Fix mypy errors (#229)

In particular, the following errors are fixed in this commit:

* Don't return False in __exit__

Returning a literal causes a mypy error when combined with the
`typing.Optional[bool]` type hint. Furthermore, exception handling is
the same when returning `False` and when returning `None` (the
exception is re-raised). Therefore, it's simpler to remove the return
statement and change the type hint to `None`.

* Correctly initialize nested tuple

Tuples of length 1 should be initialized with a trailing comma to be
properly interpreted.

* Pass correct type to use_context() in test

* Add type annotations for test helper functions

Since we have `disallow_untyped_calls = True` in our mypy config for
tests, we must add type annotations to any function that is called
from a test.


Addditionally, bump minimal mypy version to 0.740 to consistently reproduce these errors.

opentelemetry-api/src/opentelemetry/trace/__init__.py

@@ -245,13 +245,9 @@ def __exit__(
         exc_type: typing.Optional[typing.Type[BaseException]],
         exc_val: typing.Optional[BaseException],
         exc_tb: typing.Optional[python_types.TracebackType],
-    ) -> typing.Optional[bool]:
-        """Ends context manager and calls `end` on the `Span`.
-
-        Returns False.
-        """
+    ) -> None:
+        """Ends context manager and calls `end` on the `Span`."""
         self.end()
-        return False
 
 
 class TraceOptions(int):

opentelemetry-api/tests/distributedcontext/test_distributed_context.py

@@ -99,6 +99,14 @@ def test_get_current_context(self):
         self.assertIsNone(self.manager.get_current_context())
 
     def test_use_context(self):
-        expected = object()
+        expected = distributedcontext.DistributedContext(
+            (
+                distributedcontext.Entry(
+                    distributedcontext.EntryMetadata(0),
+                    distributedcontext.EntryKey("0"),
+                    distributedcontext.EntryValue(""),
+                ),
+            )
+        )
         with self.manager.use_context(expected) as output:
             self.assertIs(output, expected)

opentelemetry-api/tests/metrics/test_metrics.py

@@ -24,7 +24,7 @@ def setUp(self):
 
     def test_record_batch(self):
         counter = metrics.Counter()
-        self.meter.record_batch(("values"), ((counter, 1)))
+        self.meter.record_batch(("values"), ((counter, 1),))
 
     def test_create_metric(self):
         metric = self.meter.create_metric("", "", "", float, metrics.Counter)

opentelemetry-api/tests/test_loader.py

@@ -16,6 +16,7 @@
 import sys
 import unittest
 from importlib import reload
+from typing import Any, Callable
 
 from opentelemetry import trace
 from opentelemetry.util import loader
@@ -59,7 +60,7 @@ def test_preferred_impl(self):
 
     # NOTE: We use do_* + *_<arg> methods because subtest wouldn't run setUp,
     # which we require here.
-    def do_test_preferred_impl(self, setter):
+    def do_test_preferred_impl(self, setter: Callable[[Any], Any]) -> None:
         setter(get_opentelemetry_implementation)
         tracer = trace.tracer()
         self.assertIs(tracer, DUMMY_TRACER)
@@ -81,7 +82,7 @@ def test_try_set_again(self):
             )
         self.assertIn("already loaded", str(einfo.exception))
 
-    def do_test_get_envvar(self, envvar_suffix):
+    def do_test_get_envvar(self, envvar_suffix: str) -> None:
         global DUMMY_TRACER  # pylint:disable=global-statement
 
         # Test is not runnable with this!

tox.ini

@@ -14,7 +14,7 @@ python =
 
 [testenv]
 deps =
-  mypy,mypyinstalled: mypy~=0.711
+  mypy,mypyinstalled: mypy~=0.740
 
 setenv =
   mypy: MYPYPATH={toxinidir}/opentelemetry-api/src/