Drop support for running with Python 3.9

.github/workflows/test.yml

@@ -31,23 +31,18 @@ jobs:
         include:
         # Make sure to run mypyc compiled unit tests for both
         # the oldest and newest supported Python versions
-        - name: Test suite with py39-ubuntu, mypyc-compiled
-          python: '3.9'
-          os: ubuntu-24.04-arm
-          toxenv: py
-          tox_extra_args: "-n 4"
-          test_mypyc: true
-        - name: Test suite with py310-ubuntu
+        - name: Test suite with py310-ubuntu, mypyc-compiled
           python: '3.10'
           os: ubuntu-24.04-arm
           toxenv: py
           tox_extra_args: "-n 4"
+          test_mypyc: true
         - name: Test suite with py311-ubuntu
           python: '3.11'
           os: ubuntu-24.04-arm
           toxenv: py
           tox_extra_args: "-n 4"
-        - name: Test suite with py312-ubuntu, mypyc-compiled
+        - name: Test suite with py312-ubuntu
           python: '3.12'
           os: ubuntu-24.04-arm
           toxenv: py
@@ -101,12 +96,12 @@ jobs:
         #   tox_extra_args: "-n 4 mypyc/test/test_run.py mypyc/test/test_external.py"
         #   debug_build: true
 
-        - name: Type check our own code (py39-ubuntu)
-          python: '3.9'
+        - name: Type check our own code (py310-ubuntu)
+          python: '3.10'
           os: ubuntu-latest
           toxenv: type
-        - name: Type check our own code (py39-windows-64)
-          python: '3.9'
+        - name: Type check our own code (py310-windows-64)
+          python: '3.10'
           os: windows-latest
           toxenv: type
 

.github/workflows/test_stubgenc.yml

@@ -32,10 +32,10 @@ jobs:
       with:
         persist-credentials: false
 
-    - name: Setup 🐍 3.9
+    - name: Setup 🐍 3.10
       uses: actions/setup-python@v5
       with:
-        python-version: 3.9
+        python-version: '3.10'
 
     - name: Test stubgenc
       run: misc/test-stubgenc.sh

CHANGELOG.md

@@ -2,6 +2,15 @@
 
 ## Next Release
 
+### Drop Support for Python 3.9
+
+Mypy no longer supports running with Python 3.9, which has reached end-of-life.
+When running mypy with Python 3.10+, it is still possible to type check code
+that needs to support Python 3.9 with the `--python-version 3.9` argument.
+Support for this will be dropped in the first half of 2026!
+
+Contributed by Marc Mueller (PR [20156](https://github.com/python/mypy/pull/20156)).
+
 ## Mypy 1.18.1
 
 We’ve just uploaded mypy 1.18.1 to the Python Package Index ([PyPI](https://pypi.org/project/mypy/)).

