Add rudimentary mypy type checking

Add a very lax mypy configuration, add it to tox -e linting, and
fix/ignore the few errors that come up. The idea is to get it running
before diving in too much.

This enables:

- Progressively adding type annotations and enabling more strict
  options, which will improve the codebase (IMO).

- Annotating the public API in-line, and eventually exposing it to
  library users who use type checkers (with a py.typed file).

Though, none of this is done yet.

Refs https://github.com/pytest-dev/pytest/issues/3342.

.gitignore

@@ -35,6 +35,7 @@ env/
 .tox
 .cache
 .pytest_cache
+.mypy_cache
 .coverage
 .coverage.*
 coverage.xml

.pre-commit-config.yaml

@@ -28,6 +28,7 @@ repos:
     hooks:
     -   id: flake8
         language_version: python3
+        additional_dependencies: [flake8-typing-imports]
 -   repo: https://github.com/asottile/reorder_python_imports
     rev: v1.4.0
     hooks:
@@ -42,6 +43,17 @@ repos:
     rev: v1.4.0
     hooks:
     -   id: rst-backticks
+-   repo: https://github.com/pre-commit/mirrors-mypy
+    rev: v0.711
+    hooks:
+    -   id: mypy
+        name: mypy (src)
+        files: ^src/
+        args: []
+    -   id: mypy
+        name: mypy (testing)
+        files: ^testing/
+        args: []
 -   repo: local
     hooks:
     -   id: rst

bench/bench.py

@@ -6,7 +6,7 @@ if __name__ == "__main__":
     import pstats
 
     script = sys.argv[1:] if len(sys.argv) > 1 else ["empty.py"]
-    stats = cProfile.run("pytest.cmdline.main(%r)" % script, "prof")
+    cProfile.run("pytest.cmdline.main(%r)" % script, "prof")
     p = pstats.Stats("prof")
     p.strip_dirs()
     p.sort_stats("cumulative")

setup.cfg

@@ -61,3 +61,11 @@ ignore =
 
 [devpi:upload]
 formats = sdist.tgz,bdist_wheel
+
+[mypy]
+ignore_missing_imports = True
+no_implicit_optional = True
+strict_equality = True
+warn_redundant_casts = True
+warn_return_any = True
+warn_unused_configs = True

src/_pytest/_argcomplete.py

@@ -56,6 +56,7 @@ If things do not work right away:
 import os
 import sys
 from glob import glob
+from typing import Optional
 
 
 class FastFilesCompleter:
@@ -91,7 +92,7 @@ if os.environ.get("_ARGCOMPLETE"):
         import argcomplete.completers
     except ImportError:
         sys.exit(-1)
-    filescompleter = FastFilesCompleter()
+    filescompleter = FastFilesCompleter()  # type: Optional[FastFilesCompleter]
 
     def try_argcomplete(parser):
         argcomplete.autocomplete(parser, always_complete_options=False)

src/_pytest/_code/code.py

@@ -33,7 +33,8 @@ class Code:
     def __eq__(self, other):
         return self.raw == other.raw
 
-    __hash__ = None
+    # Ignore type because of https://github.com/python/mypy/issues/4266.
+    __hash__ = None  # type: ignore
 
     def __ne__(self, other):
         return not self == other
@@ -188,11 +189,11 @@ class TracebackEntry:
         """ path to the source code """
         return self.frame.code.path
 
-    def getlocals(self):
+    @property
+    def locals(self):
+        """ locals of underlaying frame """
         return self.frame.f_locals
 
-    locals = property(getlocals, None, None, "locals of underlaying frame")
-
     def getfirstlinesource(self):
         return self.frame.code.firstlineno
 
@@ -255,11 +256,11 @@ class TracebackEntry:
             line = "???"
         return "  File %r:%d in %s\n  %s\n" % (fn, self.lineno + 1, name, line)
 
+    @property
     def name(self):
+        """ co_name of underlaying code """
         return self.frame.code.raw.co_name
 
-    name = property(name, None, None, "co_name of underlaying code")
-
 
 class Traceback(list):
     """ Traceback objects encapsulate and offer higher level

src/_pytest/_code/source.py

@@ -44,7 +44,8 @@ class Source:
                 return str(self) == other
             return False
 
