flake8: Disable E203 check
PEP 8 recommends to treat : in slices as a binary operator with the lowest priority, and to leave an equal amount of space on either side, except if a parameter is omitted (e.g. ham[1 + 1 :]). It also states that for extended slices, both : operators have to have the same amount of spacing, except if a parameter is omitted (ham[1 + 1 ::]). Black enforces these rules consistently.
This behaviour may raise E203 whitespace before ':' warnings in style guide enforcement tools like Flake8. Since E203 is not PEP 8 compliant, you should tell Flake8 to ignore these warnings.