Build dependencies not compatible with Python 3.14

[fsc-eriker]

Describe the bug

Attempting to install AWS CLI with

uv tool install git+https://github.com/aws/aws-cli.git@v2

fails with a traceback, because ast.Str (and a bunch of other deprecated classes) was removed in 3.14.

Quoting https://docs.python.org/dev/whatsnew/3.14.html#id9:

Remove the following classes, which have been deprecated aliases of Constant since Python 3.8 and have emitted deprecation warnings since Python 3.12:

• Bytes
• Ellipsis
• NameConstant
• Num
• Str

Regression Issue

• Select this option if this issue appears to be a regression.

Expected Behavior

The above command should complete successfully without a traceback or other undesired complications.

Current Behavior

I already created a workaround for the global install, but here is a repro in a new virtual environment.

tmp$ uv venv --seed nst
Using CPython 3.14.0
Creating virtual environment with seed packages at: nst
 + pip==25.3
Activate with: source nst/bin/activate
tmp$ . nst/bin/activate
(nst) tmp$ python --version
Python 3.14.0
(nst) tmp$ uv pip install git+https://github.com/aws/aws-cli.git@v2
Using Python 3.14.0 environment at: nst
    Updated https://github.com/aws/aws-cli.git (66fddbdca2b933cbff84f18c7e5219cb34e0
error: The build backend returned an error
  Caused by: Call to `pep517.build_wheel` failed (exit status: 1)

[stderr]
Traceback (most recent call last):
  File "<string>", line 14, in <module>
    requires = get_requires_for_build({})
  File "/Users/me/.cache/uv/git-v0/checkouts/a28ee72bf38d12f6/66fddbdca/backends/pep517.py", line 101, in get_requires_for_build_wheel
    requires = flit_core.buildapi.get_requires_for_build_wheel(config_settings)
  File "/Users/me/.cache/uv/builds-v0/.tmpEcMon9/lib/python3.14/site-packages/flit_core/buildapi.py", line 32, in get_requires_for_build_wheel
    docstring, version = get_docstring_and_version_via_ast(module)
                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "/Users/me/.cache/uv/builds-v0/.tmpEcMon9/lib/python3.14/site-packages/flit_core/common.py", line 160, in get_docstring_and_version_via_ast
    and isinstance(child.value, ast.Str)
                                ^^^^^^^
AttributeError: module 'ast' has no attribute 'Str'

hint: This usually indicates a problem with the package or the build environment.

Here is the same (in the same virtual environment) with just pip, which then no longer implicates uv (though it still mentions flit):

(nst) WSAC9JJH41J9J:tmp$ pip install git+https://github.com/aws/aws-cli.git@v2
Collecting git+https://github.com/aws/aws-cli.git@v2
  Cloning https://github.com/aws/aws-cli.git (to revision v2) to ./pip-req-build-5ohmuar_
  Running command git clone --filter=blob:none --quiet https://github.com/aws/aws-cli.git /private/tmp/pip-req-build-5ohmuar_
  Running command git checkout -b v2 --track origin/v2
  Switched to a new branch 'v2'
  branch 'v2' set up to track 'origin/v2'.
  Resolved https://github.com/aws/aws-cli.git to commit 66fddbdca2b933cbff84f18c7e5219cb34e072fc
  Installing build dependencies ... done
  Getting requirements to build wheel ... error
  error: subprocess-exited-with-error
  
  × Getting requirements to build wheel did not run successfully.
  │ exit code: 1
  ╰─> [18 lines of output]
      Traceback (most recent call last):
        File "/private/tmp/nst/lib/python3.14/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 389, in <module>
          main()
          ~~~~^^
        File "/private/tmp/nst/lib/python3.14/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 373, in main
          json_out["return_val"] = hook(**hook_input["kwargs"])
                                   ~~~~^^^^^^^^^^^^^^^^^^^^^^^^
        File "/private/tmp/nst/lib/python3.14/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 143, in get_requires_for_build_wheel
          return hook(config_settings)
        File "/private/tmp/pip-req-build-5ohmuar_/backends/pep517.py", line 101, in get_requires_for_build_wheel
          requires = flit_core.buildapi.get_requires_for_build_wheel(config_settings)
        File "/private/tmp/pip-build-env-ybq9tt56/overlay/lib/python3.14/site-packages/flit_core/buildapi.py", line 32, in get_requires_for_build_wheel
          docstring, version = get_docstring_and_version_via_ast(module)
                               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
        File "/private/tmp/pip-build-env-ybq9tt56/overlay/lib/python3.14/site-packages/flit_core/common.py", line 160, in get_docstring_and_version_via_ast
          and isinstance(child.value, ast.Str)
                                      ^^^^^^^
      AttributeError: module 'ast' has no attribute 'Str'
      [end of output]
  
  note: This error originates from a subprocess, and is likely not a problem with pip.
ERROR: Failed to build 'git+https://github.com/aws/aws-cli.git@v2' when getting requirements to build wheel

Reproduction Steps

The "current behavior" section above contains two examples of reproducing this, one with uv pip install and the other with just pip install. They both work correctly with a Python 3.13 virtual environment.

Possible Solution

No response

Additional Information/Context

As a workaround, I did

uv tool install --python 3.13 git+https://github.com/aws/aws-cli.git@v2

This is with uv installed from Astral with

curl -LsSf https://astral.sh/uv/install.sh | sh

basically on a new Mac out of the box. On top of that, you need

uv python install 3.14
uv python install --default

CLI version used

aws-cli/2.31.22 Python/3.13.9 Darwin/24.6.0 source/arm64

Environment details (OS name and version, etc.)

MacOS Sequoia 15.7.1

[fsc-eriker]

2.31.22 is the one I actually managed to install yesterday; the Github HEAD now appears to be at 2.31.24

[fsc-eriker]

I asked Claude Code to investigate and it came up with this:

The AWS CLI v2 code works perfectly with Python 3.14. The installation
fails because of outdated build dependency version constraints:

1. flit_core pinned to <3.9.1

• Uses deprecated ast.Str removed in Python 3.14
• Needs >=3.12.0 for Python 3.14 support

2. ruamel.yaml.clib pinned to <=0.2.12

• Also uses deprecated ast.Str
• Needs >=0.2.13 for Python 3.14 support

[adev-code]

Hello @fsc-eriker, thank you for reaching out. We don't have plans to move the AWS CLI v2 installers to Python 3.14 at this time, but we'll upgrade to v3.13.10 when it's available. We are also tracking this issue and plans here: #9783. Closing for issue tracking duplication.