Merge pull request sympy#20590 from oscarbenjamin/pr_basic_slots

fix(core): use __slots__ for DefaultPrinting and Basic

sympy/core/_print_helpers.py

@@ -17,6 +17,11 @@ class Printable:
     This also adds support for LaTeX printing in jupyter notebooks.
     """
 
+    # Since this class is used as a mixin we set empty slots. That means that
+    # instances of any subclasses that use slots will not need to have a
+    # __dict__.
+    __slots__ = ()
+
     # Note, we always use the default ordering (lex) in __str__ and __repr__,
     # regardless of the global setting. See issue 5487.
     def __str__(self):

sympy/core/tests/test_basic.py

@@ -34,6 +34,12 @@ def test_structure():
     assert bool(b1)
 
 
+def test_immutable():
+    assert not hasattr(b1, '__dict__')
+    with raises(AttributeError):
+        b1.x = 1
+
+
 def test_equality():
     instances = [b1, b2, b3, b21, Basic(b1, b1, b1), Basic]
     for i, b_i in enumerate(instances):