refactor: simplify _find_backend implementation

Author: jcrist

ibis/expr/types/core.py

@@ -227,16 +227,15 @@ def _find_backend(self, *, use_default: bool = False) -> BaseBackend:
                     "assign a backend instance to "
                     "`ibis.options.default_backend`."
                 )
-            if (default := options.default_backend) is None and use_default:
-                default = _default_backend()
+            default = _default_backend() if use_default else None
             if default is None:
                 raise IbisError(
                     'Expression depends on no backends, and found no default'
                 )
             return default
 
         if len(backends) > 1:
-            raise ValueError('Multiple backends found')
+            raise IbisError('Multiple backends found for this expression')
 
         return backends[0]
 

ibis/tests/expr/test_table.py

@@ -1473,10 +1473,7 @@ def test_multiple_db_different_backends():
     backend2_table = con2.table('alltypes')
 
     expr = backend1_table.union(backend2_table)
-    with pytest.raises(
-        ValueError,
-        match=re.compile("multiple backends", flags=re.IGNORECASE),
-    ):
+    with pytest.raises(com.IbisError, match="Multiple backends"):
         expr.compile()