diff --git a/src/_pytest/_code/source.py b/src/_pytest/_code/source.py
index db78bbd0d..1e9dd5031 100644
--- a/src/_pytest/_code/source.py
+++ b/src/_pytest/_code/source.py
@@ -7,10 +7,17 @@ import tokenize
 import warnings
 from ast import PyCF_ONLY_AST as _AST_FLAG
 from bisect import bisect_right
+from types import FrameType
 from typing import List
+from typing import Optional
+from typing import Sequence
+from typing import Tuple
+from typing import Union
 
 import py
 
+from _pytest.compat import overload
+
 
 class Source:
     """ an immutable object holding a source code fragment,
@@ -19,7 +26,7 @@ class Source:
 
     _compilecounter = 0
 
-    def __init__(self, *parts, **kwargs):
+    def __init__(self, *parts, **kwargs) -> None:
         self.lines = lines = []  # type: List[str]
         de = kwargs.get("deindent", True)
         for part in parts:
@@ -48,7 +55,15 @@ class Source:
     # Ignore type because of https://github.com/python/mypy/issues/4266.
     __hash__ = None  # type: ignore
 
-    def __getitem__(self, key):
+    @overload
+    def __getitem__(self, key: int) -> str:
+        raise NotImplementedError()
+
+    @overload  # noqa: F811
+    def __getitem__(self, key: slice) -> "Source":
+        raise NotImplementedError()
+
+    def __getitem__(self, key: Union[int, slice]) -> Union[str, "Source"]:  # noqa: F811
         if isinstance(key, int):
             return self.lines[key]
         else:
@@ -58,10 +73,10 @@ class Source:
             newsource.lines = self.lines[key.start : key.stop]
             return newsource
 
-    def __len__(self):
+    def __len__(self) -> int:
         return len(self.lines)
 
-    def strip(self):
+    def strip(self) -> "Source":
         """ return new source object with trailing
             and leading blank lines removed.
         """
@@ -74,18 +89,20 @@ class Source:
         source.lines[:] = self.lines[start:end]
         return source
 
-    def putaround(self, before="", after="", indent=" " * 4):
+    def putaround(
+        self, before: str = "", after: str = "", indent: str = " " * 4
+    ) -> "Source":
         """ return a copy of the source object with
             'before' and 'after' wrapped around it.
         """
-        before = Source(before)
-        after = Source(after)
+        beforesource = Source(before)
+        aftersource = Source(after)
         newsource = Source()
         lines = [(indent + line) for line in self.lines]
-        newsource.lines = before.lines + lines + after.lines
+        newsource.lines = beforesource.lines + lines + aftersource.lines
         return newsource
 
-    def indent(self, indent=" " * 4):
+    def indent(self, indent: str = " " * 4) -> "Source":
         """ return a copy of the source object with
             all lines indented by the given indent-string.
         """
@@ -93,14 +110,14 @@ class Source:
         newsource.lines = [(indent + line) for line in self.lines]
         return newsource
 
-    def getstatement(self, lineno):
+    def getstatement(self, lineno: int) -> "Source":
         """ return Source statement which contains the
             given linenumber (counted from 0).
         """
         start, end = self.getstatementrange(lineno)
         return self[start:end]
 
-    def getstatementrange(self, lineno):
+    def getstatementrange(self, lineno: int):
         """ return (start, end) tuple which spans the minimal
             statement region which containing the given lineno.
         """
@@ -109,13 +126,13 @@ class Source:
         ast, start, end = getstatementrange_ast(lineno, self)
         return start, end
 
-    def deindent(self):
+    def deindent(self) -> "Source":
         """return a new source object deindented."""
         newsource = Source()
         newsource.lines[:] = deindent(self.lines)
         return newsource
 
-    def isparseable(self, deindent=True):
+    def isparseable(self, deindent: bool = True) -> bool:
         """ return True if source is parseable, heuristically
             deindenting it by default.
         """
@@ -135,11 +152,16 @@ class Source:
         else:
             return True
 
-    def __str__(self):
+    def __str__(self) -> str:
         return "\n".join(self.lines)
 
     def compile(
-        self, filename=None, mode="exec", flag=0, dont_inherit=0, _genframe=None
+        self,
+        filename=None,
+        mode="exec",
+        flag: int = 0,
+        dont_inherit: int = 0,
+        _genframe: Optional[FrameType] = None,
     ):
         """ return compiled code object. if filename is None
             invent an artificial filename which displays
@@ -183,7 +205,7 @@ class Source:
 #
 
 
-def compile_(source, filename=None, mode="exec", flags=0, dont_inherit=0):
+def compile_(source, filename=None, mode="exec", flags: int = 0, dont_inherit: int = 0):
     """ compile the given source to a raw code object,
         and maintain an internal cache which allows later
         retrieval of the source code for the code object
@@ -233,7 +255,7 @@ def getfslineno(obj):
 #
 
