diff --git a/src/_pytest/hookspec.py b/src/_pytest/hookspec.py
index c05b60791..1b4b09c85 100644
--- a/src/_pytest/hookspec.py
+++ b/src/_pytest/hookspec.py
@@ -1,5 +1,6 @@
 """ hook specifications for pytest plugins, invoked from main.py and builtin plugins.  """
 from typing import Any
+from typing import Dict
 from typing import List
 from typing import Mapping
 from typing import Optional
@@ -37,7 +38,6 @@ if TYPE_CHECKING:
     from _pytest.python import Metafunc
     from _pytest.python import Module
     from _pytest.python import PyCollector
-    from _pytest.reports import BaseReport
     from _pytest.reports import CollectReport
     from _pytest.reports import TestReport
     from _pytest.runner import CallInfo
@@ -172,7 +172,7 @@ def pytest_cmdline_preparse(config: "Config", args: List[str]) -> None:
 
 
 @hookspec(firstresult=True)
-def pytest_cmdline_main(config: "Config") -> "Optional[Union[ExitCode, int]]":
+def pytest_cmdline_main(config: "Config") -> Optional[Union["ExitCode", int]]:
     """ called for performing the main command line action. The default
     implementation will invoke the configure hooks and runtest_mainloop.
 
@@ -206,7 +206,7 @@ def pytest_load_initial_conftests(
 
 
 @hookspec(firstresult=True)
-def pytest_collection(session: "Session") -> Optional[Any]:
+def pytest_collection(session: "Session") -> Optional[object]:
     """Perform the collection protocol for the given session.
 
     Stops at first non-None result, see :ref:`firstresult`.
@@ -242,20 +242,21 @@ def pytest_collection_modifyitems(
     """
 
 
-def pytest_collection_finish(session: "Session"):
-    """ called after collection has been performed and modified.
+def pytest_collection_finish(session: "Session") -> None:
+    """Called after collection has been performed and modified.
 
     :param _pytest.main.Session session: the pytest session object
     """
 
 
 @hookspec(firstresult=True)
-def pytest_ignore_collect(path, config: "Config"):
-    """ return True to prevent considering this path for collection.
+def pytest_ignore_collect(path: py.path.local, config: "Config") -> Optional[bool]:
+    """Return True to prevent considering this path for collection.
+
     This hook is consulted for all files and directories prior to calling
     more specific hooks.
 
-    Stops at first non-None result, see :ref:`firstresult`
+    Stops at first non-None result, see :ref:`firstresult`.
 
     :param path: a :py:class:`py.path.local` - the path to analyze
     :param _pytest.config.Config config: pytest config object
@@ -263,18 +264,19 @@ def pytest_ignore_collect(path, config: "Config"):
 
 
 @hookspec(firstresult=True, warn_on_impl=COLLECT_DIRECTORY_HOOK)
-def pytest_collect_directory(path, parent):
-    """ called before traversing a directory for collection files.
+def pytest_collect_directory(path: py.path.local, parent) -> Optional[object]:
+    """Called before traversing a directory for collection files.
 
-    Stops at first non-None result, see :ref:`firstresult`
+    Stops at first non-None result, see :ref:`firstresult`.
 
     :param path: a :py:class:`py.path.local` - the path to analyze
     """
 
 
 def pytest_collect_file(path: py.path.local, parent) -> "Optional[Collector]":
-    """ return collection Node or None for the given path. Any new node
-    needs to have the specified ``parent`` as a parent.
+    """Return collection Node or None for the given path.
+
+    Any new node needs to have the specified ``parent`` as a parent.
 
     :param path: a :py:class:`py.path.local` - the path to collect
     """
@@ -287,16 +289,16 @@ def pytest_collectstart(collector: "Collector") -> None:
     """ collector starts collecting. """
 
 
-def pytest_itemcollected(item):
-    """ we just collected a test item. """
+def pytest_itemcollected(item: "Item") -> None:
+    """We just collected a test item."""
 
 
 def pytest_collectreport(report: "CollectReport") -> None:
     """ collector finished collecting. """
 
 
-def pytest_deselected(items):
-    """ called for test items deselected, e.g. by keyword. """
+def pytest_deselected(items: Sequence["Item"]) -> None:
+    """Called for deselected test items, e.g. by keyword."""
 
