Stop using pybind's implicit __hash__ (closes gh-102)

Author: inducer

gen_wrap.py

@@ -1370,6 +1370,10 @@ def write_wrapper(outf, meth):
 
 # {{{ exposer generator
 
+def is_hash(meth):
+    return meth.name == "get_hash" and len(meth.args) == 1
+
+
 def write_exposer(outf, meth, arg_names, doc_str):
     func_name = f"isl::{meth.cls}_{meth.name}"
     py_name = meth.name
@@ -1385,7 +1389,7 @@ def write_exposer(outf, meth, arg_names, doc_str):
     if meth.name == "size" and len(meth.args) == 1:
         py_name = "__len__"
 
-    if meth.name == "get_hash" and len(meth.args) == 1:
+    if is_hash(meth):
         py_name = "__hash__"
 
     extra_py_names = []
@@ -1509,6 +1513,16 @@ def gen_wrapper(include_dirs, include_barvinok=False, isl_version=None):
             for cls in classes
             for meth in fdata.classes_to_methods.get(cls, [])])
 
+        for cls in classes:
+            has_isl_hash = any(
+                    is_hash(meth) for meth in fdata.classes_to_methods.get(cls, []))
+
+            if not has_isl_hash:
+                # pybind11's C++ object base class has an object identity
+                # __hash__ that everyone inherits automatically. We don't
+                # want that.
+                expf.write(f'wrap_{cls}.attr("__hash__") = py::none();\n')
+
         expf.close()
         wrapf.close()
 

islpy/__init__.py

@@ -313,9 +313,6 @@ def generic_repr(self):
             cls.__str__ = generic_str
             cls.__repr__ = generic_repr
 
-        if not hasattr(cls, "__hash__"):
-            raise AssertionError(f"not hashable: {cls}")
-
     # }}}
 
     # {{{ Python set-like behavior
@@ -908,6 +905,12 @@ def val_to_python(self):
     # note: automatic upcasts for method arguments are provided through
     # 'implicitly_convertible' on the C++ side of the wrapper.
 
+    def make_upcasting_hash(special_method, upcast_method):
+        def wrapper(basic_instance):
+            return hash((type(basic_instance), upcast_method(basic_instance)))
+
+        return wrapper
+
     def make_new_upcast_wrapper(method, upcast):
         # This function provides a scope in which method and upcast
         # are not changed from one iteration of the enclosing for
@@ -916,7 +919,6 @@ def make_new_upcast_wrapper(method, upcast):
         def wrapper(basic_instance, *args, **kwargs):
             special_instance = upcast(basic_instance)
             return method(special_instance, *args, **kwargs)
-
         return wrapper
 
     def make_existing_upcast_wrapper(basic_method, special_method, upcast):
@@ -938,6 +940,18 @@ def wrapper(basic_instance, *args, **kwargs):
     def add_upcasts(basic_class, special_class, upcast_method):
         from functools import update_wrapper
 
+        # {{{ implicitly upcast __hash__
+
+        # We don't use hasattr() here because in the C++ part of the wrapper
+        # we overwrite pybind's unwanted default __hash__ implementation
+        # with None.
+        if (getattr(basic_class, "__hash__", None) is None
+                and getattr(special_class, "__hash__", None) is not None):
+            wrapper = make_upcasting_hash(special_class.__hash__, upcast_method)
+            basic_class.__hash__ = update_wrapper(wrapper, basic_class.__hash__)
+
+        # }}}
+
         def my_ismethod(class_, method_name):
             if method_name.startswith("_"):
                 return False

test/test_isl.py

@@ -457,6 +457,15 @@ def test_sched_constraints_set_validity():
     assert str(validity) == str(validity2)
 
 
+def test_basicset_hash():
+    # https://github.com/inducer/islpy/issues/102
+    # isl does not currently (2022-12-30) offer hashing for BasicSet.
+
+    a1 = isl.BasicSet("{[i]: 0<=i<512}")
+    a2 = isl.BasicSet("{[i]: 0<=i<512}")
+    assert hash(a1) == hash(a2)
+
+
 if __name__ == "__main__":
     import sys
     if len(sys.argv) > 1: