Repository: python/mypy

Issue:
Friendlier error when 3.10 is parsed as float in pyproject.toml
**Bug Report**

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Running mypy with Python 3.10 and `python_version` in the config file has a few wrong behaviors:
1. It lists mypy must be 3.4 or higher (it should say 3.6)
2. It truncates Python 3.10 to 3.1
3. It outputs a message when there shouldn't be one (you can invoke mypy with Python 3.10)

**To Reproduce**

Here is a subset of my `pyproject.toml`:

```toml
[tool.mypy]
# SEE: http://mypy.readthedocs.io/en/latest/config_file.html#config-file

# Specifies the Python version used to parse and check the target program.
python_version = 3.10

# Enables PEP 420 style namespace packages.
namespace_packages = true
```

I invoke mypy via: `mypy -p namespacepkg`

**Expected Behavior**

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Not to raise this message.

**Actual Behavior**

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  Did something go wrong?
  Is something broken, or not behaving as you expected?
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```
pyproject.toml: [mypy]: python_version: Python 3.1 is not supported (must be 3.4 or higher)
```

**Your Environment**

<!-- Include as many relevant details about the environment you experienced the bug in -->

- Mypy version used: 0.931
- Mypy command-line flags: n/a
- Mypy configuration options from `mypy.ini` (and other config files): see above `pyproject.toml`
- Python version used: 3.10.2
- Operating system and version: macOS v11.5.1 Big Sur

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Discussion:
You need to quote it as `"3.10"`, because it's a float! We can probably put a special-case message here to make that clear though.
Changing it to:

```toml
# Specifies the Python version used to parse and check the target program.
python_version = "3.10"
```

Indeed fixes the problem. In hindsight it makes total sense.  Yeah maybe a special message would be a good idea, thank you!
@hauntsaninja Being a first-time contributor, I would like to work on this issue. Any suggestions on where I can start with?
@hauntsaninja @jamesbraza 
May I work on this issue?
@radioactive11 @nakatomotoi yes either of you can!  Just make a fork of mypy, cut a branch, try to implement a solution + add test(s) proving it works (see [`CONTRIBUTING.md`](https://github.com/python/mypy/blob/master/CONTRIBUTING.md) for how to run tests), and open a PR back to mypy's master branch.
@jamesbraza Sure!
@jamesbraza Should I add a special message when the version mentioned has two decimal digits or a general one?
There should be a message specific to this case (though it should also cover 3.11 and other future versions).

Resolving patch:
diff --git a/mypy/config_parser.py b/mypy/config_parser.py
--- a/mypy/config_parser.py
+++ b/mypy/config_parser.py
@@ -24,8 +24,8 @@
 _INI_PARSER_CALLABLE: _TypeAlias = Callable[[Any], _CONFIG_VALUE_TYPES]
 
 
-def parse_version(v: str) -> Tuple[int, int]:
-    m = re.match(r'\A(\d)\.(\d+)\Z', v)
+def parse_version(v: Union[str, float]) -> Tuple[int, int]:
+    m = re.match(r'\A(\d)\.(\d+)\Z', str(v))
     if not m:
         raise argparse.ArgumentTypeError(
             f"Invalid python version '{v}' (expected format: 'x.y')")
@@ -36,9 +36,15 @@ def parse_version(v: str) -> Tuple[int, int]:
                 f"Python 2.{minor} is not supported (must be 2.7)")
     elif major == 3:
         if minor < defaults.PYTHON3_VERSION_MIN[1]:
-            raise argparse.ArgumentTypeError(
-                "Python 3.{} is not supported (must be {}.{} or higher)".format(minor,
-                                                                    *defaults.PYTHON3_VERSION_MIN))
+            msg = "Python 3.{0} is not supported (must be {1}.{2} or higher)".format(
+                minor,
+                *defaults.PYTHON3_VERSION_MIN
+            )
+
+            if isinstance(v, float):
+                msg += ". You may need to put quotes around your Python version"
+
+            raise argparse.ArgumentTypeError(msg)
     else:
         raise argparse.ArgumentTypeError(
             f"Python major version '{major}' out of range (must be 2 or 3)")
@@ -142,7 +148,7 @@ def check_follow_imports(choice: str) -> str:
 # Reuse the ini_config_types and overwrite the diff
 toml_config_types: Final[Dict[str, _INI_PARSER_CALLABLE]] = ini_config_types.copy()
 toml_config_types.update({
-    'python_version': lambda s: parse_version(str(s)),
+    'python_version': parse_version,
     'strict_optional_whitelist': try_split,
     'mypy_path': lambda s: [expand_path(p) for p in try_split(s, '[,:]')],
     'files': lambda s: split_and_match_files_list(try_split(s)),


Test patch:
diff --git a/test-data/unit/cmdline.test b/test-data/unit/cmdline.test
--- a/test-data/unit/cmdline.test
+++ b/test-data/unit/cmdline.test
@@ -561,6 +561,15 @@ main.py:1: error: Cannot find implementation or library stub for module named "a
 main.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
 main.py:1: error: Cannot find implementation or library stub for module named "a"
 
+[case testPythonVersionWrongFormatPyProjectTOML]
+# cmd: mypy -c pass
+[file pyproject.toml]
+\[tool.mypy]
+python_version = 3.10
+[out]
+pyproject.toml: [mypy]: python_version: Python 3.1 is not supported (must be 3.4 or higher). You may need to put quotes around your Python version
+== Return code: 0
+
 [case testPythonVersionTooOld10]
 # cmd: mypy -c pass
 [file mypy.ini]