Before:
  Save g:ale_warn_about_trailing_whitespace

  let g:ale_warn_about_trailing_whitespace = 1

  runtime ale_linters/python/prospector.vim

After:
  Restore

  call ale#linter#Reset()

  silent file something_else.py

Execute(Basic prospector errors should be handle):
  AssertEqual
  \ [
  \   {
  \     'lnum': 20,
  \     'col': 1,
  \     'text': 'Trailing whitespace',
  \     'code': '(pylint) trailing-whitespace',
  \     'type': 'E',
  \   },
  \   {
  \     'lnum': 1,
  \     'col': 1,
  \     'text': 'Missing module docstring',
  \     'code': '(pylint) missing-docstring',
  \     'type': 'E',
  \   },
  \   {
  \     'lnum': 2,
  \     'col': 1,
  \     'text': 'Missing function docstring',
  \     'code': '(pylint) missing-docstring',
  \     'type': 'E',
  \   },
  \   {
  \     'lnum': 3,
  \     'col': 5,
  \     'text': '''break'' not properly in loop',
  \     'code': '(pylint) not-in-loop',
  \     'type': 'E',
  \   },
  \   {
  \     'lnum': 4,
  \     'col': 5,
  \     'text': 'Unreachable code',
  \     'code': '(pylint) unreachable',
  \     'type': 'E',
  \   },
  \   {
  \     'lnum': 7,
  \     'col': 33,
  \     'text': 'No exception type(s) specified',
  \     'code': '(pylint) bare-except',
  \     'type': 'E',
  \   },
  \ ],
  \ ale_linters#python#prospector#Handle(bufnr(''), [
  \ '{',
  \ '  "messages": [',
  \ '    {',
  \ '      "source": "pylint",',
  \ '      "code": "trailing-whitespace",',
  \ '      "message": "Trailing whitespace",',
  \ '      "location": {',
  \ '        "character": 0,',
  \ '        "line": 20',
  \ '      }',
  \ '    },',
  \ '    {',
  \ '      "source": "pylint",',
  \ '      "code": "missing-docstring",',
  \ '      "message": "Missing module docstring",',
  \ '      "location": {',
  \ '        "character": 0,',
  \ '        "line": 1',
  \ '      }',
  \ '    },',
  \ '    {',
  \ '      "source": "pylint",',
  \ '      "code": "missing-docstring",',
  \ '      "message": "Missing function docstring",',
  \ '      "location": {',
  \ '        "character": 0,',
  \ '        "line": 2',
  \ '      }',
  \ '    },',
  \ '    {',
  \ '      "source": "pylint",',
  \ '      "code": "not-in-loop",',
  \ '      "message": "''break'' not properly in loop",',
  \ '      "location": {',
  \ '        "character": 4,',
  \ '        "line": 3',
  \ '      }',
  \ '    },',
  \ '    {',
  \ '      "source": "pylint",',
  \ '      "code": "unreachable",',
  \ '      "message": "Unreachable code",',
  \ '      "location": {',
  \ '        "character": 4,',
  \ '        "line": 4',
  \ '      }',
  \ '    },',
  \ '    {',
  \ '      "source": "pylint",',
  \ '      "code": "bare-except",',
  \ '      "message": "No exception type(s) specified",',
  \ '      "location": {',
  \ '        "character": 32,',
  \ '        "line": 7',
  \ '      }',
  \ '    }',
  \ '  ]',
  \ '}',
  \ ])

Execute(Ignoring trailing whitespace messages should work):
  let g:ale_warn_about_trailing_whitespace = 0

  AssertEqual
  \ [
  \   {
  \     'lnum': 1,
  \     'col': 1,
  \     'text': 'Missing module docstring',
  \     'code': '(pylint) missing-docstring',
  \     'type': 'E',
  \   },
  \ ],
  \ ale_linters#python#prospector#Handle(bufnr(''), [
  \ '{',
  \ '  "messages": [',
  \ '    {',
  \ '      "source": "pylint",',
  \ '      "code": "trailing-whitespace",',
  \ '      "message": "Trailing whitespace",',
  \ '      "location": {',
  \ '        "character": 0,',
  \ '        "line": 4',
  \ '      }',
  \ '    },',
  \ '    {',
  \ '      "source": "pylint",',
  \ '      "code": "missing-docstring",',
  \ '      "message": "Missing module docstring",',
  \ '      "location": {',
  \ '        "character": 0,',
  \ '        "line": 1',
  \ '      }',
  \ '    }',
  \ '  ]',
  \ '}',
  \ ])