CONTRIBUTING.md

@@ -51,7 +51,7 @@ hash -r  # This resets shell PATH cache, not necessary on Windows
 ```
 
 > **Note**
-> You'll need Python 3.9 or higher to install all requirements listed in
+> You'll need Python 3.10 or higher to install all requirements listed in
 > test-requirements.txt
 
 ### Running tests

docs/source/getting_started.rst

@@ -16,7 +16,7 @@ may not make much sense otherwise.
 Installing and running mypy
 ***************************
 
-Mypy requires Python 3.9 or later to run.  You can install mypy using pip:
+Mypy requires Python 3.10 or later to run.  You can install mypy using pip:
 
 .. code-block:: shell
 

mypy/defaults.py

@@ -6,7 +6,7 @@
 # Earliest fully supported Python 3.x version. Used as the default Python
 # version in tests. Mypy wheels should be built starting with this version,
 # and CI tests should be run on this version (and later versions).
-PYTHON3_VERSION: Final = (3, 9)
+PYTHON3_VERSION: Final = (3, 10)
 
 # Earliest Python 3.x version supported via --python-version 3.x. To run
 # mypy, at least version PYTHON3_VERSION is needed.

mypy/test/meta/test_parse_data.py

@@ -50,13 +50,15 @@ def test_bad_ge_version_check(self) -> None:
             """
             [case abc]
             s: str
-            [out version>=3.9]
+            [out version>=3.10]
             abc
             """
         )
 
         # Assert
-        assert "version>=3.9 always true since minimum runtime version is (3, 9)" in actual.stdout
+        assert (
+            "version>=3.10 always true since minimum runtime version is (3, 10)" in actual.stdout
+        )
 
     def test_bad_eq_version_check(self) -> None:
         # Act
@@ -70,4 +72,6 @@ def test_bad_eq_version_check(self) -> None:
         )
 
         # Assert
-        assert "version==3.7 always false since minimum runtime version is (3, 9)" in actual.stdout
+        assert (
+            "version==3.7 always false since minimum runtime version is (3, 10)" in actual.stdout
+        )

mypy/test/testpythoneval.py

@@ -71,6 +71,9 @@ def test_python_evaluation(testcase: DataDrivenTestCase, cache_dir: str) -> None
                     return
         mypy_cmdline.extend(additional_flags)
 
+    if "--no-force-union-syntax" not in mypy_cmdline:
+        mypy_cmdline.append("--force-union-syntax")
+
     # Write the program to a file.
     program = "_" + testcase.name + ".py"
     program_path = os.path.join(test_temp_dir, program)

mypy/util.py

@@ -482,10 +482,10 @@ def get_unique_redefinition_name(name: str, existing: Container[str]) -> str:
 def check_python_version(program: str) -> None:
     """Report issues with the Python used to run mypy, dmypy, or stubgen"""
     # Check for known bad Python versions.
-    if sys.version_info[:2] < (3, 9):  # noqa: UP036, RUF100
+    if sys.version_info[:2] < (3, 10):  # noqa: UP036, RUF100
         sys.exit(
-            "Running {name} with Python 3.8 or lower is not supported; "
-            "please upgrade to 3.9 or newer".format(name=program)
+            "Running {name} with Python 3.9 or lower is not supported; "
+            "please upgrade to 3.10 or newer".format(name=program)
         )
 
 

mypy_self_check.ini

@@ -7,7 +7,7 @@ show_traceback = True
 pretty = True
 always_false = MYPYC
 plugins = mypy.plugins.proper_plugin
-python_version = 3.9
+python_version = 3.10
 exclude = mypy/typeshed/|mypyc/test-data/|mypyc/lib-rt/
 enable_error_code = ignore-without-code,redundant-expr
 enable_incomplete_feature = PreciseTupleTypes

mypyc/doc/getting_started.rst

@@ -38,7 +38,7 @@ Installation
