Stubgen giving error about self documenting expressions in f-strings

[cquick01]

Bug Report

Hi, I am trying to run stubgen but am running into an error

Critical error during semantic analysis: path/to/script.py:140: error: f-string: self documenting expressions are only supported in Python 3.8 and greater

But, I'm running Python 3.9.6, and mypy 0.910 which should definitely support 3.8+ since mypy 0.800 added support for 3.9.

To Reproduce

Paths to binaries

(venv) $ which python
/home/cquick/test/venv/bin/python
(venv) $ which mypy
/home/cquick/test/venv/bin/mypy
(venv) $ which stubgen
/home/cquick/test/venv/bin/stubgen

Versions

(venv) $ python -V
Python 3.9.6

(venv) $ python -m pip install -U mypy
Requirement already satisfied: mypy in ./venv/lib/python3.9/site-packages (0.910)
Requirement already satisfied: typing-extensions>=3.7.4 in ./venv/lib/python3.9/site-packages (from mypy) (3.10.0.0)
Requirement already satisfied: toml in ./venv/lib/python3.9/site-packages (from mypy) (0.10.2)
Requirement already satisfied: mypy-extensions<0.5.0,>=0.4.3 in ./venv/lib/python3.9/site-packages (from mypy) (0.4.3)

(venv) $ mypy -V
mypy 0.910

Simple test script

(venv) $ cat test.py
#!/usr/bin/env python3

def foo(x):
    print(f"{x = }")

if __name__ == "__main__":
    foo(1)

Test script successfully prints self documenting expression

(venv) $ python test.py
x = 1

But running stubgen fails

(venv) $ stubgen test.py
Critical error during semantic analysis: test.py:4: error: f-string: self documenting expressions are only supported in Python 3.8 and greater

Expected Behavior

Stubgen would run successfully.

Actual Behavior

Stubgen produces Critical error during semantic analysis

Your Environment

• Mypy version used: 0.910
• MypyStubgen command-line flags: stubgen test.py
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.9.6
• Operating system and version: Same behavior both on Ubuntu 18.04.5 LTS (WSL2) and with latest python:3-buster Docker images (currently 3.9.6) with mypy 0.910 installed.

[cquick01]

Looking into it a bit, I think the issue is related to picking a "default" Python 3 version of (3, 6) here

pyversion = defaults.PYTHON2_VERSION if ns.py2 else defaults.PYTHON3_VERSION

This little patch seems to get stubgen working successfully with self documenting f-strings

diff --git a/mypy/stubgen.py b/mypy/stubgen.py
index 91f461b84..920d6405b 100755
--- a/mypy/stubgen.py
+++ b/mypy/stubgen.py
@@ -1623,6 +1623,9 @@ def parse_options(args: List[str]) -> Options:
     if ns.quiet and ns.verbose:
         parser.error('Cannot specify both quiet and verbose messages')

+    if sys.version_info[:2] > pyversion:
+        pyversion = sys.version_info[:2]
+
     # Create the output folder if it doesn't already exist.
     if not os.path.exists(ns.output_dir):
         os.makedirs(ns.output_dir)

But I doubt that's really the right way to fix it... Should defaults.py get updated to point to a newer default python 3 version?

Edit: or maybe add a --python-version argument to stubgen?