 
 @hookspec(firstresult=True)
@@ -312,13 +314,14 @@ def pytest_make_collect_report(collector: "Collector") -> "Optional[CollectRepor
 
 
 @hookspec(firstresult=True)
-def pytest_pycollect_makemodule(path: py.path.local, parent) -> "Optional[Module]":
-    """ return a Module collector or None for the given path.
+def pytest_pycollect_makemodule(path: py.path.local, parent) -> Optional["Module"]:
+    """Return a Module collector or None for the given path.
+
     This hook will be called for each matching test module path.
     The pytest_collect_file hook needs to be used if you want to
     create test modules for files that do not match as a test module.
 
-    Stops at first non-None result, see :ref:`firstresult`
+    Stops at first non-None result, see :ref:`firstresult`.
 
     :param path: a :py:class:`py.path.local` - the path of module to collect
     """
@@ -326,11 +329,12 @@ def pytest_pycollect_makemodule(path: py.path.local, parent) -> "Optional[Module
 
 @hookspec(firstresult=True)
 def pytest_pycollect_makeitem(
-    collector: "PyCollector", name: str, obj
-) -> "Union[None, Item, Collector, List[Union[Item, Collector]]]":
-    """ return custom item/collector for a python object in a module, or None.
+    collector: "PyCollector", name: str, obj: object
+) -> Union[None, "Item", "Collector", List[Union["Item", "Collector"]]]:
+    """Return a custom item/collector for a Python object in a module, or None.
 
-    Stops at first non-None result, see :ref:`firstresult` """
+    Stops at first non-None result, see :ref:`firstresult`.
+    """
 
 
 @hookspec(firstresult=True)
@@ -466,7 +470,7 @@ def pytest_runtest_call(item: "Item") -> None:
     """
 
 
-def pytest_runtest_teardown(item: "Item", nextitem: "Optional[Item]") -> None:
+def pytest_runtest_teardown(item: "Item", nextitem: Optional["Item"]) -> None:
     """Called to perform the teardown phase for a test item.
 
     The default implementation runs the finalizers and calls ``teardown()``
@@ -505,7 +509,9 @@ def pytest_runtest_logreport(report: "TestReport") -> None:
 
 
 @hookspec(firstresult=True)
-def pytest_report_to_serializable(config: "Config", report: "BaseReport"):
+def pytest_report_to_serializable(
+    config: "Config", report: Union["CollectReport", "TestReport"],
+) -> Optional[Dict[str, Any]]:
     """
     Serializes the given report object into a data structure suitable for sending
     over the wire, e.g. converted to JSON.
@@ -513,7 +519,9 @@ def pytest_report_to_serializable(config: "Config", report: "BaseReport"):
 
 
 @hookspec(firstresult=True)
-def pytest_report_from_serializable(config: "Config", data):
+def pytest_report_from_serializable(
+    config: "Config", data: Dict[str, Any],
+) -> Optional[Union["CollectReport", "TestReport"]]:
     """
     Restores a report object previously serialized with pytest_report_to_serializable().
     """
@@ -528,11 +536,11 @@ def pytest_report_from_serializable(config: "Config", data):
 def pytest_fixture_setup(
     fixturedef: "FixtureDef", request: "SubRequest"
 ) -> Optional[object]:
-    """ performs fixture setup execution.
+    """Performs fixture setup execution.
 
-    :return: The return value of the call to the fixture function
+    :return: The return value of the call to the fixture function.
 
-    Stops at first non-None result, see :ref:`firstresult`
+    Stops at first non-None result, see :ref:`firstresult`.
 
