checked author's solution. it will return 4. not an issue.

That test doesn't exist in either the sample or full test suites. There is a similar one -5x^2+10x+4, for which the gradient is indeed -20; I think you've modified the tests and are now failing them because you modified them. Probably not a kata issue.

is it just me or are some of the python tests just wrong?

this test (test.assert_equals(differentiate("-x^2+10x+4", 3), -20)) says that the slope of -x^2+10x+4 at point 3 is 20 when in reality it should be 4.

I calculated this myself and also let wolfram alpha do it: https://www.wolframalpha.com/input?i=derivative+of+-x%5E2%2B10x%2B4+at+3

I passed most tests, but a big part of them is still completely wrong. Am i not getting something here?