Add --nocolor and --forcecolor options (#85)

Default to nocolor when not running in a tty.

docs/usage.rst

@@ -104,6 +104,23 @@ By default nox will continue to run all sessions even if one fails. You can use
     nox --stop-on-first-error
 
 
+Controlling color output
+------------------------
+
+By default, Nox will output colorful logs if you're using in an interactive
+terminal. However, if you are redirecting ``stderr`` to a file or otherwise
+not using an interactive terminal, nox will output in plaintext.
+
+You can manually control Nox's output using the ``--nocolor`` and ``--forcecolor`` flags.
+
+For example, this will always output colorful logs::
+
+    nox --forcecolor
+
+However, this will never output colorful logs::
+
+    nox --nocolor
+
 Windows
 -------
 

nox/logger.py

@@ -36,25 +36,33 @@ def success(self, msg, *args, **kwargs):
 logger.setLevel(logging.DEBUG)
 
 
-def setup_logging():  # pragma: no cover
+def setup_logging(color):  # pragma: no cover
+    """Setup logging.
+
+    Args:
+        color (bool): If true, the output will be colored using
+            colorlog. Otherwise, it will be plaintext.
+    """
     root_logger = logging.getLogger()
     root_logger.setLevel(logging.DEBUG)
     handler = logging.StreamHandler()
 
-    formatter = ColoredFormatter(
-        "%(cyan)s%(name)s > %(log_color)s%(message)s",
-        reset=True,
-        log_colors={
-            'DEBUG': 'cyan',
-            'INFO': 'blue',
-            'WARNING': 'yellow',
-            'ERROR': 'red',
-            'CRITICAL': 'red,bg_white',
-            'SUCCESS': 'green'
-        }
-    )
-
-    handler.setFormatter(formatter)
+    if color is True:
+        formatter = ColoredFormatter(
+            "%(cyan)s%(name)s > %(log_color)s%(message)s",
+            reset=True,
+            log_colors={
+                'DEBUG': 'cyan',
+                'INFO': 'blue',
+                'WARNING': 'yellow',
+                'ERROR': 'red',
+                'CRITICAL': 'red,bg_white',
+                'SUCCESS': 'green'
+            }
+        )
+
+        handler.setFormatter(formatter)
+
     root_logger.addHandler(handler)
 
     # Silence noisy loggers

nox/main.py

@@ -75,6 +75,13 @@ def main():
     parser.add_argument(
         '--report', default=None,
         help='Output a report of all sessions.')
+    parser.add_argument(
+        '--nocolor', default=not sys.stderr.isatty(), action='store_true',
+        help='Disable all color output.')
+    parser.add_argument(
+        '--forcecolor', default=False, action='store_true',
+        help=('Force color output, even if stdout is not an interactive '
+              'terminal.'))
     parser.add_argument(
         'posargs', nargs=argparse.REMAINDER,
         help='Arguments that are passed through to the sessions.')
@@ -90,7 +97,7 @@ def main():
         return
 
     global_config = GlobalConfig(args)
-    setup_logging()
+    setup_logging(color=not args.nocolor or args.forcecolor)
 
     # Execute the appropriate tasks.
     exit_code = workflow.execute(