Python test to check instance type

Question

I want to use unittest in python to check if a method returns object of the right class.

Every example in the web shows tests for \'type\' returned.

For example,

Answer 1

This should work:

self.assertIsInstance(result, sqlalchemy.orm.query.Query)

You need to have import sqlalchemy in the file.

Answer 2

You could use assertIsInstance(), presumably using isinstance() which is the recommended function for testing types. You could also assertIs() or assertTrue() combined with type() depending on the context:

#assert.py
import unittest

class TestType(unittest.TestCase):

  def setUp(self):
      self.number = 1

  def test_assert_true(self):
      self.assertTrue(type(self.number) is int)

  def test_assert_is_instance(self):
      self.assertIsInstance(self.number, int)

  def test_assert_is_with_type(self):
      self.assertIs(type(self.number), int)

  def test_assert_is(self):
      self.assertIs(self.number, int)

if __name__ == '__main__':
    unittest.main()


$ python assert.py 

test_assert_is (__main__.TestType) ... FAIL
test_assert_is_instance (__main__.TestType) ... ok
test_assert_is_with_type (__main__.TestType) ... ok
test_assert_true (__main__.TestType) ... ok

======================================================================
FAIL: test_assert_is (__main__.TestType)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "assert.py", line 19, in test_assert_is
    self.assertIs(self.number, int)
AssertionError: 1 is not <type 'int'>

----------------------------------------------------------------------
Ran 4 tests in 0.000s

FAILED (failures=1)

The assertion error of the test_assert_is(self) might lead one to believe the type of 1 is not integer however it's comparing the object represented by 1 and the object that describes the type of an integer. This is most likely why isinstance() is preferred since it's more verbose about what it's checking and less typing is involved, so in general less error prone.