fix: fix the python detector regex + tests [backport 3.17] (#15078)

Backport 2a69e20 from #15068 to 3.17.

## Description

- Fix the Python Detector regex so it also detects paths ending with the
major version number (/foo/bar/python3).
- Use `re.compile`. 
- Add tests.

## Testing

- Add a new test for positive and negative matches of the Python
Detector.

Signed-off-by: Juanjo Alvarez <juanjo.alvarezmartinez@datadoghq.com>
Co-authored-by: Juanjo Alvarez Martinez <juanjo.alvarezmartinez@datadoghq.com>

Author: dd-octo-sts[bot]

ddtrace/settings/_inferred_base_service.py

@@ -36,8 +36,8 @@ def __init__(self, environ: Dict[str, str]):
         # - Starts with an optional directory (anything before the last '/' or '')
         # - Ends with the expected command name, possibly followed by a version
         # - Ensures that it does not end with .py
-        # - Match /python, /python3.7, etc.
-        self.pattern = r"(^|/)(?!.*\.py$)(" + re.escape("python") + r"(\d+\.\d+)?$)"
+        # - Match /python, /python3, /python3.7, etc.
+        self.pattern = re.compile(r"(^|/)(?!.*\.py$)(" + re.escape("python") + r"(\d+(\.\d+)?)?$)")
 
     def detect(self, args: List[str], skip_args_preceded_by_flags=True) -> Optional[ServiceMetadata]:
         """
@@ -127,7 +127,7 @@ def find_nearest_top_level(self, fp: pathlib.Path) -> str:
 
     def matches(self, command: str) -> bool:
         # Returns if the command matches the regex pattern for finding python executables / commands.
-        return bool(re.search(self.pattern, command))
+        return bool(self.pattern.search(command))
 
 
 def detect_service(args: List[str]) -> Optional[str]:

releasenotes/notes/pythondetector-better-regex-5a0148ccade51b0e.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    Fix the Python Detector regular expression so it also detects paths ending with only the major version number.

tests/internal/service_name/test_inferred_base_service.py

@@ -8,6 +8,7 @@
 
 import pytest
 
+from ddtrace.settings._inferred_base_service import PythonDetector
 from ddtrace.settings._inferred_base_service import _module_exists
 from ddtrace.settings._inferred_base_service import detect_service
 
@@ -240,3 +241,38 @@ def test_get_service(cmd, default, expected, testdir):
     )
 
     assert "AssertionError" not in result.stderr, "AssertionError found in stderr"
+
+
+@pytest.mark.parametrize(
+    "command,should_match,expected_capture",
+    [
+        ("python", True, "python"),
+        ("python3", True, "python3"),
+        ("python3.11", True, "python3.11"),
+        ("/usr/local/bin/python", True, "/python"),
+        ("/usr/local/bin/python3", True, "/python3"),
+        ("/usr/local/bin/python3.11", True, "/python3.11"),
+        ("python2", True, "python2"),
+        ("python3.9", True, "python3.9"),
+        ("/python", True, "/python"),
+        ("/python3", True, "/python3"),
+        ("python.py", False, None),  # Should not match .py files
+        ("/path/to/python.py", False, None),  # Should not match .py files
+        ("not-python", False, None),  # Should not match
+        ("pythonic", False, None),  # Should not match
+    ],
+)
+def test_python_detector_pattern_matching(command, should_match, expected_capture):
+    """Test that the PythonDetector regex pattern correctly matches various Python executable formats."""
+    detector = PythonDetector(dict(os.environ))
+
+    match = detector.pattern.search(command)
+
+    if should_match:
+        assert match is not None, f"Expected '{command}' to match but it didn't"
+        # The full match should contain the expected capture
+        assert expected_capture in match.group(
+            0
+        ), f"Expected capture '{expected_capture}' not found in match '{match.group(0)}'"
+    else:
+        assert match is None, f"Expected '{command}' not to match but it did: {match.group(0) if match else None}"