Allow use without virtualenv activation

Fixes: #1507

src/ansiblelint/__main__.py

@@ -25,6 +25,7 @@
 import logging
 import os
 import pathlib
+import shutil
 import subprocess
 import sys
 from contextlib import contextmanager
@@ -138,6 +139,9 @@ def _do_list(rules: "RulesCollection") -> int:
 
 def main(argv: Optional[List[str]] = None) -> int:
     """Linter CLI entry point."""
+    # alter PATH if needed (venv support)
+    path_inject()
+
     if argv is None:
         argv = sys.argv
     initialize_options(argv[1:])
@@ -250,5 +254,26 @@ def _run_cli_entrypoint() -> None:
         raise SystemExit(str(e))
 
 
+def path_inject() -> None:
+    """Add python interpreter path to top of PATH to fix outside venv calling."""
+    # This make it possible to call ansible-lint that was installed inside a
+    # virtualenv without having to pre-activate it. Otherwise subprocess will
+    # either fail to find ansible executables or call the wrong ones.
+    #
+    # This must be run before we do run any subprocesses, and loading config
+    # does this as part of the ansible detection.
+    paths = [x for x in os.environ.get("PATH", "").split(os.pathsep) if x]
+    ansible_path = shutil.which("ansible")
+    if ansible_path:
+        ansible_path = os.path.dirname(ansible_path)
+    py_path = os.path.dirname(sys.executable)
+    # Determine if we need to manipulate PATH
+    if ansible_path not in paths or py_path != ansible_path:  # pragma: no cover
+        # tested by test_call_from_outside_venv but coverage cannot detect it
+        paths.insert(0, py_path)
+        os.environ["PATH"] = os.pathsep.join(paths)
+        print(f"WARNING: PATH altered to include {py_path}", file=sys.stderr)
+
+
 if __name__ == "__main__":
     _run_cli_entrypoint()

test/test_main.py

@@ -0,0 +1,33 @@
+"""Tests related to ansiblelint.__main__ module."""
+import os
+import subprocess
+import sys
+from pathlib import Path
+
+import pytest
+
+
+@pytest.mark.parametrize(
+    ("expected_warning"),
+    (False, True),
+    ids=("normal", "isolated"),
+)
+def test_call_from_outside_venv(expected_warning: bool) -> None:
+    """Asserts ability to be called w/ or w/o venv activation."""
+    env = None
+    if expected_warning:
+        env = {"HOME": Path.home()}
+    py_path = os.path.dirname(sys.executable)
+    # Passing custom env prevents the process from inheriting PATH or other
+    # environment variables from the current process, so we emulate being
+    # called from outside the venv.
+    proc = subprocess.run(
+        [f"{py_path}/ansible-lint", "--version"],
+        check=True,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+        universal_newlines=True,
+        env=env,
+    )
+    warning_found = "PATH altered to include" in proc.stderr
+    assert warning_found is expected_warning