Fix hashing for custom eq objects (#909)

* Add hashing for eq'd objects

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Add coverage to test

* oops

* Add changelog message

* Revert "Add changelog message"

This reverts commit 0bd3f5c.

* Address comments

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Hynek Schlawack <hs@ox.cx>

changelog.d/909.change.rst

@@ -0,0 +1 @@
+If an eq key is defined, it is also used before hashing the attribute.

src/attr/_make.py

@@ -325,13 +325,11 @@ def _compile_and_eval(script, globs, locs=None, filename=""):
     eval(bytecode, globs, locs)
 
 
-def _make_method(name, script, filename, globs=None):
+def _make_method(name, script, filename, globs):
     """
     Create the method with the script given and return the method object.
     """
     locs = {}
-    if globs is None:
-        globs = {}
 
     # In order of debuggers like PDB being able to step through the code,
     # we add a fake linecache entry.
@@ -1680,6 +1678,8 @@ def _make_hash(cls, attrs, frozen, cache_hash):
 
     unique_filename = _generate_unique_filename(cls, "hash")
     type_hash = hash(unique_filename)
+    # If eq is custom generated, we need to include the functions in globs
+    globs = {}
 
     hash_def = "def __hash__(self"
     hash_func = "hash(("
@@ -1714,7 +1714,14 @@ def append_hash_computation_lines(prefix, indent):
         )
 
         for a in attrs:
-            method_lines.append(indent + "        self.%s," % a.name)
+            if a.eq_key:
+                cmp_name = "_%s_key" % (a.name,)
+                globs[cmp_name] = a.eq_key
+                method_lines.append(
+                    indent + "        %s(self.%s)," % (cmp_name, a.name)
+                )
+            else:
+                method_lines.append(indent + "        self.%s," % a.name)
 
         method_lines.append(indent + "    " + closing_braces)
 
@@ -1734,7 +1741,7 @@ def append_hash_computation_lines(prefix, indent):
         append_hash_computation_lines("return ", tab)
 
     script = "\n".join(method_lines)
-    return _make_method("__hash__", script, unique_filename)
+    return _make_method("__hash__", script, unique_filename, globs)
 
 
 def _add_hash(cls, attrs):

tests/test_make.py

@@ -2057,6 +2057,27 @@ def __repr__(self):
 
         assert "hi" == repr(C(42))
 
+    @pytest.mark.parametrize("slots", [True, False])
+    @pytest.mark.parametrize("frozen", [True, False])
+    def test_hash_uses_eq(self, slots, frozen):
+        """
+        If eq is passed in, then __hash__ should use the eq callable
+        to generate the hash code.
+        """
+
+        @attr.s(slots=slots, frozen=frozen, hash=True)
+        class C(object):
+            x = attr.ib(eq=str)
+
+        @attr.s(slots=slots, frozen=frozen, hash=True)
+        class D(object):
+            x = attr.ib()
+
+        # These hashes should be the same because 1 is turned into
+        # string before hashing.
+        assert hash(C("1")) == hash(C(1))
+        assert hash(D("1")) != hash(D(1))
+
     @pytest.mark.parametrize("slots", [True, False])
     @pytest.mark.parametrize("frozen", [True, False])
     def test_detect_auto_hash(self, slots, frozen):