diff --git a/cirq-core/cirq/study/sweeps.py b/cirq-core/cirq/study/sweeps.py
index 880ee18c082..c4c41016d4e 100644
--- a/cirq-core/cirq/study/sweeps.py
+++ b/cirq-core/cirq/study/sweeps.py
@@ -98,10 +98,10 @@ def __add__(self, other: 'Sweep') -> 'Sweep':
         return Zip(*sweeps)
 
     @abc.abstractmethod
-    def __eq__(self, other):
+    def __eq__(self, other) -> bool:
         pass
 
-    def __ne__(self, other):
+    def __ne__(self, other) -> bool:
         return not self == other
 
     @property
diff --git a/cirq-core/cirq/value/abc_alt_test.py b/cirq-core/cirq/value/abc_alt_test.py
index f59947b000e..33ff9c4ce52 100644
--- a/cirq-core/cirq/value/abc_alt_test.py
+++ b/cirq-core/cirq/value/abc_alt_test.py
@@ -13,6 +13,8 @@
 # limitations under the License.
 
 import abc
+from typing import NoReturn, Optional
+
 import pytest
 
 from cirq import ABCMetaImplementAnyOneOf, alternative
@@ -21,7 +23,7 @@
 def test_regular_abstract():
     class RegularAbc(metaclass=ABCMetaImplementAnyOneOf):
         @abc.abstractmethod
-        def my_method(self):
+        def my_method(self) -> str:
             """Docstring."""
 
     with pytest.raises(TypeError, match='abstract'):
@@ -38,11 +40,11 @@ def my_method(self, arg, kw=99):
             """my_method doc."""
 
         @abc.abstractmethod
-        def alt(self):
+        def alt(self) -> str:
             pass
 
     class SingleAlternativeChild(SingleAlternative):
-        def alt(self):
+        def alt(self) -> str:
             return 'alt'
 
     class SingleAlternativeOverride(SingleAlternative):
@@ -80,22 +82,22 @@ def _default_impl(self, arg, kw=99):
             """Default implementation."""
 
         @alternative(requires='alt', implementation=_default_impl)
-        def my_method(self, arg, kw=99):
+        def my_method(self, arg, kw=99) -> None:
             """my_method doc."""
 
         @abc.abstractmethod
-        def alt(self):
+        def alt(self) -> None:
             pass
 
     class SingleAlternativeChild(SingleAlternative):
-        def alt(self):
+        def alt(self) -> None:
             """Alternative method."""
 
     class SingleAlternativeOverride(SingleAlternative):
-        def my_method(self, arg, kw=99):
+        def my_method(self, arg, kw=99) -> None:
             """my_method override."""
 
-        def alt(self):
+        def alt(self) -> None:
             """Unneeded alternative method."""
 
     assert SingleAlternative.my_method.__doc__ == 'my_method doc.'
@@ -151,32 +153,32 @@ def _default_impl2(self, arg, kw=99):
 
         @alternative(requires='alt1', implementation=_default_impl1)
         @alternative(requires='alt2', implementation=_default_impl2)
-        def my_method(self, arg, kw=99):
+        def my_method(self, arg, kw=99) -> str:
             """Docstring."""
 
         @abc.abstractmethod
-        def alt1(self):
+        def alt1(self) -> Optional[str]:
             pass
 
         @abc.abstractmethod
-        def alt2(self):
+        def alt2(self) -> Optional[str]:
             pass
 
     class TwoAlternativesChild(TwoAlternatives):
-        def alt1(self):
+        def alt1(self) -> str:
             return 'alt1'
 
-        def alt2(self):
+        def alt2(self) -> NoReturn:
             raise RuntimeError  # coverage: ignore
 
     class TwoAlternativesOverride(TwoAlternatives):
-        def my_method(self, arg, kw=99):
+        def my_method(self, arg, kw=99) -> str:
             return 'override'
 
-        def alt1(self):
+        def alt1(self) -> NoReturn:
             raise RuntimeError  # coverage: ignore
 
-        def alt2(self):
+        def alt2(self) -> NoReturn:
             raise RuntimeError  # coverage: ignore
 
     class TwoAlternativesForceSecond(TwoAlternatives):
diff --git a/dev_tools/check.py b/dev_tools/check.py
index e9bad9a20f0..e5d0054af6b 100644
--- a/dev_tools/check.py
+++ b/dev_tools/check.py
@@ -64,7 +64,7 @@ def perform_check(self, env: env_tools.PreparedEnv, verbose: bool) -> Tuple[bool
             A tuple containing a pass/fail boolean and then a details message.
         """
 
-    def needs_python2_env(self):
+    def needs_python2_env(self) -> bool:
         return False
 
     def run(