Drop support for running with Python 3.9

Author: cdce8p

.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