 ------------
 
 Mypyc is shipped as part of the mypy distribution. Install mypy like
-this (you need Python 3.9 or later):
+this (you need Python 3.10 or later):
 
 .. code-block::
 

pyproject.toml

@@ -38,7 +38,6 @@ classifiers = [
   "Intended Audience :: Developers",
   "License :: OSI Approved :: MIT License",
   "Programming Language :: Python :: 3",
-  "Programming Language :: Python :: 3.9",
   "Programming Language :: Python :: 3.10",
   "Programming Language :: Python :: 3.11",
   "Programming Language :: Python :: 3.12",
@@ -47,7 +46,7 @@ classifiers = [
   "Topic :: Software Development",
   "Typing :: Typed",
 ]
-requires-python = ">=3.9"
+requires-python = ">=3.10"
 dependencies = [
   # When changing this, also update build-system.requires and mypy-requirements.txt
   "typing_extensions>=4.6.0",
@@ -107,7 +106,7 @@ mypyc = [
 
 [tool.black]
 line-length = 99
-target-version = ["py39", "py310", "py311", "py312", "py313", "py314"]
+target-version = ["py310", "py311", "py312", "py313", "py314"]
 skip-magic-trailing-comma = true
 force-exclude = '''
 ^/mypy/typeshed|

setup.py

@@ -8,8 +8,8 @@
 import sys
 from typing import TYPE_CHECKING, Any
 
-if sys.version_info < (3, 9, 0):  # noqa: UP036, RUF100
-    sys.stderr.write("ERROR: You need Python 3.9 or later to use mypy.\n")
+if sys.version_info < (3, 10, 0):  # noqa: UP036, RUF100
+    sys.stderr.write("ERROR: You need Python 3.10 or later to use mypy.\n")
     exit(1)
 
 # we'll import stuff from the source tree, let's ensure is on the sys path

test-data/unit/cmdline.test

@@ -1038,10 +1038,6 @@ public static void main(String[] args)
 [file pkg/y.py]
 x: str = 0
 [out]
-pkg/x.py:1: error: Invalid syntax
-Found 1 error in 1 file (errors prevented further checking)
-== Return code: 2
-[out version>=3.10]
 pkg/x.py:1: error: Invalid syntax. Perhaps you forgot a comma?
 Found 1 error in 1 file (errors prevented further checking)
 == Return code: 2

test-data/unit/fine-grained-blockers.test

@@ -19,12 +19,6 @@ def f(x: int) -> None: pass
 def f() -> None: pass
 [out]
 ==
-a.py:1: error: Invalid syntax
-==
-main:2: error: Missing positional argument "x" in call to "f"
-==
-[out version>=3.10]
-==
 a.py:1: error: Expected ':'
 ==
 main:2: error: Missing positional argument "x" in call to "f"
@@ -44,16 +38,6 @@ def f(x: int) -> None: pass
 def f() -> None: pass
 [out]
 ==
-a.py:1: error: Invalid syntax  [syntax]
-    def f(x: int) ->
-                    ^
-==
-main:3: error: Missing positional argument "x" in call to "f"  [call-arg]
-    a.f()
-    ^~~~~
-==
-[out version>=3.10]
-==
 a.py:1: error: Expected ':'  [syntax]
     def f(x: int) ->
                    ^
@@ -77,13 +61,6 @@ def f(x: int
 def f(x: int) -> None: pass
 [out]
 ==
-a.py:1: error: Invalid syntax
-==
-a.py:2: error: Invalid syntax
-==
-main:2: error: Missing positional argument "x" in call to "f"
-[out version>=3.10]
-==
 a.py:1: error: Expected ':'
 ==
 a.py:2: error: Expected ':'
@@ -124,14 +101,6 @@ def f() -> None: pass
 main:3: error: Too many arguments for "f"
 main:5: error: Too many arguments for "f"
 ==
-a.py:1: error: Invalid syntax
-==
-main:3: error: Too many arguments for "f"
-main:5: error: Too many arguments for "f"
-[out version>=3.10]
-main:3: error: Too many arguments for "f"
-main:5: error: Too many arguments for "f"
-==
 a.py:1: error: Expected ':'
 ==
 main:3: error: Too many arguments for "f"

test-data/unit/fine-grained-suggest.test

@@ -1117,11 +1117,6 @@ def foo():
 
 (
 [out]
-foo.py:4: error: Unexpected EOF while parsing