-    __hash__ = None
+    # Ignore type because of https://github.com/python/mypy/issues/4266.
+    __hash__ = None  # type: ignore
 
     def __getitem__(self, key):
         if isinstance(key, int):

src/_pytest/assertion/rewrite.py

@@ -12,6 +12,10 @@ import struct
 import sys
 import tokenize
 import types
+from typing import Dict
+from typing import List
+from typing import Optional
+from typing import Set
 
 import atomicwrites
 
@@ -459,17 +463,18 @@ def set_location(node, lineno, col_offset):
     return node
 
 
-def _get_assertion_exprs(src: bytes):  # -> Dict[int, str]
+def _get_assertion_exprs(src: bytes) -> Dict[int, str]:
     """Returns a mapping from {lineno: "assertion test expression"}"""
-    ret = {}
+    ret = {}  # type: Dict[int, str]
 
     depth = 0
-    lines = []
-    assert_lineno = None
-    seen_lines = set()
+    lines = []  # type: List[str]
+    assert_lineno = None  # type: Optional[int]
+    seen_lines = set()  # type: Set[int]
 
     def _write_and_reset() -> None:
         nonlocal depth, lines, assert_lineno, seen_lines
+        assert assert_lineno is not None
         ret[assert_lineno] = "".join(lines).rstrip().rstrip("\\")
         depth = 0
         lines = []
@@ -477,21 +482,21 @@ def _get_assertion_exprs(src: bytes):  # -> Dict[int, str]
         seen_lines = set()
 
     tokens = tokenize.tokenize(io.BytesIO(src).readline)
-    for tp, src, (lineno, offset), _, line in tokens:
-        if tp == tokenize.NAME and src == "assert":
+    for tp, source, (lineno, offset), _, line in tokens:
+        if tp == tokenize.NAME and source == "assert":
             assert_lineno = lineno
         elif assert_lineno is not None:
             # keep track of depth for the assert-message `,` lookup
-            if tp == tokenize.OP and src in "([{":
+            if tp == tokenize.OP and source in "([{":
                 depth += 1
-            elif tp == tokenize.OP and src in ")]}":
+            elif tp == tokenize.OP and source in ")]}":
                 depth -= 1
 
             if not lines:
                 lines.append(line[offset:])
                 seen_lines.add(lineno)
             # a non-nested comma separates the expression from the message
-            elif depth == 0 and tp == tokenize.OP and src == ",":
+            elif depth == 0 and tp == tokenize.OP and source == ",":
                 # one line assert with message
                 if lineno in seen_lines and len(lines) == 1:
                     offset_in_trimmed = offset + len(lines[-1]) - len(line)

src/_pytest/capture.py

@@ -547,6 +547,8 @@ class FDCaptureBinary:
             self.start = lambda: None
             self.done = lambda: None
         else:
+            self.start = self._start
+            self.done = self._done
             if targetfd == 0:
                 assert not tmpfile, "cannot set tmpfile with stdin"
                 tmpfile = open(os.devnull, "r")
@@ -568,7 +570,7 @@ class FDCaptureBinary:
             self.targetfd, getattr(self, "targetfd_save", None), self._state
         )
 
-    def start(self):
+    def _start(self):
         """ Start capturing on targetfd using memorized tmpfile. """
         try:
             os.fstat(self.targetfd_save)
@@ -585,7 +587,7 @@ class FDCaptureBinary:
         self.tmpfile.truncate()
         return res
 
-    def done(self):
+    def _done(self):
         """ stop capturing, restore streams, return original capture file,
         seeked to position zero. """
         targetfd_save = self.__dict__.pop("targetfd_save")
@@ -618,7 +620,8 @@ class FDCapture(FDCaptureBinary):
     snap() produces text
     """
 
-    EMPTY_BUFFER = str()
+    # Ignore type because it doesn't match the type in the superclass (bytes).
+    EMPTY_BUFFER = str()  # type: ignore
 
     def snap(self):
         res = super().snap()
@@ -679,7 +682,8 @@ class SysCapture:
 
 
 class SysCaptureBinary(SysCapture):
-    EMPTY_BUFFER = b""
+    # Ignore type because it doesn't match the type in the superclass (str).
+    EMPTY_BUFFER = b""  # type: ignore
 
     def snap(self):
         res = self.tmpfile.buffer.getvalue()

src/_pytest/debugging.py

@@ -74,7 +74,7 @@ class pytestPDB:
 
