Pytest test dicovery triggers sqlite3.OperationalError: unable to open database file

[Ch41r05]

I've got the following code:

# config.py

from yaml import load, SafeLoader
from loguru import logger

""" Populate custom configuration for the
    SERVICE
    component
"""

APPNAME="acceloop-search-engine"

@logger.catch
def _get_field(config, fieldname):
  """Get specified field from yaml config file

  Args:
      config (dict): dictionary containing the parsed config yaml
      fieldname (str): the name of the field to be searched

  Returns:
      str: the value corresponding to the given field name
  """
  if fieldname not in config:
    raise SystemExit(f"No field named {fieldname} is present in yaml configuration file, please add it.")
  
  if not config[fieldname]:
    raise SystemExit(f"Field {fieldname} is present in yaml configuration file, but must not be empty string, null or None. Please correctly evaluate it.")

  return config[fieldname]

@logger.catch
def _get_loader():
  """Get customizable yaml loader

  Returns:
      loader: customized yaml loader
  """
  loader = SafeLoader
  return loader

@logger.catch
def _get_config(config_filename):
  """Get dictionary containing the parsed yaml config file

  Returns:
      dict: dictionary containing the parsed config yaml
  """
  return load(open(config_filename, "rb"), Loader=_get_loader())


class Config(object):

  def __new__(cls):
    if not hasattr(cls, "instance"):
      cls.instance = super(Config, cls).__new__(cls)
      cls.instance._init_singleton()
    return cls.instance
  
  @logger.catch
  def _init_singleton(self):
    try:
      __config = _get_config("config.yml")
    except Exception as error:
      raise SystemExit(f"Error while loading yaml config file: {error}")

    __app_config = _get_field(__config, "app")
    
    self.__table_db_path = _get_field(__app_config, "table-db-path")

  @property
  def table_db_path(self):
    return self.__table_db_path

This file declares a singleton config object that reads various fields (stripped for sake of brevity here) including table-db-path.

# database.py

import sqlite3
from urllib.request import pathname2url
from .config import Config

try:
    dburi = "file:{}?mode=r".format(pathname2url(Config().table_db_path))
    LOCAL_SESSION = sqlite3.connect(dburi, uri=True)
except sqlite3.OperationalError:
    raise SystemExit(f"No db {Config().table_db_path} found. Please create it using the correct service.")

Simple class that creates a connection to a sqlite3 database.

# test_database.py

from src.database import LOCAL_SESSION

def test_should_return_true_when_read_from_db_is_given():
    with LOCAL_SESSION.cursor() as cursor:
        cursor.execute("""
                        SELECT count(*) AS tot
                        FROM table
                        """)
        assert cursor.fetchone()[0] == 1

Simple test file that should checks that there's exactly one record in table.

# conftest.py

import pytest
from unittest.mock import patch
from src.config import Config

@pytest.fixture(scope="session", autouse=True)
def mock_config_table_db_path():
    with patch.object(Config, "table_db_path", "./tests/resources/ase_service.db"):
        yield

print("CONFTEST Loaded")

Lastly, I try to patch the Config().table_db_path class property in conftest.py.

Conftest gets correctly loaded for other parameters, but this parameter strangely triggers

sqlite3.OperationalError: unable to open database file

Error when pytest tries to discover tests, making said test not runnable. The strange thing is that if I check the value of Config().table_db_path, it's the unpatched value, instead of the patched one.
Does anyone have any clue on why this is happening?

[RonnyPfannschmidt]

This is a user error

Monkey patching the config after you already used it to make the database is too late

[Ch41r05]

Thanks for the reply @RonnyPfannschmidt , I think I'm missing something, the database.py is imported in the test, which should be triggered after the conftest.py. Would you please mind to elaborate your answer with a bit of examples? Thanks a lot

[RonnyPfannschmidt]

The import in the test file Happens at collection time,which is way before a fixture is applied

Managing actual resources ought not to be done as global variables

[Ch41r05]

Thanks for the quick and clear answer @RonnyPfannschmidt , this really helped me. If you could go the extra mile and link me some best practice for managing resources that works well with pytest you'd really help me a lot.

Thanks again