     .. note::
         If the fixture function returns None, other implementations of
@@ -555,7 +563,7 @@ def pytest_fixture_post_finalizer(
 
 
 def pytest_sessionstart(session: "Session") -> None:
-    """ called after the ``Session`` object has been created and before performing collection
+    """Called after the ``Session`` object has been created and before performing collection
     and entering the run test loop.
 
     :param _pytest.main.Session session: the pytest session object
@@ -563,9 +571,9 @@ def pytest_sessionstart(session: "Session") -> None:
 
 
 def pytest_sessionfinish(
-    session: "Session", exitstatus: "Union[int, ExitCode]"
+    session: "Session", exitstatus: Union[int, "ExitCode"],
 ) -> None:
-    """ called after whole test run finished, right before returning the exit status to the system.
+    """Called after whole test run finished, right before returning the exit status to the system.
 
     :param _pytest.main.Session session: the pytest session object
     :param int exitstatus: the status which pytest will return to the system
@@ -573,7 +581,7 @@ def pytest_sessionfinish(
 
 
 def pytest_unconfigure(config: "Config") -> None:
-    """ called before test process is exited.
+    """Called before test process is exited.
 
     :param _pytest.config.Config config: pytest config object
     """
@@ -587,7 +595,7 @@ def pytest_unconfigure(config: "Config") -> None:
 def pytest_assertrepr_compare(
     config: "Config", op: str, left: object, right: object
 ) -> Optional[List[str]]:
-    """return explanation for comparisons in failing assert expressions.
+    """Return explanation for comparisons in failing assert expressions.
 
     Return None for no custom explanation, otherwise return a list
     of strings.  The strings will be joined by newlines but any newlines
@@ -598,7 +606,7 @@ def pytest_assertrepr_compare(
     """
 
 
-def pytest_assertion_pass(item, lineno: int, orig: str, expl: str) -> None:
+def pytest_assertion_pass(item: "Item", lineno: int, orig: str, expl: str) -> None:
     """
     **(Experimental)**
 
@@ -665,12 +673,12 @@ def pytest_report_header(
 
 
 def pytest_report_collectionfinish(
-    config: "Config", startdir: py.path.local, items: "Sequence[Item]"
+    config: "Config", startdir: py.path.local, items: Sequence["Item"],
 ) -> Union[str, List[str]]:
     """
     .. versionadded:: 3.2
 
-    return a string or list of strings to be displayed after collection has finished successfully.
+    Return a string or list of strings to be displayed after collection has finished successfully.
 
     These strings will be displayed after the standard "collected X items" message.
 
@@ -689,7 +697,7 @@ def pytest_report_collectionfinish(
 
 @hookspec(firstresult=True)
 def pytest_report_teststatus(
-    report: "BaseReport", config: "Config"
+    report: Union["CollectReport", "TestReport"], config: "Config"
 ) -> Tuple[
     str, str, Union[str, Mapping[str, bool]],
 ]:
@@ -734,7 +742,7 @@ def pytest_terminal_summary(
 def pytest_warning_captured(
     warning_message: "warnings.WarningMessage",
     when: "Literal['config', 'collect', 'runtest']",
-    item: "Optional[Item]",
+    item: Optional["Item"],
     location: Optional[Tuple[str, int, str]],
 ) -> None:
     """(**Deprecated**) Process a warning captured by the internal pytest warnings plugin.
@@ -831,7 +839,9 @@ def pytest_keyboard_interrupt(
 
 
 def pytest_exception_interact(
-    node: "Node", call: "CallInfo[object]", report: "Union[CollectReport, TestReport]"
+    node: "Node",
+    call: "CallInfo[object]",
+    report: Union["CollectReport", "TestReport"],
 ) -> None:
     """Called when an exception was raised which can potentially be
     interactively handled.
diff --git a/src/_pytest/main.py b/src/_pytest/main.py
index b7a3a958a..98dabaf87 100644
--- a/src/_pytest/main.py
+++ b/src/_pytest/main.py
@@ -302,8 +302,8 @@ def _main(config: Config, session: "Session") -> Optional[Union[int, ExitCode]]:
     return None
 