     _pluginmanager = None
     _config = None
-    _saved = []
+    _saved = []  # type: list
     _recursive_debug = 0
     _wrapped_pdb_cls = None
 

src/_pytest/fixtures.py

@@ -6,6 +6,8 @@ import warnings
 from collections import defaultdict
 from collections import deque
 from collections import OrderedDict
+from typing import Dict
+from typing import Tuple
 
 import attr
 import py
@@ -31,6 +33,9 @@ from _pytest.deprecated import FIXTURE_NAMED_REQUEST
 from _pytest.outcomes import fail
 from _pytest.outcomes import TEST_OUTCOME
 
+if False:  # TYPE_CHECKING
+    from typing import Type
+
 
 @attr.s(frozen=True)
 class PseudoFixtureDef:
@@ -54,10 +59,10 @@ def pytest_sessionstart(session):
     session._fixturemanager = FixtureManager(session)
 
 
-scopename2class = {}
+scopename2class = {}  # type: Dict[str, Type[nodes.Node]]
 
 
-scope2props = dict(session=())
+scope2props = dict(session=())  # type: Dict[str, Tuple[str, ...]]
 scope2props["package"] = ("fspath",)
 scope2props["module"] = ("fspath", "module")
 scope2props["class"] = scope2props["module"] + ("cls",)
@@ -960,7 +965,8 @@ class FixtureFunctionMarker:
     scope = attr.ib()
     params = attr.ib(converter=attr.converters.optional(tuple))
     autouse = attr.ib(default=False)
-    ids = attr.ib(default=None, converter=_ensure_immutable_ids)
+    # Ignore type because of https://github.com/python/mypy/issues/6172.
+    ids = attr.ib(default=None, converter=_ensure_immutable_ids)  # type: ignore
     name = attr.ib(default=None)
 
     def __call__(self, function):

src/_pytest/mark/__init__.py

@@ -91,7 +91,8 @@ def pytest_cmdline_main(config):
         return 0
 
 
-pytest_cmdline_main.tryfirst = True
+# Ignore type because of https://github.com/python/mypy/issues/2087.
+pytest_cmdline_main.tryfirst = True  # type: ignore
 
 
 def deselect_by_keyword(items, config):

src/_pytest/mark/structures.py

@@ -3,6 +3,7 @@ import warnings
 from collections import namedtuple
 from collections.abc import MutableMapping
 from operator import attrgetter
+from typing import Set
 
 import attr
 
@@ -298,7 +299,7 @@ class MarkGenerator:
     on the ``test_function`` object. """
 
     _config = None
-    _markers = set()
+    _markers = set()  # type: Set[str]
 
     def __getattr__(self, name):
         if name[0] == "_":

src/_pytest/nodes.py

@@ -280,7 +280,8 @@ class Node:
             truncate_locals=truncate_locals,
         )
 
-    repr_failure = _repr_failure_py
+    def repr_failure(self, excinfo, style=None):
+        return self._repr_failure_py(excinfo, style)
 
 
 def get_fslocation_from_item(item):

src/_pytest/outcomes.py

@@ -70,7 +70,8 @@ def exit(msg, returncode=None):
     raise Exit(msg, returncode)
 
 
-exit.Exception = Exit
+# Ignore type because of https://github.com/python/mypy/issues/2087.
+exit.Exception = Exit  # type: ignore
 
 
 def skip(msg="", *, allow_module_level=False):
@@ -96,7 +97,8 @@ def skip(msg="", *, allow_module_level=False):
     raise Skipped(msg=msg, allow_module_level=allow_module_level)
 
 
-skip.Exception = Skipped
+# Ignore type because of https://github.com/python/mypy/issues/2087.
+skip.Exception = Skipped  # type: ignore
 
 
 def fail(msg="", pytrace=True):
@@ -111,7 +113,8 @@ def fail(msg="", pytrace=True):
     raise Failed(msg=msg, pytrace=pytrace)
 
 
-fail.Exception = Failed
+# Ignore type because of https://github.com/python/mypy/issues/2087.
+fail.Exception = Failed  # type: ignore
 
 
 class XFailed(Failed):
@@ -132,7 +135,8 @@ def xfail(reason=""):
     raise XFailed(reason)
 
