How to mock psycopg2 cursor object?

Question

I have this code segment in Python2:

def super_cool_method():
    con = psycopg2.connect(**connection_stuff)
    cur = con.cursor(cursor_factory=DictCursor)
    cur.execute("Super duper SQL query")
    rows = cur.fetchall()

    for row in rows:
        # do some data manipulation on row
    return rows

that I’d like to write some unittests for. I’m wondering how to use mock.patch in order to patch out the cursor and connection variables so that they return a fake set of data? I’ve tried the following segment of code for my unittests but to no avail:

@mock.patch("psycopg2.connect")
@mock.patch("psycopg2.extensions.cursor.fetchall")
def test_super_awesome_stuff(self, a, b):
    testing = super_cool_method()

But I seem to get the following error:

TypeError: can't set attributes of built-in/extension type 'psycopg2.extensions.cursor'

Asked By: zyshara

||

Source

Answer 1

Since the cursor is the return value of con.cursor, you only need to mock the connection, then configure it properly. For example,

query_result = [("field1a", "field2a"), ("field1b", "field2b")]
with mock.patch('psycopg2.connect') as mock_connect:
    mock_connect.cursor.return_value.fetchall.return_value = query_result
    super_cool_method()

Answered By: chepner

Answer 2

You have a series of chained calls, each returning a new object. If you mock just the psycopg2.connect() call, you can follow that chain of calls (each producing mock objects) via .return_value attributes, which reference the returned mock for such calls:

@mock.patch("psycopg2.connect")
def test_super_awesome_stuff(self, mock_connect):
    expected = [['fake', 'row', 1], ['fake', 'row', 2]]

    mock_con = mock_connect.return_value  # result of psycopg2.connect(**connection_stuff)
    mock_cur = mock_con.cursor.return_value  # result of con.cursor(cursor_factory=DictCursor)
    mock_cur.fetchall.return_value = expected  # return this when calling cur.fetchall()

    result = super_cool_method()
    self.assertEqual(result, expected)

Because you hold onto references for the mock connect function, as well as the mock connection and cursor objects you can then also assert if they were called correctly:

mock_connect.assert_called_with(**connection_stuff)
mock_con.cursor.asset_called_with(cursor_factory=DictCursor)
mock_cur.execute.assert_called_with("Super duper SQL query")

If you don’t need to test these, you could just chain up the return_value references to go straight to the result of cursor() call on the connection object:

@mock.patch("psycopg2.connect")
def test_super_awesome_stuff(self, mock_connect):
    expected = [['fake', 'row', 1], ['fake', 'row' 2]]
    mock_connect.return_value.cursor.return_value.fetchall.return_value = expected

    result = super_cool_method()
    self.assertEqual(result, expected)

Note that if you are using the connection as a context manager to automatically commit the transaction and you use as to bind the object returned by __enter__() to a new name (so with psycopg2.connect(...) as conn: # ...) then you’ll need to inject an additional __enter__.return_value in the call chain:

mock_con_cm = mock_connect.return_value  # result of psycopg2.connect(**connection_stuff)
mock_con = mock_con_cm.__enter__.return_value  # object assigned to con in with ... as con    
mock_cur = mock_con.cursor.return_value  # result of con.cursor(cursor_factory=DictCursor)
mock_cur.fetchall.return_value = expected  # return this when calling cur.fetchall()

The same applies to the result of with conn.cursor() as cursor:, the conn.cursor.return_value.__enter__.return_value object is assigned to the as target.

Answered By: Martijn Pieters

Answer 3

The following answer is the variation of above answers.
I was using django.db.connections cursor object.

So following code worked for me

@patch('django.db.connections')
def test_supercool_method(self, mock_connections):
    query_result = [("field1a", "field2a"), ("field1b", "field2b")]
    mock_connections.__getitem__.return_value.cursor.return_value.__enter__.return_value.fetchall.return_value = query_result

    result = supercool_method()
    self.assertIsInstance(result, list)

Answered By: Shubham Agrawal

Answer 4

@patch("psycopg2.connect")
async def test_update_task_after_launch(fake_connection):
    """
    
    """
    fake_update_count =4
    fake_connection.return_value = Mock(cursor=lambda : Mock(execute=lambda x,y :"",  
fetch_all=lambda:['some','fake','rows'],rowcount=fake_update_count,close=lambda:""))

Answered By: Otobong Jerome

How to mock psycopg2 cursor object?

Question:

Answers: