Parse --jobs as a custom argparse type. Fixes #567

src/flake8/checker.py

@@ -137,19 +137,12 @@ class Manager(object):
             return 0
 
         jobs = self.options.jobs
-        if jobs != "auto" and not jobs.isdigit():
-            LOG.warning(
-                '"%s" is not a valid parameter to --jobs. Must be one '
-                'of "auto" or a numerical value, e.g., 4.',
-                jobs,
-            )
-            return 0
 
         # If the value is "auto", we want to let the multiprocessing library
         # decide the number based on the number of CPUs. However, if that
         # function is not implemented for this particular value of Python we
         # default to 1
-        if jobs == "auto":
+        if jobs.is_auto:
             try:
                 return multiprocessing.cpu_count()
             except NotImplementedError:
@@ -157,7 +150,7 @@ class Manager(object):
 
         # Otherwise, we know jobs should be an integer and we can just convert
         # it to an integer
-        return int(jobs)
+        return jobs.n_jobs
 
     def _handle_results(self, filename, results):
         style_guide = self.style_guide

src/flake8/main/options.py

@@ -62,6 +62,27 @@ def register_preliminary_options(parser):
     )
 
 
+class JobsArgument:
+    """Type callback for the --jobs argument."""
+
+    def __init__(self, arg):  # type: (str) -> None
+        """Parse and validate the --jobs argument.
+
+        :param str arg:
+            The argument passed by argparse for validation
+        """
+        self.is_auto = False
+        self.n_jobs = -1
+        if arg == "auto":
+            self.is_auto = True
+        elif arg.isdigit():
+            self.n_jobs = int(arg)
+        else:
+            raise argparse.ArgumentTypeError(
+                "{!r} must be 'auto' or an integer.".format(arg),
+            )
+
+
 def register_default_options(option_manager):
     """Register the default options on our OptionManager.
 
@@ -293,6 +314,7 @@ def register_default_options(option_manager):
         "--jobs",
         default="auto",
         parse_from_config=True,
+        type=JobsArgument,
         help="Number of subprocesses to use to run checks in parallel. "
         'This is ignored on Windows. The default, "auto", will '
         "auto-detect the number of processors available to use."