pip_audit, test: warn on Python path confusion

CHANGELOG.md

@@ -16,6 +16,11 @@ All versions prior to 0.0.9 are untracked.
   specifier for its `requires-python` version
   ([#447](https://github.com/pypa/pip-audit/pull/447))
 
+* Users are now warned if a `pip-audit` invocation is ambiguous, e.g.
+  if they've installed `pip-audit` globally but are asking for an audit
+  of a loaded virtual environment
+  ([#451](https://github.com/pypa/pip-audit/pull/451))
+
 ## [2.4.10]
 
 ### Fixed

pip_audit/_cache.py

@@ -67,7 +67,7 @@ def _get_cache_dir(custom_cache_dir: Path | None, *, use_pip: bool = True) -> Pa
             return pip_cache_dir
         else:
             logger.warning(
-                f"Warning: pip {_PIP_VERSION} doesn't support the `cache dir` subcommand, "
+                f"pip {_PIP_VERSION} doesn't support the `cache dir` subcommand, "
                 f"using {_PIP_AUDIT_INTERNAL_CACHE} instead"
             )
             return _PIP_AUDIT_INTERNAL_CACHE

pip_audit/_dependency_source/pip.py

@@ -4,6 +4,7 @@
 """
 
 import logging
+import os
 import subprocess
 import sys
 from pathlib import Path
@@ -64,9 +65,35 @@ def __init__(
         self._skip_editable = skip_editable
         self.state = state
 
+        # NOTE: By default `pip_api` invokes `pip` through `sys.executable`, like so:
+        #
+        #    {sys.executable} -m pip [args ...]
+        #
+        # This is the right decision 99% of the time, but it can result in unintuitive audits
+        # for users who have installed `pip-audit` globally but are trying to audit
+        # a loaded virtual environment, since `pip-audit`'s `sys.executable` will be the global
+        # Python and not the virtual environment's Python.
+        #
+        # To check for this, we check whether the Python that `pip_api` plans to use
+        # matches the active virtual environment's prefix. We do this instead of comparing
+        # against the $PATH-prioritized Python because that might be the same "effective"
+        # Python but with a different symlink (e.g. `<path>/python{,3,3.7}`). We *could*
+        # handle that case by resolving the symlinks, but that would then piece the
+        # virtual environment that we're attempting to detect.
+        effective_python = os.environ.get("PIPAPI_PYTHON_LOCATION", sys.executable)
+        venv_prefix = os.getenv("VIRTUAL_ENV")
+        if venv_prefix is not None and not effective_python.startswith(venv_prefix):
+            logger.warning(
+                f"pip-audit will run pip against {effective_python}, but you have "
+                f"a virtual environment loaded at {venv_prefix}. This may result in "
+                "unintuitive audits, since your local environment will not be audited. "
+                "You can forcefully override this behavior by setting PIPAPI_PYTHON_LOCATION "
+                "to the location of your virtual environment's Python interpreter."
+            )
+
         if _PIP_VERSION < _MINIMUM_RELIABLE_PIP_VERSION:
             logger.warning(
-                f"Warning: pip {_PIP_VERSION} is very old, and may not provide reliable "
+                f"pip {_PIP_VERSION} is very old, and may not provide reliable "
                 "dependency information! You are STRONGLY encouraged to upgrade to a "
                 "newer version of pip."
             )

test/dependency_source/test_pip.py

@@ -26,6 +26,26 @@ def test_pip_source():
     assert pytest_spec in specs
 
 
+def test_pip_source_warns_about_confused_python(monkeypatch):
+    monkeypatch.setenv("PIPAPI_PYTHON_LOCATION", "/definitely/fake/path/python")
+    monkeypatch.setenv("VIRTUAL_ENV", "/definitely/fake/env")
+    logger = pretend.stub(warning=pretend.call_recorder(lambda s: None))
+    monkeypatch.setattr(pip, "logger", logger)
+
+    pip.PipSource()
+
+    assert logger.warning.calls == [
+        pretend.call(
+            "pip-audit will run pip against /definitely/fake/path/python, but you have "
+            "a virtual environment loaded at /definitely/fake/env. "
+            "This may result in unintuitive audits, since your local environment will not "
+            "be audited. You can forcefully override this behavior by setting "
+            "PIPAPI_PYTHON_LOCATION to the location of your virtual environment's Python "
+            "interpreter."
+        )
+    ]
+
+
 def test_pip_source_warns_about_old_pip(monkeypatch):
     # Rather than hack around with virtualenvs and install a very old pip,
     # simply lie about how old ours is.
@@ -34,7 +54,12 @@ def test_pip_source_warns_about_old_pip(monkeypatch):
     monkeypatch.setattr(pip, "logger", logger)
 
     pip.PipSource()
-    assert len(logger.warning.calls) == 1
+    assert logger.warning.calls == [
+        pretend.call(
+            "pip 1.0.0 is very old, and may not provide reliable dependency information! "
+            "You are STRONGLY encouraged to upgrade to a newer version of pip."
+        )
+    ]
 
 
 def test_pip_source_pip_api_failure(monkeypatch):