Use some more dogfooding (python#7027)

This PR does essentially two (related) things:
* `TYPE_CHECKING` is the official way for type-checking-only imports to avoid import cycles. This PR removes all uses of `if MYPY: ...` except for couple absolutely necessary files that are imported from `setup.py`.
* `typing_extensions` was designed in part as a drop-in replacement for `typing` on Python versions where some names are not yet available in `typing`.

I propose that mypy code adopts these itself. IMO this makes code look tidier (also it is 170 negative lines).

mypy/binder.py

@@ -1,10 +1,8 @@
-from typing import Dict, List, Set, Iterator, Union, Optional, Tuple, cast
 from contextlib import contextmanager
 from collections import defaultdict
 
-MYPY = False
-if MYPY:
-    from typing import DefaultDict
+from typing import Dict, List, Set, Iterator, Union, Optional, Tuple, cast
+from typing_extensions import DefaultDict
 
 from mypy.types import Type, AnyType, PartialType, UnionType, TypeOfAny, NoneType
 from mypy.subtypes import is_subtype
@@ -35,10 +33,7 @@ def __init__(self) -> None:
         self.unreachable = False
 
 
-if MYPY:
-    # This is the type of stored assignments for union type rvalues.
-    # We use 'if MYPY: ...' since typing-3.5.1 does not have 'DefaultDict'
-    Assigns = DefaultDict[Expression, List[Tuple[Type, Optional[Type]]]]
+Assigns = DefaultDict[Expression, List[Tuple[Type, Optional[Type]]]]
 
 
 class ConditionalTypeBinder:

mypy/build.py

@@ -25,11 +25,7 @@
 
 from typing import (AbstractSet, Any, Dict, Iterable, Iterator, List,
                     Mapping, NamedTuple, Optional, Set, Tuple, Union, Callable, TextIO)
-MYPY = False
-if MYPY:
-    from typing import ClassVar
-    from typing_extensions import Final
-
+from typing_extensions import ClassVar, Final, TYPE_CHECKING
 from mypy_extensions import TypedDict
 
 from mypy.nodes import MypyFile, ImportBase, Import, ImportFrom, ImportAll, SymbolTable
@@ -45,7 +41,7 @@
 from mypy.util import (
     DecodeError, decode_python_encoding, is_sub_path, get_mypy_comments, module_prefix
 )
-if MYPY:
+if TYPE_CHECKING:
     from mypy.report import Reports  # Avoid unconditional slow import
 from mypy import moduleinfo
 from mypy.fixup import fixup_module

mypy/checker.py

@@ -8,6 +8,7 @@
     Dict, Set, List, cast, Tuple, TypeVar, Union, Optional, NamedTuple, Iterator, Iterable,
     Sequence
 )
+from typing_extensions import Final
 
 from mypy.errors import Errors, report_internal_error
 from mypy.nodes import (
@@ -68,11 +69,6 @@
 from mypy import state
 from mypy.traverser import has_return_statement
 
-MYPY = False
-if MYPY:
-    from typing_extensions import Final
-
-
 T = TypeVar('T')
 
 DEFAULT_LAST_PASS = 1  # type: Final  # Pass numbers start at 0

mypy/checkexpr.py

@@ -5,10 +5,7 @@
 from typing import (
     cast, Dict, Set, List, Tuple, Callable, Union, Optional, Sequence, Iterator
 )
-MYPY = False
-if MYPY:
-    from typing import ClassVar
-    from typing_extensions import Final
+from typing_extensions import ClassVar, Final
 
 from mypy.errors import report_internal_error
 from mypy.typeanal import (

mypy/checkmember.py

@@ -1,6 +1,7 @@
 """Type checking of attribute access"""
 
 from typing import cast, Callable, List, Optional, TypeVar, Union
+from typing_extensions import TYPE_CHECKING
 
 from mypy.types import (
     Type, Instance, AnyType, TupleType, TypedDictType, CallableType, FunctionLike, TypeVarDef,
@@ -25,8 +26,7 @@
 from mypy import meet
 from mypy.typeops import tuple_fallback
 
-MYPY = False
-if MYPY:  # import for forward declaration only
+if TYPE_CHECKING:  # import for forward declaration only
     import mypy.checker
 
 from mypy import state

mypy/checkstrformat.py

@@ -3,6 +3,7 @@
 import re
 
 from typing import cast, List, Tuple, Dict, Callable, Union, Optional, Pattern
+from typing_extensions import Final, TYPE_CHECKING
 
 from mypy.types import (
     Type, AnyType, TupleType, Instance, UnionType, TypeOfAny
@@ -11,12 +12,10 @@
     StrExpr, BytesExpr, UnicodeExpr, TupleExpr, DictExpr, Context, Expression, StarExpr
 )
 