 
-def pytest_collection(session: "Session") -> Sequence[nodes.Item]:
-    return session.perform_collect()
+def pytest_collection(session: "Session") -> None:
+    session.perform_collect()
 
 
 def pytest_runtestloop(session: "Session") -> bool:
@@ -343,9 +343,7 @@ def _in_venv(path: py.path.local) -> bool:
     return any([fname.basename in activates for fname in bindir.listdir()])
 
 
-def pytest_ignore_collect(
-    path: py.path.local, config: Config
-) -> "Optional[Literal[True]]":
+def pytest_ignore_collect(path: py.path.local, config: Config) -> Optional[bool]:
     ignore_paths = config._getconftest_pathlist("collect_ignore", path=path.dirpath())
     ignore_paths = ignore_paths or []
     excludeopt = config.getoption("ignore")
diff --git a/src/_pytest/python.py b/src/_pytest/python.py
index f3c42f421..7fbd59add 100644
--- a/src/_pytest/python.py
+++ b/src/_pytest/python.py
@@ -422,7 +422,7 @@ class PyCollector(PyobjMixin, nodes.Collector):
         return values
 
     def _makeitem(
-        self, name: str, obj
+        self, name: str, obj: object
     ) -> Union[
         None, nodes.Item, nodes.Collector, List[Union[nodes.Item, nodes.Collector]]
     ]:
diff --git a/src/_pytest/reports.py b/src/_pytest/reports.py
index 8b213ed13..7aba0b024 100644
--- a/src/_pytest/reports.py
+++ b/src/_pytest/reports.py
@@ -1,6 +1,7 @@
 from io import StringIO
 from pprint import pprint
 from typing import Any
+from typing import Dict
 from typing import Iterable
 from typing import Iterator
 from typing import List
@@ -69,7 +70,7 @@ class BaseReport:
         def __getattr__(self, key: str) -> Any:
             raise NotImplementedError()
 
-    def toterminal(self, out) -> None:
+    def toterminal(self, out: TerminalWriter) -> None:
         if hasattr(self, "node"):
             out.line(getworkerinfoline(self.node))
 
@@ -187,7 +188,7 @@ class BaseReport:
         )
         return verbose
 
-    def _to_json(self):
+    def _to_json(self) -> Dict[str, Any]:
         """
         This was originally the serialize_report() function from xdist (ca03269).
 
@@ -199,7 +200,7 @@ class BaseReport:
         return _report_to_json(self)
 
     @classmethod
-    def _from_json(cls: "Type[_R]", reportdict) -> _R:
+    def _from_json(cls: "Type[_R]", reportdict: Dict[str, object]) -> _R:
         """
         This was originally the serialize_report() function from xdist (ca03269).
 
@@ -382,11 +383,13 @@ class CollectErrorRepr(TerminalRepr):
     def __init__(self, msg) -> None:
         self.longrepr = msg
 
-    def toterminal(self, out) -> None:
+    def toterminal(self, out: TerminalWriter) -> None:
         out.line(self.longrepr, red=True)
 
 
-def pytest_report_to_serializable(report: BaseReport):
+def pytest_report_to_serializable(
+    report: Union[CollectReport, TestReport]
+) -> Optional[Dict[str, Any]]:
     if isinstance(report, (TestReport, CollectReport)):
         data = report._to_json()
         data["$report_type"] = report.__class__.__name__
@@ -394,7 +397,9 @@ def pytest_report_to_serializable(report: BaseReport):
     return None
 
 
-def pytest_report_from_serializable(data) -> Optional[BaseReport]:
+def pytest_report_from_serializable(
+    data: Dict[str, Any],
+) -> Optional[Union[CollectReport, TestReport]]:
     if "$report_type" in data:
         if data["$report_type"] == "TestReport":
             return TestReport._from_json(data)
@@ -406,7 +411,7 @@ def pytest_report_from_serializable(data) -> Optional[BaseReport]:
     return None
 
 
-def _report_to_json(report: BaseReport):
+def _report_to_json(report: BaseReport) -> Dict[str, Any]:
     """
     This was originally the serialize_report() function from xdist (ca03269).
 
