diff --git a/tools/lint b/tools/lint
index 021ca0ca00..61628ca139 100755
--- a/tools/lint
+++ b/tools/lint
@@ -4,11 +4,9 @@ from __future__ import absolute_import
 from contextlib import contextmanager
 import logging
 import os
-import re
 import sys
 import optparse
 import subprocess
-import traceback
 
 # check for the venv
 from lib import sanity_check
@@ -206,459 +204,6 @@ def run_parallel(lint_functions):
             failed = True
     return failed
 
-def build_custom_checkers(by_lang):
-    # type: (Dict[str, List[str]]) -> Tuple[Callable[[], bool], Callable[[], bool]]
-    RuleList = List[Dict[str, Any]]
-
-    def custom_check_file(fn, rules, skip_rules=None, max_length=None):
-        # type: (str, RuleList, Optional[Any], Optional[int]) -> bool
-        failed = False
-        lineFlag = False
-        for i, line in enumerate(open(fn)):
-            line_newline_stripped = line.strip('\n')
-            line_fully_stripped = line_newline_stripped.strip()
-            skip = False
-            lineFlag = True
-            for rule in skip_rules or []:
-                if re.match(rule, line):
-                    skip = True
-            if line_fully_stripped.endswith('  # nolint'):
-                continue
-            if skip:
-                continue
-            for rule in rules:
-                exclude_list = rule.get('exclude', set())
-                if fn in exclude_list or os.path.dirname(fn) in exclude_list:
-                    continue
-                exclude_list = rule.get('exclude_line', set())
-                if (fn, line_fully_stripped) in exclude_list:
-                    continue
-                if rule.get("include_only"):
-                    found = False
-                    for item in rule.get("include_only", set()):
-                        if item in fn:
-                            found = True
-                    if not found:
-                        continue
-                try:
-                    line_to_check = line_fully_stripped
-                    if rule.get('strip') is not None:
-                        if rule['strip'] == '\n':
-                            line_to_check = line_newline_stripped
-                        else:
-                            raise Exception("Invalid strip rule")
-                    if re.search(rule['pattern'], line_to_check):
-                        sys.stdout.write(rule['description'] + ' at %s line %s:\n' % (fn, i+1))
-                        print(line)
-                        failed = True
-                except Exception:
-                    print("Exception with %s at %s line %s" % (rule['pattern'], fn, i+1))
-                    traceback.print_exc()
-            if isinstance(line, bytes):
-                line_length = len(line.decode("utf-8"))
-            else:
-                line_length = len(line)
-            if (max_length is not None and line_length > max_length and
-                '# type' not in line and 'test' not in fn and 'example' not in fn and
-                not re.match("\[[ A-Za-z0-9_:,&()-]*\]: http.*", line) and
-                    "#ignorelongline" not in line and 'migrations' not in fn):
-                print("Line too long (%s) at %s line %s: %s" % (len(line), fn, i+1, line_newline_stripped))
-                failed = True
-            lastLine = line
-        if lineFlag and '\n' not in lastLine:
-            print("No newline at the end of file.  Fix with `sed -i '$a\\' %s`" % (fn,))
-            failed = True
-        return failed
-
-    whitespace_rules = [
-        # This linter should be first since bash_rules depends on it.
-        {'pattern': '\s+$',
-         'strip': '\n',
-         'description': 'Fix trailing whitespace'},
-        {'pattern': '\t',
-         'strip': '\n',
-         'exclude': set(['zerver/lib/bugdown/codehilite.py',
-                         'tools/travis/success-http-headers.txt']),
-         'description': 'Fix tab-based whitespace'},
-    ]  # type: RuleList
-    markdown_whitespace_rules = list([rule for rule in whitespace_rules if rule['pattern'] != '\s+$']) + [
-        # Two spaces trailing a line with other content is okay--it's a markdown line break.
-        # This rule finds one space trailing a non-space, three or more trailing spaces, and
-        # spaces on an empty line.
-        {'pattern': '((?<!\s)\s$)|(\s\s\s+$)|(^\s+$)',
-         'strip': '\n',
-         'description': 'Fix trailing whitespace'},
-        {'pattern': '^#+[A-Za-z0-9]',
-         'strip': '\n',
-         'description': 'Missing space after # in heading'},
-    ]  # type: RuleList
-    js_rules = cast(RuleList, [
-        {'pattern': '[^_]function\(',
-         'description': 'The keyword "function" should be followed by a space'},
-        {'pattern': '.*blueslip.warning\(.*',
-         'description': 'The module blueslip has no function warning, try using blueslip.warn'},
-        {'pattern': '[)]{$',
-         'description': 'Missing space between ) and {'},
-        {'pattern': '["\']json/',
-         'description': 'Relative URL for JSON route not supported by i18n'},
-        # This rule is constructed with + to avoid triggering on itself
-        {'pattern': " =" + '[^ =>~"]',
-         'description': 'Missing whitespace after "="'},
-        {'pattern': '^[ ]*//[A-Za-z0-9]',
-         'description': 'Missing space after // in comment'},
-        {'pattern': 'if[(]',
-         'description': 'Missing space between if and ('},
-        {'pattern': 'else{$',
-         'description': 'Missing space between else and {'},
-        {'pattern': '^else {$',
-         'description': 'Write JS else statements on same line as }'},
-        {'pattern': '^else if',
-         'description': 'Write JS else statements on same line as }'},
-        {'pattern': 'console[.][a-z]',
-         'exclude': set(['static/js/blueslip.js',
-                         'frontend_tests/zjsunit',
-                         'frontend_tests/casper_lib/common.js',
