> s = "nothin is impossible."
> key = "nothing"
> assert key in s, "Key: '{}' is not in Target: '{}'".format(key, s)

Traceback (most recent call last):
 File "<input>", line 1, in <module>
AssertionError: Key: 'nothing' is not in Target: 'nothin is impossible.'

import pytest

def test_case():
  expected = "Hello"
  actual = "hello"
  assert expected == actual

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

"""
================================== FAILURES ===================================
__________________________________ test_case __________________________________

  def test_case():
    expected = "Hello"
    actual = "hello"
>    assert expected == actual
E    assert 'Hello' == 'hello'
E     - Hello
E     """"

import unittest

class TestStringMethods(unittest.TestCase):

  def test_upper(self):
    self.assertEqual('foo'.upper(), 'FoO')

if __name__ == '__main__':
  unittest.main()
  
"""
Failure
Expected :'FOO'
Actual  :'FoO'

Traceback (most recent call last):
 File "assertion_in_python.py", line 6, in test_upper
  self.assertEqual('foo'.upper(), 'FoO')
AssertionError: 'FOO' != 'FoO'
"""

from ptest.decorator import *
from ptest.assertion import *

@TestClass()
class TestCases:
  @Test()
  def test1(self):
    actual = 'foo'
    expected = 'bar'
    assert_that(expected).is_equal_to(actual)

"""
Start to run following 1 tests:
------------------------------
...
[demo.assertion_in_python.TestCases.test1@Test] Failed with following message:
...
AssertionError: Unexpectedly that the str <bar> is not equal to str <foo>.
"""

pip install assertpy

from assertpy import assert_that

def test_something():
  assert_that(1 + 2).is_equal_to(3)
  assert_that('foobar')    .is_length(6)    .starts_with('foo')    .ends_with('bar')
  assert_that(['a', 'b', 'c'])    .contains('a')    .does_not_contain('x')

Expected <foo> to be of length <4>, but was <3>.
Expected <foo> to be empty string, but was not.
Expected <False>, but was not.
Expected <foo> to contain only digits, but did not.
Expected <123> to contain only alphabetic chars, but did not.
Expected <foo> to contain only uppercase chars, but did not.
Expected <FOO> to contain only lowercase chars, but did not.
Expected <foo> to be equal to <bar>, but was not.
Expected <foo> to be not equal to <foo>, but was.
Expected <foo> to be case-insensitive equal to <BAR>, but was not.