mock postgresql using pytest

Question

I have code like below

 engine = create_engine(URL(
        host="host",
        username="username",
        password="password",
        database="dbname",
        drivername="drivername",
        port="port"
    )) 

 conn = engine.connect()

 table_obj = Table(
        table_name,
        metadata,
        autoload=True,
        autoload_with=engine)

 def check_something(conn, table_obj, u_id):
 stmt = select(*[c for c in table_obj.c])
 result = conn.execute(stmt).fetchall()
 return result

Here i need to test the check_something method without connecting to postgresql database using pytest. Had gone through links available in google but everyone connecting to DB.

any option available to test the method by mocking DB connection like we mock AWS connections(S3, etc..)

Imre_G · Accepted Answer · 2023-02-15 09:04:00Z

1

You can use the standard unittest.mock module from Python to decorate your test with a mock function replacing the pg method with whatever you want to return from postgres.

from unittest.mock import patch

def my_pg_mock():
    return []

@patch('module.check_something', my_pg_mock)
def test_something(mock): # include it here or it will override your other fixtures. Can have any name. 
    pass

The docs can be found here even though I always find them somewhat confusing: https://docs.python.org/dev/library/unittest.mock.html. The module.check_something is important because you need to patch the module were it's used, not where it is defined. So if your code imports check_something in main.py, the string to pass to patch is main.check_something and not for example postgres.check_something.

You can also pass the return value directly with:

from unittest.mock import patch

@patch('module.check_something', return_value=[])
def test_something(mock):  # include it here or it will override your other fixtures. Can have any name. 
    pass

edited Feb 15, 2023 at 9:04

answered Nov 24, 2021 at 15:37

Imre_G

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5 Comments

Sai Over a year ago

Any option available with 'pytest'

Imre_G Over a year ago

This is 100% pytest compatible and included in the python standard library. So if you define pytest tests, this runs

Imre_G Over a year ago

Using it with pytest myself. All pytest libraries are wrapping this anyway

Sai Over a year ago

In my case i need to compare the results. would it be better to mock the return result or create an in memory sqllite engine

Imre_G Over a year ago

It depends. How in-memory is it? If the sqlite approach means that you create persistent state somewhere f.e. a data file, I would advise against that, because it is going to bite you later. Better to just mock the return result. Mocking the result is better in almost all cases I think as it's simple and specific to your test. It is even better (if possible) to structure your code in a way that the pg interface is separate from the other methods, because then you can just call your method with the expected result from pg and you don't need to mock anything.

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