 
-def findsource(obj):
+def findsource(obj) -> Tuple[Optional[Source], int]:
     try:
         sourcelines, lineno = inspect.findsource(obj)
     except Exception:
@@ -243,7 +265,7 @@ def findsource(obj):
     return source, lineno
 
 
-def getsource(obj, **kwargs):
+def getsource(obj, **kwargs) -> Source:
     from .code import getrawcode
 
     obj = getrawcode(obj)
@@ -255,21 +277,21 @@ def getsource(obj, **kwargs):
     return Source(strsrc, **kwargs)
 
 
-def deindent(lines):
+def deindent(lines: Sequence[str]) -> List[str]:
     return textwrap.dedent("\n".join(lines)).splitlines()
 
 
-def get_statement_startend2(lineno, node):
+def get_statement_startend2(lineno: int, node: ast.AST) -> Tuple[int, Optional[int]]:
     import ast
 
     # flatten all statements and except handlers into one lineno-list
     # AST's line numbers start indexing at 1
-    values = []
+    values = []  # type: List[int]
     for x in ast.walk(node):
         if isinstance(x, (ast.stmt, ast.ExceptHandler)):
             values.append(x.lineno - 1)
             for name in ("finalbody", "orelse"):
-                val = getattr(x, name, None)
+                val = getattr(x, name, None)  # type: Optional[List[ast.stmt]]
                 if val:
                     # treat the finally/orelse part as its own statement
                     values.append(val[0].lineno - 1 - 1)
@@ -283,7 +305,12 @@ def get_statement_startend2(lineno, node):
     return start, end
 
 
-def getstatementrange_ast(lineno, source: Source, assertion=False, astnode=None):
+def getstatementrange_ast(
+    lineno: int,
+    source: Source,
+    assertion: bool = False,
+    astnode: Optional[ast.AST] = None,
+) -> Tuple[ast.AST, int, int]:
     if astnode is None:
         content = str(source)
         # See #4260:
diff --git a/src/_pytest/pytester.py b/src/_pytest/pytester.py
index 14db8409e..6b45e077b 100644
--- a/src/_pytest/pytester.py
+++ b/src/_pytest/pytester.py
@@ -1,4 +1,5 @@
 """(disabled by default) support for testing pytest and pytest plugins."""
+import collections.abc
 import gc
 import importlib
 import os
@@ -8,9 +9,15 @@ import subprocess
 import sys
 import time
 import traceback
-from collections.abc import Sequence
 from fnmatch import fnmatch
 from io import StringIO
+from typing import Callable
+from typing import Dict
+from typing import Iterable
+from typing import List
+from typing import Optional
+from typing import Sequence
+from typing import Tuple
 from typing import Union
 from weakref import WeakKeyDictionary
 
@@ -21,10 +28,16 @@ from _pytest._code import Source
 from _pytest._io.saferepr import saferepr
 from _pytest.capture import MultiCapture
 from _pytest.capture import SysCapture
+from _pytest.fixtures import FixtureRequest
 from _pytest.main import ExitCode
 from _pytest.main import Session
 from _pytest.monkeypatch import MonkeyPatch
 from _pytest.pathlib import Path
+from _pytest.reports import TestReport
+
+if False:  # TYPE_CHECKING
+    from typing import Type
+
 
 IGNORE_PAM = [  # filenames added when obtaining details about the current user
     "/var/lib/sss/mc/passwd"
@@ -142,7 +155,7 @@ class LsofFdLeakChecker:
 
 
 @pytest.fixture
-def _pytest(request):
+def _pytest(request: FixtureRequest) -> "PytestArg":
     """Return a helper which offers a gethookrecorder(hook) method which
     returns a HookRecorder instance which helps to make assertions about called
     hooks.
@@ -152,10 +165,10 @@ def _pytest(request):
 
 
 class PytestArg:
-    def __init__(self, request):
+    def __init__(self, request: FixtureRequest) -> None:
         self.request = request
 
-    def gethookrecorder(self, hook):
+    def gethookrecorder(self, hook) -> "HookRecorder":
         hookrecorder = HookRecorder(hook._pm)
         self.request.addfinalizer(hookrecorder.finish_recording)
         return hookrecorder
@@ -176,6 +189,11 @@ class ParsedCall:
         del d["_name"]
         return "<ParsedCall {!r}(**{!r})>".format(self._name, d)
 
+    if False:  # TYPE_CHECKING
+        # The class has undetermined attributes, this tells mypy about it.
+        def __getattr__(self, key):
+            raise NotImplementedError()
+
 
 class HookRecorder:
     """Record all hooks called in a plugin manager.
@@ -185,27 +203,27 @@ class HookRecorder:
 