 
-xfail.Exception = XFailed
+# Ignore type because of https://github.com/python/mypy/issues/2087.
+xfail.Exception = XFailed  # type: ignore
 
 
 def importorskip(modname, minversion=None, reason=None):

src/_pytest/python_api.py

@@ -9,6 +9,7 @@ from collections.abc import Sized
 from decimal import Decimal
 from itertools import filterfalse
 from numbers import Number
+from typing import Union
 
 from more_itertools.more import always_iterable
 
@@ -58,7 +59,8 @@ class ApproxBase:
             a == self._approx_scalar(x) for a, x in self._yield_comparisons(actual)
         )
 
-    __hash__ = None
+    # Ignore type because of https://github.com/python/mypy/issues/4266.
+    __hash__ = None  # type: ignore
 
     def __ne__(self, actual):
         return not (actual == self)
@@ -202,8 +204,10 @@ class ApproxScalar(ApproxBase):
     Perform approximate comparisons where the expected value is a single number.
     """
 
-    DEFAULT_ABSOLUTE_TOLERANCE = 1e-12
-    DEFAULT_RELATIVE_TOLERANCE = 1e-6
+    # Using Real should be better than this Union, but not possible yet:
+    # https://github.com/python/typeshed/pull/3108
+    DEFAULT_ABSOLUTE_TOLERANCE = 1e-12  # type: Union[float, Decimal]
+    DEFAULT_RELATIVE_TOLERANCE = 1e-6  # type: Union[float, Decimal]
 
     def __repr__(self):
         """
@@ -261,7 +265,8 @@ class ApproxScalar(ApproxBase):
         # Return true if the two numbers are within the tolerance.
         return abs(self.expected - actual) <= self.tolerance
 
-    __hash__ = None
+    # Ignore type because of https://github.com/python/mypy/issues/4266.
+    __hash__ = None  # type: ignore
 
     @property
     def tolerance(self):
@@ -691,7 +696,7 @@ def raises(expected_exception, *args, **kwargs):
     fail(message)
 
 
-raises.Exception = fail.Exception
+raises.Exception = fail.Exception  # type: ignore
 
 
 class RaisesContext:

src/_pytest/reports.py

@@ -1,4 +1,5 @@
 from pprint import pprint
+from typing import Optional
 
 import py
 
@@ -28,7 +29,7 @@ def getslaveinfoline(node):
 
 
 class BaseReport:
-    when = None
+    when = None  # type: Optional[str]
     location = None
 
     def __init__(self, **kw):

src/_pytest/tmpdir.py

@@ -26,7 +26,10 @@ class TempPathFactory:
         # using os.path.abspath() to get absolute path instead of resolve() as it
         # does not work the same in all platforms (see #4427)
         # Path.absolute() exists, but it is not public (see https://bugs.python.org/issue25012)
-        converter=attr.converters.optional(lambda p: Path(os.path.abspath(str(p))))
+        # Ignore type because of https://github.com/python/mypy/issues/6172.
+        converter=attr.converters.optional(
+            lambda p: Path(os.path.abspath(str(p)))  # type: ignore
+        )
     )
     _trace = attr.ib()
     _basetemp = attr.ib(default=None)

testing/code/test_excinfo.py

@@ -589,7 +589,8 @@ raise ValueError()
 
     def test_repr_local_with_exception_in_class_property(self):
         class ExceptionWithBrokenClass(Exception):
-            @property
+            # Type ignored because it's bypassed intentionally.
+            @property  # type: ignore
             def __class__(self):
                 raise TypeError("boom!")
 

testing/python/raises.py

@@ -265,7 +265,8 @@ class TestRaises:
         """Test current behavior with regard to exceptions via __class__ (#4284)."""
 
         class CrappyClass(Exception):
-            @property
+            # Type ignored because it's bypassed intentionally.
+            @property  # type: ignore
             def __class__(self):
                 assert False, "via __class__"
 

testing/test_compat.py

@@ -141,7 +141,8 @@ def test_safe_isclass():
     assert safe_isclass(type) is True
 
     class CrappyClass(Exception):
-        @property
+        # Type ignored because it's bypassed intentionally.
+        @property  # type: ignore
         def __class__(self):
             assert False, "Should be ignored"
 

testing/test_pdb.py

@@ -6,7 +6,8 @@ import pytest
 from _pytest.debugging import _validate_usepdb_cls
 
 try:
-    breakpoint
+    # Type ignored for Python <= 3.6.
+    breakpoint  # type: ignore
 except NameError:
     SUPPORTS_BREAKPOINT_BUILTIN = False
 else: