How to run unittest test cases in the order they are declared

The solution is to create a TestSuite explicitly, instead of letting unittest.main() follow all its default test discovery and ordering behavior. Here's how I got it to work:

import unittest

class TestCaseB(unittest.TestCase):
    def runTest(self):
        print("running test case B")

class TestCaseA(unittest.TestCase):
    def runTest(self):
        print("running test case A")


import inspect
def get_decl_line_no(cls):
    return inspect.getsourcelines(cls)[1]

# get all test cases defined in this module
test_case_classes = list(filter(lambda c: c.__name__ in globals(), 
                                unittest.TestCase.__subclasses__()))

# sort them by decl line no
test_case_classes.sort(key=get_decl_line_no)

# make into a suite and run it
suite = unittest.TestSuite(cls() for cls in test_case_classes)
unittest.TextTestRunner().run(suite)

This gives the desired output:

running test case B
.running test case A
.
----------------------------------------------------------------------
Ran 2 tests in 0.000s

OK

It is important to note that the test method in each class must be named runTest.

You can manually build a TestSuite where your TestCases and all tests inside them run by line number:

# Python 3.8.3
import unittest
import sys
import inspect


def isTestClass(x):
    return inspect.isclass(x) and issubclass(x, unittest.TestCase)


def isTestFunction(x):
    return inspect.isfunction(x) and x.__name__.startswith("test")


class TestB(unittest.TestCase):
    def test_B(self):
        print("Running test_B")
        self.assertEqual((2+2), 4)

    def test_A(self):
        print("Running test_A")
        self.assertEqual((2+2), 4)

    def setUpClass():
        print("TestB Class Setup")


class TestA(unittest.TestCase):
    def test_A(self):
        print("Running test_A")
        self.assertEqual((2+2), 4)

    def test_B(self):
        print("Running test_B")
        self.assertEqual((2+2), 4)

    def setUpClass():
        print("TestA Class Setup")


def suite():

    # get current module object
    module = sys.modules[__name__]

    # get all test className,class tuples in current module
    testClasses = [
        tup for tup in
        inspect.getmembers(module, isTestClass)
    ]

    # sort classes by line number
    testClasses.sort(key=lambda t: inspect.getsourcelines(t[1])[1])

    testSuite = unittest.TestSuite()

    for testClass in testClasses:
        # get list of testFunctionName,testFunction tuples in current class
        classTests = [
            tup for tup in
            inspect.getmembers(testClass[1], isTestFunction)
        ]

        # sort TestFunctions by line number
        classTests.sort(key=lambda t: inspect.getsourcelines(t[1])[1])

        # create TestCase instances and add to testSuite;
        for test in classTests:
            testSuite.addTest(testClass[1](test[0]))

    return testSuite


if __name__ == '__main__':

    runner = unittest.TextTestRunner()
    runner.run(suite())

Output:

TestB Class Setup
Running test_B
.Running test_A
.TestA Class Setup
Running test_A
.Running test_B
.
----------------------------------------------------------------------
Ran 4 tests in 0.000s

OK