     """
 
-    def __init__(self, pluginmanager):
+    def __init__(self, pluginmanager) -> None:
         self._pluginmanager = pluginmanager
-        self.calls = []
+        self.calls = []  # type: List[ParsedCall]
 
-        def before(hook_name, hook_impls, kwargs):
+        def before(hook_name: str, hook_impls, kwargs) -> None:
             self.calls.append(ParsedCall(hook_name, kwargs))
 
-        def after(outcome, hook_name, hook_impls, kwargs):
+        def after(outcome, hook_name: str, hook_impls, kwargs) -> None:
             pass
 
         self._undo_wrapping = pluginmanager.add_hookcall_monitoring(before, after)
 
-    def finish_recording(self):
+    def finish_recording(self) -> None:
         self._undo_wrapping()
 
-    def getcalls(self, names):
+    def getcalls(self, names: Union[str, Iterable[str]]) -> List[ParsedCall]:
         if isinstance(names, str):
             names = names.split()
         return [call for call in self.calls if call._name in names]
 
-    def assert_contains(self, entries):
+    def assert_contains(self, entries) -> None:
         __tracebackhide__ = True
         i = 0
         entries = list(entries)
@@ -226,7 +244,7 @@ class HookRecorder:
             else:
                 pytest.fail("could not find {!r} check {!r}".format(name, check))
 
-    def popcall(self, name):
+    def popcall(self, name: str) -> ParsedCall:
         __tracebackhide__ = True
         for i, call in enumerate(self.calls):
             if call._name == name:
@@ -236,20 +254,27 @@ class HookRecorder:
         lines.extend(["  %s" % x for x in self.calls])
         pytest.fail("\n".join(lines))
 
-    def getcall(self, name):
+    def getcall(self, name: str) -> ParsedCall:
         values = self.getcalls(name)
         assert len(values) == 1, (name, values)
         return values[0]
 
     # functionality for test reports
 
-    def getreports(self, names="pytest_runtest_logreport pytest_collectreport"):
+    def getreports(
+        self,
+        names: Union[
+            str, Iterable[str]
+        ] = "pytest_runtest_logreport pytest_collectreport",
+    ) -> List[TestReport]:
         return [x.report for x in self.getcalls(names)]
 
     def matchreport(
         self,
-        inamepart="",
-        names="pytest_runtest_logreport pytest_collectreport",
+        inamepart: str = "",
+        names: Union[
+            str, Iterable[str]
+        ] = "pytest_runtest_logreport pytest_collectreport",
         when=None,
     ):
         """return a testreport whose dotted import path matches"""
@@ -275,13 +300,20 @@ class HookRecorder:
             )
         return values[0]
 
-    def getfailures(self, names="pytest_runtest_logreport pytest_collectreport"):
+    def getfailures(
+        self,
+        names: Union[
+            str, Iterable[str]
+        ] = "pytest_runtest_logreport pytest_collectreport",
+    ) -> List[TestReport]:
         return [rep for rep in self.getreports(names) if rep.failed]
 
-    def getfailedcollections(self):
+    def getfailedcollections(self) -> List[TestReport]:
         return self.getfailures("pytest_collectreport")
 
-    def listoutcomes(self):
+    def listoutcomes(
+        self
+    ) -> Tuple[List[TestReport], List[TestReport], List[TestReport]]:
         passed = []
         skipped = []
         failed = []
@@ -296,31 +328,31 @@ class HookRecorder:
                 failed.append(rep)
         return passed, skipped, failed
 
-    def countoutcomes(self):
+    def countoutcomes(self) -> List[int]:
         return [len(x) for x in self.listoutcomes()]
 
-    def assertoutcome(self, passed=0, skipped=0, failed=0):
+    def assertoutcome(self, passed: int = 0, skipped: int = 0, failed: int = 0) -> None:
         realpassed, realskipped, realfailed = self.listoutcomes()
         assert passed == len(realpassed)
         assert skipped == len(realskipped)
         assert failed == len(realfailed)
 
-    def clear(self):
+    def clear(self) -> None:
         self.calls[:] = []
 
 
 @pytest.fixture
-def linecomp(request):
+def linecomp(request: FixtureRequest) -> "LineComp":
     return LineComp()
 
 
 @pytest.fixture(name="LineMatcher")
-def LineMatcher_fixture(request):
+def LineMatcher_fixture(request: FixtureRequest) -> "Type[LineMatcher]":
     return LineMatcher
 
 
 @pytest.fixture
-def testdir(request, tmpdir_factory):
+def testdir(request: FixtureRequest, tmpdir_factory) -> "Testdir":
     return Testdir(request, tmpdir_factory)
 
 
@@ -363,7 +395,13 @@ class RunResult:
     :ivar duration: duration in seconds
     """
 