-MYPY = False
-if MYPY:
+if TYPE_CHECKING:
     # break import cycle only needed for mypy
     import mypy.checker
     import mypy.checkexpr
-    from typing_extensions import Final
 from mypy import message_registry
 from mypy.messages import MessageBuilder
 

mypy/config_parser.py

@@ -6,15 +6,11 @@
 import re
 import sys
 
-from mypy import defaults
-from mypy.options import Options, PER_MODULE_OPTIONS
-
 from typing import Any, Dict, List, Mapping, Optional, Tuple, TextIO
+from typing_extensions import Final
 
-
-MYPY = False
-if MYPY:
-    from typing_extensions import Final
+from mypy import defaults
+from mypy.options import Options, PER_MODULE_OPTIONS
 
 
 def parse_version(v: str) -> Tuple[int, int]:

mypy/constraints.py

@@ -1,6 +1,7 @@
 """Type inference constraints."""
 
 from typing import Iterable, List, Optional, Sequence
+from typing_extensions import Final
 
 from mypy.types import (
     CallableType, Type, TypeVisitor, UnboundType, AnyType, NoneType, TypeVarType, Instance,
@@ -15,11 +16,6 @@
 from mypy.nodes import COVARIANT, CONTRAVARIANT
 from mypy.argmap import ArgTypeExpander
 
-MYPY = False
-if MYPY:
-    from typing_extensions import Final
-
-
 SUBTYPE_OF = 0  # type: Final[int]
 SUPERTYPE_OF = 1  # type: Final[int]
 

mypy/defaults.py

@@ -1,8 +1,6 @@
 import os
 
-MYPY = False
-if MYPY:
-    from typing_extensions import Final
+from typing_extensions import Final
 
 PYTHON2_VERSION = (2, 7)  # type: Final
 PYTHON3_VERSION = (3, 6)  # type: Final

mypy/dmypy_server.py

@@ -17,6 +17,7 @@
 from contextlib import redirect_stderr, redirect_stdout
 
 from typing import AbstractSet, Any, Callable, Dict, List, Mapping, Optional, Sequence, Tuple
+from typing_extensions import Final
 
 import mypy.build
 import mypy.errors
@@ -33,11 +34,6 @@
 from mypy.typestate import reset_global_state
 from mypy.version import __version__
 
-
-MYPY = False
-if MYPY:
-    from typing_extensions import Final
-
 MEM_PROFILE = False  # type: Final  # If True, dump memory profile after initialization
 
 if sys.platform == 'win32':

mypy/dmypy_util.py

@@ -6,13 +6,10 @@
 import json
 
 from typing import Any
+from typing_extensions import Final
 
 from mypy.ipc import IPCBase
 
-MYPY = False
-if MYPY:
-    from typing_extensions import Final
-
 DEFAULT_STATUS_FILE = '.dmypy.json'  # type: Final
 
 

mypy/errors.py

@@ -4,15 +4,12 @@
 from collections import OrderedDict, defaultdict
 
 from typing import Tuple, List, TypeVar, Set, Dict, Optional, TextIO
+from typing_extensions import Final
 
 from mypy.scope import Scope
 from mypy.options import Options
 from mypy.version import __version__ as mypy_version
 
-MYPY = False
-if MYPY:
-    from typing_extensions import Final
-
 T = TypeVar('T')
 allowed_duplicates = ['@overload', 'Got:', 'Expected:']  # type: Final
 

mypy/fastparse.py

@@ -1,13 +1,11 @@
 import re
 import sys
 
+import typing  # for typing.Type, which conflicts with types.Type
 from typing import (
     Tuple, Union, TypeVar, Callable, Sequence, Optional, Any, Dict, cast, List, overload, Set
 )
-MYPY = False
-if MYPY:
-    import typing  # for typing.Type, which conflicts with types.Type
-    from typing_extensions import Final, Literal
+from typing_extensions import Final, Literal
 
 from mypy.sharedparse import (
     special_function_elide_names, argument_elide_name,

mypy/fastparse2.py

@@ -16,11 +16,9 @@
 """
 import sys
 
+import typing  # for typing.Type, which conflicts with types.Type
 from typing import Tuple, Union, TypeVar, Callable, Sequence, Optional, Any, Dict, cast, List, Set
-MYPY = False
-if MYPY:
-    import typing  # for typing.Type, which conflicts with types.Type
-    from typing_extensions import Final, Literal
+from typing_extensions import Final, Literal
 
 from mypy.sharedparse import (
     special_function_elide_names, argument_elide_name,

mypy/find_sources.py

@@ -3,15 +3,12 @@
 import os.path
 
 from typing import List, Sequence, Set, Tuple, Optional, Dict
+from typing_extensions import Final
 
 from mypy.modulefinder import BuildSource, PYTHON_EXTENSIONS
 from mypy.fscache import FileSystemCache
 from mypy.options import Options
 
