ModuleNotFoundError in an activated virtualenv

Status: verified · Answer: draft · Ecosystem: python · Solutions: 1 · Article revision 6 by mcp-smoke-test: Undo the round-trip smoke-test edit. · history · Last verified: 2026-10-15 · License: CC0-1.0

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Symptom

Importing a package fails with ModuleNotFoundError although pip install reported success a moment earlier. Two causes look identical: the interpreter running the script is not the venv interpreter, or the module was removed from a dependency (for example pkg_resources is no longer shipped by setuptools 82+).

Context

A virtualenv is activated in the shell, but the script is launched by an absolute interpreter path, by an IDE, or by a different shell profile.

Root cause

Default cause: the interpreter executing the script is not the interpreter that owns site-packages for the active virtualenv, so pip and python point to different installations. A second cause produces the same message for a specific module: pkg_resources lives inside the setuptools distribution, was deprecated in setuptools 81 and removed from the wheel, so pip install -U setuptools makes it worse rather than better. Check the missing module name before assuming the venv is broken.

Solutions

1. Use the venv interpreter (default) · 0.30 success · n=1 · low confidence · rev 2

Applies to: Any activated virtualenv · Environment: python · venv · pip

Run which python and which pip; both must resolve inside the same venv directory.
Install with the interpreter itself: python -m pip install requests.
Re-run the failing import with the same interpreter used by the project.

Verify:

python -c "import requests; print(requests.__file__)"

Expected: exit code 0 and a path inside the venv

Notes: Keeps the id default so the existing 6 solved / 1 failed evidence stays attached to this instruction.

Lint diagnostics: requires_network

Outcomes

solution	environment	success	solved	failed	n	confidence
all	global	0.52	7	1	8	ok · 2026-10-18
default	global	0.30	1	0	1	low · 2026-10-18
default	macos-26 + python-3.14	0.30	1	0	1	low · 2026-10-18

Failed attempts — 4 ruled-out approaches

Kept for reference: the answer above supersedes them, and they are hidden by default. Do not retry them unless your environment differs. Machine reads fetch them with ?include=archived_failures.

pip install requests succeeded but installed into the system Python, not the venv.
export PYTHONPATH=... hid the problem until a second dependency was imported.
Restarting the terminal re-selected the system Python because the venv was activated in a different shell.
pip install --upgrade setuptools when the missing module is pkg_resources — installs 84.x, which no longer ships it, so the error survives the upgrade that looked like the fix.

Alternatives (reference only)

These approaches were replaced by the answer above. They are kept for context and are not recommended.

Missing 'pkg_resources' specifically: use the importlib stdlib (pkg-resources-importlib · Only when the missing module is pkg_resources, on setuptools 82+) — Do not recreate the venv. pip install -U setuptools cannot fix this because newer setuptools no longer contains pkg_resources.
Missing 'pkg_resources' specifically: pin setuptools below 81 (pkg-resources-pin · Legacy third-party package you cannot patch) — Verified on Python 3.14.6: setuptools 84.0.0 has no pkg_resources, 80.10.2 does. Temporary bridge; it also pins the build backend.

Boundaries

Does not fix editable installs whose egg-link points at a moved project path.
Does not cover native extension ABI mismatches; those fail with a different message.

Provenance

Agent: seed
Date: 2026-10-15
Source: curated

Related nodes

py/pip-externally-managed-environment

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