-    def __init__(self, ret: Union[int, ExitCode], outlines, errlines, duration) -> None:
+    def __init__(
+        self,
+        ret: Union[int, ExitCode],
+        outlines: Sequence[str],
+        errlines: Sequence[str],
+        duration: float,
+    ) -> None:
         try:
             self.ret = pytest.ExitCode(ret)  # type: Union[int, ExitCode]
         except ValueError:
@@ -374,13 +412,13 @@ class RunResult:
         self.stderr = LineMatcher(errlines)
         self.duration = duration
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return (
             "<RunResult ret=%s len(stdout.lines)=%d len(stderr.lines)=%d duration=%.2fs>"
             % (self.ret, len(self.stdout.lines), len(self.stderr.lines), self.duration)
         )
 
-    def parseoutcomes(self):
+    def parseoutcomes(self) -> Dict[str, int]:
         """Return a dictionary of outcomestring->num from parsing the terminal
         output that the test process produced.
 
@@ -393,8 +431,14 @@ class RunResult:
         raise ValueError("Pytest terminal summary report not found")
 
     def assert_outcomes(
-        self, passed=0, skipped=0, failed=0, error=0, xpassed=0, xfailed=0
-    ):
+        self,
+        passed: int = 0,
+        skipped: int = 0,
+        failed: int = 0,
+        error: int = 0,
+        xpassed: int = 0,
+        xfailed: int = 0,
+    ) -> None:
         """Assert that the specified outcomes appear with the respective
         numbers (0 means it didn't occur) in the text output from a test run.
 
@@ -420,19 +464,19 @@ class RunResult:
 
 
 class CwdSnapshot:
-    def __init__(self):
+    def __init__(self) -> None:
         self.__saved = os.getcwd()
 
-    def restore(self):
+    def restore(self) -> None:
         os.chdir(self.__saved)
 
 
 class SysModulesSnapshot:
-    def __init__(self, preserve=None):
+    def __init__(self, preserve: Optional[Callable[[str], bool]] = None):
         self.__preserve = preserve
         self.__saved = dict(sys.modules)
 
-    def restore(self):
+    def restore(self) -> None:
         if self.__preserve:
             self.__saved.update(
                 (k, m) for k, m in sys.modules.items() if self.__preserve(k)
@@ -442,10 +486,10 @@ class SysModulesSnapshot:
 
 
 class SysPathsSnapshot:
-    def __init__(self):
+    def __init__(self) -> None:
         self.__saved = list(sys.path), list(sys.meta_path)
 
-    def restore(self):
+    def restore(self) -> None:
         sys.path[:], sys.meta_path[:] = self.__saved
 
 
@@ -1357,7 +1401,7 @@ class LineMatcher:
         :param str match_nickname: the nickname for the match function that
             will be logged to stdout when a match occurs
         """
-        assert isinstance(lines2, Sequence)
+        assert isinstance(lines2, collections.abc.Sequence)
         lines2 = self._getlines(lines2)
         lines1 = self.lines[:]
         nextline = None
diff --git a/src/_pytest/warning_types.py b/src/_pytest/warning_types.py
index 80353ccbc..22cb17dba 100644
--- a/src/_pytest/warning_types.py
+++ b/src/_pytest/warning_types.py
@@ -1,6 +1,14 @@
+from typing import Any
+from typing import Generic
+from typing import TypeVar
+
 import attr
 
 
+if False:  # TYPE_CHECKING
+    from typing import Type  # noqa: F401 (used in type string)
+
+
 class PytestWarning(UserWarning):
     """
     Bases: :class:`UserWarning`.
@@ -72,7 +80,7 @@ class PytestExperimentalApiWarning(PytestWarning, FutureWarning):
     __module__ = "pytest"
 
     @classmethod
-    def simple(cls, apiname):
+    def simple(cls, apiname: str) -> "PytestExperimentalApiWarning":
         return cls(
             "{apiname} is an experimental api that may change over time".format(
                 apiname=apiname
@@ -103,17 +111,20 @@ class PytestUnknownMarkWarning(PytestWarning):
     __module__ = "pytest"
 
 
+_W = TypeVar("_W", bound=PytestWarning)
+
+
 @attr.s
-class UnformattedWarning:
+class UnformattedWarning(Generic[_W]):
     """Used to hold warnings that need to format their message at runtime, as opposed to a direct message.
 
     Using this class avoids to keep all the warning types and messages in this module, avoiding misuse.
     """
 
-    category = attr.ib()
-    template = attr.ib()
+    category = attr.ib(type="Type[_W]")
+    template = attr.ib(type=str)
 
-    def format(self, **kwargs):
+    def format(self, **kwargs: Any) -> _W:
         """Returns an instance of the warning category, formatted with given kwargs"""
         return self.category(self.template.format(**kwargs))