-Command 'suggest' is only valid after a 'check' command (that produces no parse errors)
-==
-foo.py:4: error: Unexpected EOF while parsing
-[out version>=3.10]
 foo.py:4: error: '(' was never closed
 Command 'suggest' is only valid after a 'check' command (that produces no parse errors)
 ==

test-data/unit/parse-errors.test

@@ -154,8 +154,6 @@ file:2: error: Syntax error in type comment "A B"
 [case testMissingBracket]
 def foo(
 [out]
-file:1: error: Unexpected EOF while parsing
-[out version>=3.10]
 file:1: error: '(' was never closed
 
 [case testInvalidSignatureInComment1]

test-data/unit/pythoneval.test

@@ -1651,8 +1651,8 @@ def foo(x: T) -> T:
     return x
 [out]
 _testTypeAliasWithNewStyleUnion.py:5: note: Revealed type is "typing._SpecialForm"
-_testTypeAliasWithNewStyleUnion.py:25: note: Revealed type is "type[builtins.int] | builtins.str"
-_testTypeAliasWithNewStyleUnion.py:28: note: Revealed type is "type[builtins.int] | builtins.str"
+_testTypeAliasWithNewStyleUnion.py:25: note: Revealed type is "Union[type[builtins.int], builtins.str]"
+_testTypeAliasWithNewStyleUnion.py:28: note: Revealed type is "Union[type[builtins.int], builtins.str]"
 
 [case testTypeAliasWithNewStyleUnionInStub]
 import m
@@ -1704,7 +1704,7 @@ CU3 = int | Callable[[str | bool], str]
 CU4: TypeAlias = int | Callable[[str | bool], str]
 [out]
 m.pyi:5: note: Revealed type is "typing._SpecialForm"
-m.pyi:22: note: Revealed type is "typing._SpecialForm"
+m.pyi:22: note: Revealed type is "types.UnionType[type[builtins.int], builtins.str]"
 _testTypeAliasWithNewStyleUnionInStub.py:3: note: Revealed type is "Union[type[builtins.int], builtins.str]"
 _testTypeAliasWithNewStyleUnionInStub.py:5: note: Revealed type is "Union[type[builtins.int], builtins.str]"
 _testTypeAliasWithNewStyleUnionInStub.py:7: note: Revealed type is "Union[type[builtins.int], builtins.str]"
@@ -1730,7 +1730,7 @@ reveal_type(e.foo)
 reveal_type(E.Y.foo)
 [out]
 _testEnumNameWorkCorrectlyOn311.py:11: note: Revealed type is "builtins.str"
-_testEnumNameWorkCorrectlyOn311.py:12: note: Revealed type is "Literal[1]? | Literal[2]?"
+_testEnumNameWorkCorrectlyOn311.py:12: note: Revealed type is "Union[Literal[1]?, Literal[2]?]"
 _testEnumNameWorkCorrectlyOn311.py:13: note: Revealed type is "Literal['X']?"
 _testEnumNameWorkCorrectlyOn311.py:14: note: Revealed type is "builtins.int"
 _testEnumNameWorkCorrectlyOn311.py:15: note: Revealed type is "builtins.int"
@@ -1799,9 +1799,9 @@ WrongEllipsis = tuple[float, float, ...] | str  # Error
 
 reveal_type(tuple[int, str]((1, "x")))
 [out]
-_testTupleWithDifferentArgsPy310.py:15: note: Revealed type is "builtins.str | tuple[builtins.float, builtins.float, builtins.str]"
-_testTupleWithDifferentArgsPy310.py:16: note: Revealed type is "tuple[builtins.float] | builtins.str"
-_testTupleWithDifferentArgsPy310.py:17: note: Revealed type is "builtins.tuple[builtins.float, ...] | builtins.str"
+_testTupleWithDifferentArgsPy310.py:15: note: Revealed type is "Union[builtins.str, tuple[builtins.float, builtins.float, builtins.str]]"
+_testTupleWithDifferentArgsPy310.py:16: note: Revealed type is "Union[tuple[builtins.float], builtins.str]"
+_testTupleWithDifferentArgsPy310.py:17: note: Revealed type is "Union[builtins.tuple[builtins.float, ...], builtins.str]"
 _testTupleWithDifferentArgsPy310.py:18: note: Revealed type is "tuple[builtins.float, builtins.str]"
 _testTupleWithDifferentArgsPy310.py:19: note: Revealed type is "builtins.tuple[builtins.float, ...]"
 _testTupleWithDifferentArgsPy310.py:20: note: Revealed type is "builtins.list[tuple[builtins.int, builtins.str]]"