydac.api.ast.utilities import variables, math_atoms_as_str, get_name
 from psydac.api.utilities     import flatten
 from psydac.api.ast.utilities import build_pythran_types_header
 from psydac.api.ast.utilities import build_pyccel_types_decorator
@@ -1233,7 +1233,7 @@ def _visit_ComputeKernelExpr(self, expr, op=None, lhs=None, **kwargs):
         lhs = lhs[:]
 
         # Create a new name for the temporaries used in each patch
-        name=lhs[0]._name[12:-8]
+        name = get_name(lhs)
         temps, rhs = cse_main.cse(rhs, symbols=cse_main.numbered_symbols(prefix=f'temp{name}'))
 
         normal_vec_stmts = []

psydac/api/ast/utilities.py

@@ -1040,3 +1040,28 @@ def math_atoms_as_str(expr, lib='math'):
                 sqrt = True
 
     return set.union(math_functions, math_constants)
+
+def get_name(lhs):
+    """
+    Given a list of variable return the meaningful part of the name of the
+    first variable that has a _name attribute.
+
+    Was added to solve issue #327 caused by trying to access the name of a 
+    variable that has not such attribute.
+
+    Parameters
+    ----------
+    lhs : list
+        list from whom we need to extract a name.
+
+    Returns
+    -------
+    str
+        meaningful part of the name of the variable or "zero term" if no 
+        variable has a name.
+
+    """
+    for term in lhs:
+        if hasattr(term, '_name'):
+            return term._name[12:-8]
+    return "zero_term"

psydac/api/tests/test_assembly.py

@@ -10,6 +10,7 @@
 from sympde.core     import Constant
 from sympde.expr     import LinearForm, BilinearForm, Functional, Norm
 from sympde.expr     import integral
+from sympde.calculus import Inner
 
 from psydac.linalg.solvers     import inverse
 from psydac.api.discretization import discretize
@@ -443,6 +444,32 @@ def test_non_symmetric_BilinearForm(backend):
 
     print("PASSED")
 
+#==============================================================================
+def test_non_symmetric_different_space_BilinearForm(backend):
+
+    kwargs = {'backend': PSYDAC_BACKENDS[backend]} if backend else {}
+
+    domain = Square()
+    V = VectorFunctionSpace('V', domain, kind='Hdiv')
+    X = VectorFunctionSpace('X', domain, kind='h1')
+
+    u = element_of(X, name='u')    
+    w = element_of(V, name='w')
+
+    A = BilinearForm((u, w), integral(domain, Inner(u, w)))
+
+    ncells = [4, 4]
+    degree = [2, 2]
+
+    domain_h = discretize(domain, ncells=ncells)
+    Vh = discretize(V, domain_h, degree=degree)
+    Xh = discretize(X, domain_h, degree=degree)
+
+    ah = discretize(A, domain_h, (Xh, Vh), **kwargs)
+    A = ah.assemble()
+
+    print("PASSED")
+
 #==============================================================================
 def test_assembly_no_synchr_args(backend):
 
@@ -509,4 +536,5 @@ def test_assembly_no_synchr_args(backend):
     test_multiple_fields(None)
     test_math_imports(None)
     test_non_symmetric_BilinearForm(None)
+    test_non_symmetric_different_space_BilinearForm(None)
     test_assembly_no_synchr_args(None)