@@ -414,7 +419,9 @@ def _report_to_json(report: BaseReport):
     serialization.
     """
 
-    def serialize_repr_entry(entry: Union[ReprEntry, ReprEntryNative]):
+    def serialize_repr_entry(
+        entry: Union[ReprEntry, ReprEntryNative]
+    ) -> Dict[str, Any]:
         data = attr.asdict(entry)
         for key, value in data.items():
             if hasattr(value, "__dict__"):
@@ -422,25 +429,28 @@ def _report_to_json(report: BaseReport):
         entry_data = {"type": type(entry).__name__, "data": data}
         return entry_data
 
-    def serialize_repr_traceback(reprtraceback: ReprTraceback):
+    def serialize_repr_traceback(reprtraceback: ReprTraceback) -> Dict[str, Any]:
         result = attr.asdict(reprtraceback)
         result["reprentries"] = [
             serialize_repr_entry(x) for x in reprtraceback.reprentries
         ]
         return result
 
-    def serialize_repr_crash(reprcrash: Optional[ReprFileLocation]):
+    def serialize_repr_crash(
+        reprcrash: Optional[ReprFileLocation],
+    ) -> Optional[Dict[str, Any]]:
         if reprcrash is not None:
             return attr.asdict(reprcrash)
         else:
             return None
 
-    def serialize_longrepr(rep):
+    def serialize_longrepr(rep: BaseReport) -> Dict[str, Any]:
+        assert rep.longrepr is not None
         result = {
             "reprcrash": serialize_repr_crash(rep.longrepr.reprcrash),
             "reprtraceback": serialize_repr_traceback(rep.longrepr.reprtraceback),
             "sections": rep.longrepr.sections,
-        }
+        }  # type: Dict[str, Any]
         if isinstance(rep.longrepr, ExceptionChainRepr):
             result["chain"] = []
             for repr_traceback, repr_crash, description in rep.longrepr.chain:
@@ -473,7 +483,7 @@ def _report_to_json(report: BaseReport):
     return d
 
 
-def _report_kwargs_from_json(reportdict):
+def _report_kwargs_from_json(reportdict: Dict[str, Any]) -> Dict[str, Any]:
     """
     This was originally the serialize_report() function from xdist (ca03269).
 
diff --git a/src/_pytest/terminal.py b/src/_pytest/terminal.py
index 6a58260e9..9b10e5ffe 100644
--- a/src/_pytest/terminal.py
+++ b/src/_pytest/terminal.py
@@ -467,9 +467,9 @@ class TerminalReporter:
     def line(self, msg: str, **kw: bool) -> None:
         self._tw.line(msg, **kw)
 
-    def _add_stats(self, category: str, items: List) -> None:
+    def _add_stats(self, category: str, items: Sequence) -> None:
         set_main_color = category not in self.stats
-        self.stats.setdefault(category, []).extend(items[:])
+        self.stats.setdefault(category, []).extend(items)
         if set_main_color:
             self._set_main_color()
 
@@ -499,7 +499,7 @@ class TerminalReporter:
             #     which garbles our output if we use self.write_line
             self.write_line(msg)
 
-    def pytest_deselected(self, items) -> None:
+    def pytest_deselected(self, items: Sequence[Item]) -> None:
         self._add_stats("deselected", items)
 
     def pytest_runtest_logstart(
diff --git a/src/_pytest/unittest.py b/src/_pytest/unittest.py
index a90b56c29..0e4a31311 100644
--- a/src/_pytest/unittest.py
+++ b/src/_pytest/unittest.py
@@ -44,7 +44,7 @@ if TYPE_CHECKING:
 
 
 def pytest_pycollect_makeitem(
-    collector: PyCollector, name: str, obj
+    collector: PyCollector, name: str, obj: object
 ) -> Optional["UnitTestCase"]:
     # has unittest been imported and is obj a subclass of its TestCase?
     try: