Flask: the difference flow between py.test and python in patch and mock

Sorry for only providing simple code here, I just translate my complex code into simple code for you to know my point.

For example:

main.py

from lib import testlib

print testlib.func1()

lib/testlib.py

def func1():
    return 'unexpected result'

test_mock.py

from lib import testlib
from mock import Mock

class TestCase(unittest.TestCase):
    def test_mock(self):
        testlib.func1 = Mock(return_value=('expected result')

test_patch.py

from lib import testlib
from mock import patch

class TestCase(unittest.TestCase):
    def test_patch(self):
        with patch('lib.testlib.func1') as p:
            p.return_value = ('expected result')

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In case test_mock.py, below commands will get different result

First is wrong, the function Mock don't change the function return value correctly. I also use pdb to check, only before debugger enter into main function, all the things will work well.

python test_mock.py

Second is correct

py.test test_mock.py

However in case test_patch.py, both commands python and py.test will get same and correct result. I finally select the patch solution.

---

I hope those clues are enough for you. I need someone explain the reason.

IMO, I guess the flow of py.test and python is different.