Small changes + added docs

docs/source/kinds_of_types.rst

@@ -241,6 +241,95 @@ more specific type:
     since the caller may have to use :py:func:`isinstance` before doing anything
     interesting with the value.
 
+.. _alternative_union_syntax:
+
+Alternative union syntax
+------------------------
+
+`PEP 604 <https://www.python.org/dev/peps/pep-0604/>`_ introduced an alternative way
+for writing union types. Starting with **Python 3.10** it is possible to write
+``Union[int, str]`` as ``int | str``. Any of the following options is possible
+
+.. code-block:: python
+
+    from typing import List
+
+    # Use as Union
+    t1: int | str  # equivalent to Union[int, str]
+
+    # Use as Optional
+    t2: int | None  # equivalent to Optional[int]
+
+    # Use in generics
+    t3: List[int | str]  # equivalent to List[Union[int, str]]
+
+    # Use in type aliases
+    T4 = int | None
+    x: T4
+
+    # Quoted variable annotations
+    t5: "int | str"
+
+    # Quoted function annotations
+    def f(t6: "int | str") -> None: ...
+
+    # Type comments
+    t6 = 42  # type: int | str
+
+It is possible to use most of these even for earlier versions. However there are some
+limitations to be aware of.
+
+.. _alternative_union_syntax_stub_files:
+
+Stub files
+""""""""""
+
+All options are supported, regardless of the Python version the project uses.
+
+.. _alternative_union_syntax_37:
+
+Python 3.7 - 3.9
+""""""""""""""""
+
+It is necessary to add ``from __future__ import annotations`` to delay the evaluation
+of type annotations. Not using it would result in a ``TypeError``.
+This does not apply for **type comments**, **quoted function** and **quoted variable** annotations,
+as those also work for earlier versions, see :ref:`below <alternative_union_syntax_older_version>`.
+
+.. warning::
+
+    Type aliases are **NOT** supported! Those result in a ``TypeError`` regardless
+    if the evaluation of type annotations is delayed.
+
+    Dynamic evaluation of annotations is **NOT** possible (e.g. ``typing.get_type_hints`` and ``eval``).
+    See `note PEP 604 <https://www.python.org/dev/peps/pep-0604/#change-only-pep-484-type-hints-to-accept-the-syntax-type1-type2>`_. Use ``typing.Union`` instead!
+
+.. code-block:: python
+
+    from __future__ import annotations
+
+    t1: int | None
+
+    # Type aliases
+    T2 = int | None  # TypeError!
+
+.. _alternative_union_syntax_older_version:
+
+Older versions
+""""""""""""""
+
++------------------------------------------+-----------+-----------+-----------+
+| Python Version                           | 3.6       | 3.0 - 3.5 | 2.7       |
++==========================================+===========+===========+===========+
+| Type comments                            | yes       | yes       | yes       |
++------------------------------------------+-----------+-----------+-----------+
+| Quoted function annotations              | yes       | yes       |           |
++------------------------------------------+-----------+-----------+-----------+
+| Quoted variable annotations              | yes       |           |           |
++------------------------------------------+-----------+-----------+-----------+
+| Everything else                          |           |           |           |
++------------------------------------------+-----------+-----------+-----------+
+
 .. _strict_optional:
 
 Optional types and the None type

mypy/fastparse.py

@@ -242,7 +242,7 @@ def parse_type_comment(type_comment: str,
                                   line=line,
                                   override_column=column,
                                   assume_str_is_unicode=assume_str_is_unicode,
-                                  is_type_comment=True).visit(typ.body)
+                                  is_evaluated=False).visit(typ.body)
         return ignored, converted
 
 
@@ -1279,14 +1279,14 @@ def __init__(self,
                  line: int = -1,
                  override_column: int = -1,
                  assume_str_is_unicode: bool = True,
-                 is_type_comment: bool = False,
+                 is_evaluated: bool = True,
                  ) -> None:
         self.errors = errors
         self.line = line
         self.override_column = override_column
         self.node_stack = []  # type: List[AST]
         self.assume_str_is_unicode = assume_str_is_unicode
-        self.is_type_comment = is_type_comment
+        self.is_evaluated = is_evaluated
 
     def convert_column(self, column: int) -> int:
         """Apply column override if defined; otherwise return column.
@@ -1436,7 +1436,7 @@ def visit_BinOp(self, n: ast3.BinOp) -> Type:
         return UnionType([left, right],
                          line=self.line,
                          column=self.convert_column(n.col_offset),
-                         is_evaluated=(not self.is_type_comment),
+                         is_evaluated=self.is_evaluated,
                          uses_pep604_syntax=True)
 
     def visit_NameConstant(self, n: NameConstant) -> Type:

test-data/unit/check-union-or-syntax.test

@@ -74,21 +74,41 @@ reveal_type(x)  # N: Revealed type is 'builtins.list[Union[builtins.int, builtin
 [builtins fixtures/list.pyi]
 
 
-[case testUnionOrSyntaxWithQuotedTypes]
-# flags: --python-version 3.10
+[case testUnionOrSyntaxWithQuotedFunctionTypes]
+# flags: --python-version 3.4
 from typing import Union
 def f(x: 'Union[int, str, None]') -> 'Union[int, None]':
     reveal_type(x)  # N: Revealed type is 'Union[builtins.int, builtins.str, None]'
     return 42
 reveal_type(f)  # N: Revealed type is 'def (x: Union[builtins.int, builtins.str, None]) -> Union[builtins.int, None]'
 
-# flags: --python-version 3.10
 def g(x: "int | str | None") -> "int | None":
     reveal_type(x)  # N: Revealed type is 'Union[builtins.int, builtins.str, None]'
     return 42
 reveal_type(g)  # N: Revealed type is 'def (x: Union[builtins.int, builtins.str, None]) -> Union[builtins.int, None]'
 
 
+[case testUnionOrSyntaxWithQuotedVariableTypes]
+# flags: --python-version 3.6
+y: "int | str" = 42
+reveal_type(y)  # N: Revealed type is 'Union[builtins.int, builtins.str]'
+
+
+[case testUnionOrSyntaxWithTypeAliasWorking]
+# flags: --python-version 3.10
+from typing import Union
+T = Union[int, str]
+x: T
+reveal_type(x)  # N: Revealed type is 'Union[builtins.int, builtins.str]'
+
+
+[case testUnionOrSyntaxWithTypeAliasNotAllowed]
+# flags: --python-version 3.9
+from __future__ import annotations
+T = int | str  # E: Unsupported left operand type for | ("Type[int]")
+[builtins fixtures/tuple.pyi]
+
+
 [case testUnionOrSyntaxInComment]
 # flags: --python-version 3.6
 x = 1  # type: int | str