Hoist passing through sys.argv at the CLI layer

`flake8.main.cli.main()` is the primary entry point for the command-line
implementation of flake8 (invoked via `__main__` or `console_scripts`).
Therefore, it is reasonable for the entry point to be responsible for
obtaining command line arguments from `sys.argv` there.

Note that `sys.argv[1:]` is necessary in order to strip off the script
name.  Formerly, this was not needed in
`Application.parse_preliminary_options_and_args()`, which was using
`sys.argv[:]` because the result of the argument parsing was just for
determining additional configuration to be loaded.  Then, the *real* CLI
argument parsing was forwarding the original `None` value to
`argparse.ArgumentParser.parse_args()`, which internally was obtaining
arguments as `sys.argv[1:]`.

Additionally, the contract for various argument parsing methods to be
guaranteed a `List[str]`.

src/flake8/main/application.py

@@ -97,8 +97,8 @@ class Application(object):
         #: The parsed diff information
         self.parsed_diff = {}  # type: Dict[str, Set[int]]
 
-    def parse_preliminary_options_and_args(self, argv=None):
-        # type: (Optional[List[str]]) -> None
+    def parse_preliminary_options_and_args(self, argv):
+        # type: (List[str]) -> None
         """Get preliminary options and args from CLI, pre-plugin-loading.
 
         We need to know the values of a few standard options and args now, so
@@ -121,7 +121,7 @@ class Application(object):
         # do not need to worry and we can continue. If it is, we successfully
         # defer printing the version until just a little bit later.
         # Similarly we have to defer printing the help text until later.
-        args = (argv if argv is not None else sys.argv)[:]
+        args = argv[:]
         try:
             args.remove("--version")
         except ValueError:
@@ -136,8 +136,8 @@ class Application(object):
             pass
 
         opts, args = self.option_manager.parse_known_args(args)
-        # parse_known_args includes program name and unknown options as args
-        args = [a for a in args[1:] if not a.startswith("-")]
+        # parse_known_args includes unknown options as args
+        args = [a for a in args if not a.startswith("-")]
         self.prelim_opts, self.prelim_args = opts, args
 
     def exit(self):
@@ -344,7 +344,7 @@ class Application(object):
         self.formatter.show_statistics(self.guide.stats)
 
     def initialize(self, argv):
-        # type: (Optional[List[str]]) -> None
+        # type: (List[str]) -> None
         """Initialize the application to be run.
 
         This finds the plugins, registers their options, and parses the
@@ -373,13 +373,13 @@ class Application(object):
         self.formatter.stop()
 
     def _run(self, argv):
-        # type: (Optional[List[str]]) -> None
+        # type: (List[str]) -> None
         self.initialize(argv)
         self.run_checks()
         self.report()
 
-    def run(self, argv=None):
-        # type: (Optional[List[str]]) -> None
+    def run(self, argv):
+        # type: (List[str]) -> None
         """Run our application.
 
         This method will also handle KeyboardInterrupt exceptions for the

src/flake8/main/cli.py

@@ -1,4 +1,5 @@
 """Command-line implementation of flake8."""
+import sys
 from typing import List, Optional
 
 from flake8.main import application
@@ -14,6 +15,9 @@ def main(argv=None):
     :param list argv:
         The arguments to be passed to the application for parsing.
     """
+    if argv is None:
+        argv = sys.argv[1:]
+
     app = application.Application()
     app.run(argv)
     app.exit()

tests/integration/test_main.py

@@ -126,3 +126,13 @@ def test_bug_report_successful(capsys):
     out, err = capsys.readouterr()
     assert json.loads(out)
     assert err == ''
+
+
+def test_obtaining_args_from_sys_argv_when_not_explicity_provided(capsys):
+    """Test that arguments are obtained from 'sys.argv'."""
+    with mock.patch('sys.argv', ['flake8', '--help']):
+        _call_main(None)
+
+    out, err = capsys.readouterr()
+    assert out.startswith('usage: flake8 [options] file file ...\n')
+    assert err == ''