Pytest discovery, fixture resolution and port conflicts — DedSec Project

pytest collected 0 items

Meaning and differential diagnosis

Pytest may find no tests because filenames, function names or configured testpaths do not match discovery rules. A collection error is different from an empty test suite, as is running pytest in the wrong directory. Do not make a CI job green by silently accepting zero collected tests.

Reproduction / nondestructive inspection

# Save as test_arithmetic.py; requires pytest.
def add(a, b):
    return a + b

def test_add_integers():
    assert add(2, 3) == 5

def test_add_negative():
    assert add(-2, 2) == 0

Interpret evidence and fix the cause

Run python3 -m pytest --collect-only -q from the repository root. Inspect pyproject.toml, pytest.ini or setup.cfg for python_files, python_functions and testpaths. Verify the desired tests appear by node ID. Exit code 5 generally indicates no tests collected; a different nonzero code may mean import/collection errors. Avoid renaming arbitrary production files just to satisfy discovery.

Practical lab

Rename the file to arithmetic_checks.py and confirm default discovery no longer sees it; run pytest --collect-only -q against its explicit path and explain the difference. Add an assertion in CI that the expected critical test node is present.

pytest fixture not found

Meaning and differential diagnosis

Pytest interprets names in a test function's parameters as fixture requests. A typo, a fixture defined in an inaccessible conftest.py, an optional plugin not installed or a mismatched fixture scope may break setup. A regular local variable with the same spelling does not become a pytest fixture automatically.

Reproduction / nondestructive inspection

# Save as test_catalog.py; requires pytest.
import pytest

@pytest.fixture
def catalog():
    return {"books": ["Algorithms", "Networks"]}

def test_catalog_has_books(catalog):
    assert "Algorithms" in catalog["books"]
    assert len(catalog["books"]) == 2

Interpret evidence and fix the cause

Read the name of the missing fixture and list available ones using python3 -m pytest --fixtures -q. Trace where conftest.py is located relative to the test module; fixtures in a sibling subtree are not automatically visible. Plugin fixtures depend on the selected interpreter. Prefer narrowly scoped fixtures with explicit setup and teardown over mutable module-global state.

Practical lab

Rename catalog to catalogue in the decorator function without updating the test parameter; inspect the missing-fixture message. Restore it, then move the fixture into conftest.py and confirm both tests in that directory can reuse it.

address already in use

Meaning and differential diagnosis

EADDRINUSE usually means that the requested local address/port combination is already bound. Causes include a second server instance, a listener bound to all interfaces, or address-family differences. Depending on options and OS, recently closed sockets can also affect rebinding. Killing an arbitrary process by port number is unsafe.

Reproduction / nondestructive inspection

import errno
import socket
first = socket.socket()
second = socket.socket()
try:
    first.bind(("127.0.0.1", 0))  # OS chooses a free port.
    port = first.getsockname()[1]
    try:
        second.bind(("127.0.0.1", port))
    except OSError as exc:
        assert exc.errno == errno.EADDRINUSE
    else:
        raise AssertionError("duplicate bind accepted")
finally:
    first.close()
    second.close()

Interpret evidence and fix the cause

Inspect listeners with ss -ltnp on Linux/Termux where supported, or lsof -iTCP -sTCP:LISTEN. Confirm process ownership and its working directory before stopping a service. Investigate interface binding (127.0.0.1 versus 0.0.0.0), container port mapping, and IPv6 behavior. SO_REUSEADDR is not a universal fix for two actively listening servers.

Practical lab

Rerun the fixture on the same host, recording the dynamically chosen port and exception errno. Next run one local test server, try to start a second on that port, then stop only the known first server and verify the second can bind.