linter: Create error printing library.

For performance reasons, we spawn each linter in a separate OS thread.
The downside of this is that all lints would end up in stdout without
much visual separation, resulting in confusing error log. This commit
introduce the `print_err` function, which shows which linter each line
of lint is from.

scripts/lib/zulip_tools.py

@@ -25,6 +25,13 @@ FAIL = '\033[91m'
 ENDC = '\033[0m'
 BLACKONYELLOW = '\x1b[0;30;43m'
 WHITEONRED = '\x1b[0;37;41m'
+BOLDRED = '\x1B[1;31m'
+
+GREEN = '\x1b[32m'
+YELLOW = '\x1b[33m'
+BLUE = '\x1b[34m'
+MAGENTA = '\x1b[35m'
+CYAN = '\x1b[36m'
 
 def get_deployment_version(extract_path):
     # type: (str) -> str

tools/linter_lib/printer.py

@@ -0,0 +1,33 @@
+from __future__ import print_function, absolute_import
+
+import sys
+import os
+from itertools import cycle
+
+sys.path.append(os.path.join(os.path.dirname(__file__), '../..'))
+from scripts.lib.zulip_tools import ENDC, BOLDRED, GREEN, YELLOW, BLUE, MAGENTA, CYAN
+
+from typing import Union, Text
+
+colors = cycle([GREEN, YELLOW, BLUE, MAGENTA, CYAN])
+
+
+def print_err(name, color, line):
+    # type: (str, str, Union[Text, bytes]) -> None
+
+    # Decode with UTF-8 if in Python 3 and `line` is of bytes type.
+    # (Python 2 does this automatically)
+    if sys.version_info[0] == 3 and isinstance(line, bytes):
+        line = line.decode('utf-8')
+
+    print('{}{}{}|{end} {}{}{end}'.format(
+        color,
+        name,
+        ' ' * max(0, 10 - len(name)),
+        BOLDRED,
+        line.rstrip(),
+        end=ENDC)
+    )
+
+    # Python 2's print function does not have a `flush` option.
+    sys.stdout.flush()