-MYPY = False
-if MYPY:
-    from typing_extensions import Final
-
 PY_EXTENSIONS = tuple(PYTHON_EXTENSIONS)  # type: Final
 
 

mypy/interpreted_plugin.py

@@ -1,13 +1,14 @@
 """Hack for handling non-mypyc compiled plugins with a mypyc-compiled mypy"""
 
 from typing import Optional, Callable, Any, Dict, List, Tuple
+from typing_extensions import TYPE_CHECKING
+
 from mypy.options import Options
 from mypy.types import Type, CallableType
 from mypy.nodes import SymbolTableNode, MypyFile
 from mypy.lookup import lookup_fully_qualified
 
-MYPY = False
-if MYPY:
+if TYPE_CHECKING:
     import mypy.plugin
 
 

mypy/ipc.py

@@ -11,11 +11,7 @@
 import tempfile
 
 from typing import Optional, Callable
-
-MYPY = False
-if MYPY:
-    from typing import Type
-    from typing_extensions import Final
+from typing_extensions import Final, Type
 
 from types import TracebackType
 

mypy/literals.py

@@ -1,4 +1,5 @@
 from typing import Optional, Union, Any, Tuple, Iterable
+from typing_extensions import Final
 
 from mypy.nodes import (
     Expression, ComparisonExpr, OpExpr, MemberExpr, UnaryExpr, StarExpr, IndexExpr, LITERAL_YES,
@@ -11,10 +12,6 @@
 )
 from mypy.visitor import ExpressionVisitor
 
-MYPY = False
-if MYPY:
-    from typing_extensions import Final
-
 # [Note Literals and literal_hash]
 # ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 #

mypy/main.py

@@ -7,6 +7,7 @@
 import time
 
 from typing import Any, Dict, List, Optional, Tuple, TextIO
+from typing_extensions import Final
 
 from mypy import build
 from mypy import defaults
@@ -22,13 +23,7 @@
 
 from mypy.version import __version__
 
-MYPY = False
-if MYPY:
-    from typing_extensions import Final
-
-
 orig_stat = os.stat  # type: Final
-
 MEM_PROFILE = False  # type: Final  # If True, dump memory profile
 
 

mypy/message_registry.py

@@ -6,18 +6,12 @@
 add a method to MessageBuilder and call this instead.
 """
 
-MYPY = False
-if MYPY:
-    from typing_extensions import Final
-
+from typing_extensions import Final
 
 # Invalid types
-
 INVALID_TYPE_RAW_ENUM_VALUE = "Invalid type: try using Literal[{}.{}] instead?"  # type: Final
 
-
-# Type checker error message constants --
-
+# Type checker error message constants
 NO_RETURN_VALUE_EXPECTED = 'No return value expected'  # type: Final
 MISSING_RETURN_STATEMENT = 'Missing return statement'  # type: Final
 INVALID_IMPLICIT_RETURN = 'Implicit return in function which does not return'  # type: Final

mypy/messages.py

@@ -15,6 +15,7 @@
 from textwrap import dedent
 
 from typing import cast, List, Dict, Any, Sequence, Iterable, Tuple, Set, Optional, Union
+from typing_extensions import Final
 
 from mypy.erasetype import erase_type
 from mypy.errors import Errors
@@ -32,11 +33,6 @@
 from mypy.util import unmangle
 from mypy import message_registry
 
-MYPY = False
-if MYPY:
-    from typing_extensions import Final
-
-
 ARG_CONSTRUCTOR_NAMES = {
     ARG_POS: "Arg",
     ARG_OPT: "DefaultArg",

mypy/modulefinder.py

@@ -11,10 +11,7 @@
 import sys
 
 from typing import Dict, List, NamedTuple, Optional, Set, Tuple
-
-MYPY = False
-if MYPY:
-    from typing_extensions import Final
+from typing_extensions import Final
 
 from mypy.defaults import PYTHON3_VERSION_MIN
 from mypy.fscache import FileSystemCache