fix: empty for linear expressions with terms (#424)

Author: coroa

linopy/expressions.py

@@ -9,13 +9,12 @@
 
 import functools
 import logging
-from collections.abc import Hashable, Iterator, Mapping, Sequence
+from collections.abc import Callable, Hashable, Iterator, Mapping, Sequence
 from dataclasses import dataclass, field
 from itertools import product, zip_longest
 from typing import (
     TYPE_CHECKING,
     Any,
-    Callable,
 )
 from warnings import warn
 
@@ -1352,7 +1351,7 @@ def empty(self) -> bool:
         """
         Get whether the linear expression is empty.
         """
-        return self.shape == (0,)
+        return not self.size
 
     def densify_terms(self) -> LinearExpression:
         """

test/test_linear_expression.py

@@ -539,6 +539,12 @@ def test_linear_expression_loc(x: Variable, y: Variable) -> None:
     assert expr.loc[0].size < expr.loc[:5].size
 
 
+def test_linear_expression_empty(v: Variable) -> None:
+    expr = 7 * v
+    assert not expr.empty()
+    assert expr.loc[[]].empty()
+
+
 def test_linear_expression_isnull(v: Variable) -> None:
     expr = np.arange(20) * v
     filter = (expr.coeffs >= 10).any(TERM_DIM)