-                         'frontend_tests/node_tests',
-                         'static/js/debug.js']),
-         'description': 'console.log and similar should not be used in webapp'},
-        {'pattern': 'i18n[.]t',
-         'include_only': set(['static/js/portico']),
-         'description': 'i18n.t is not available in portico pages yet'},
-        {'pattern': '[.]text\(["\'][a-zA-Z]',
-         'description': 'Strings passed to $().text should be wrapped in i18n.t() for internationalization'},
-        {'pattern': 'compose_error\(["\']',
-         'exclude': set(['tools/lint']),
-         'description': 'Argument to compose_error should be a literal string enclosed '
-                        'by i18n.t()'},
-        {'pattern': 'ui.report_success\(',
-         'exclude': set(['tools/lint']),
-         'description': 'Deprecated function, use ui_report.success.'},
-        {'pattern': 'report.success\(["\']',
-         'exclude': set(['tools/lint']),
-         'description': 'Argument to report_success should be a literal string enclosed '
-                        'by i18n.t()'},
-        {'pattern': 'ui.report_error\(',
-         'exclude': set(['tools/lint']),
-         'description': 'Deprecated function, use ui_report.error.'},
-        {'pattern': 'report.error\(["\']',
-         'exclude': set(['tools/lint']),
-         'description': 'Argument to report_error should be a literal string enclosed '
-                        'by i18n.t()'},
-    ]) + whitespace_rules
-    python_rules = cast(RuleList, [
-        {'pattern': '^(?!#)@login_required',
-         'description': '@login_required is unsupported; use @zulip_login_required'},
-        {'pattern': '".*"%\([a-z_].*\)?$',
-         'description': 'Missing space around "%"'},
-        {'pattern': "'.*'%\([a-z_].*\)?$",
-         'exclude': set(['tools/lint',
-                         'analytics/lib/counts.py',
-                         'analytics/tests/test_counts.py',
-                         ]),
-         'exclude_line': set([
-             ('zerver/views/users.py',
-              "return json_error(_(\"Email '%(email)s' not allowed for realm '%(realm)s'\") %"),
-             ('zproject/settings.py',
-              "'format': '%(asctime)s %(levelname)-8s %(message)s'"),
-         ]),
-         'description': 'Missing space around "%"'},
-        # This rule is constructed with + to avoid triggering on itself
-        {'pattern': " =" + '[^ =>~"]',
-         'description': 'Missing whitespace after "="'},
-        {'pattern': '":\w[^"]*$',
-         'description': 'Missing whitespace after ":"'},
-        {'pattern': "':\w[^']*$",
-         'description': 'Missing whitespace after ":"'},
-        {'pattern': "^\s+[#]\w",
-         'strip': '\n',
-         'description': 'Missing whitespace after "#"'},
-        {'pattern': "assertEquals[(]",
-         'description': 'Use assertEqual, not assertEquals (which is deprecated).'},
-        {'pattern': "== None",
-         'exclude': 'tools/lint',
-         'description': 'Use `is None` to check whether something is None'},
-        {'pattern': "type:[(]",
-         'description': 'Missing whitespace after ":" in type annotation'},
-        {'pattern': "# type [(]",
-         'description': 'Missing : after type in type annotation'},
-        {'pattern': "#type",
-         'exclude': 'tools/lint',
-         'description': 'Missing whitespace after "#" in type annotation'},
-        {'pattern': 'if[(]',
-         'description': 'Missing space between if and ('},
-        {'pattern': ", [)]",
-         'description': 'Unnecessary whitespace between "," and ")"'},
-        {'pattern': "%  [(]",
-         'description': 'Unnecessary whitespace between "%" and "("'},
-        # This next check could have false positives, but it seems pretty
-        # rare; if we find any, they can be added to the exclude list for
-        # this rule.
-        {'pattern': ' % [a-zA-Z0-9_.]*\)?$',
-         'exclude_line': set([
-             ('tools/tests/test_template_parser.py', '{% foo'),
-         ]),
-         'description': 'Used % comprehension without a tuple'},
-        {'pattern': '.*%s.* % \([a-zA-Z0-9_.]*\)$',
-         'description': 'Used % comprehension without a tuple'},
-        {'pattern': 'django.utils.translation',
-         'include_only': set(['test']),
-         'description': 'Test strings should not be tagged for translationx'},
-        {'pattern': 'json_success\({}\)',
-         'description': 'Use json_success() to return nothing'},
-        # To avoid json_error(_variable) and json_error(_(variable))
-        {'pattern': '\Wjson_error\(_\(?\w+\)',
-         'exclude': set(['tools/lint', 'zerver/tests']),
-         'description': 'Argument to json_error should be a literal string enclosed by _()'},
-        {'pattern': '\Wjson_error\([\'"].+[),]$',
-         'exclude': set(['tools/lint', 'zerver/tests']),
-         'exclude_line': set([
-             # We don't want this string tagged for translation.
-             ('zerver/views/compatibility.py', 'return json_error("Client is too old")'),
-         ]),
-         'description': 'Argument to json_error should a literal string enclosed by _()'},
-        # To avoid JsonableError(_variable) and JsonableError(_(variable))
-        {'pattern': '\WJsonableError\(_\(?\w.+\)',
-         'exclude': set(['tools/lint', 'zerver/tests']),
-         'description': 'Argument to JsonableError should be a literal string enclosed by _()'},
-        {'pattern': '\WJsonableError\(["\'].+\)',
-         'exclude': set(['tools/lint', 'zerver/tests']),
-         'description': 'Argument to JsonableError should be a literal string enclosed by _()'},
-        {'pattern': '([a-zA-Z0-9_]+)=REQ\([\'"]\\1[\'"]',
-         'description': 'REQ\'s first argument already defaults to parameter name'},
-        {'pattern': 'self\.client\.(get|post|patch|put|delete)',
-         'exclude': set(['zilencer/tests.py']),
-         'description': \
-         '''Do not call self.client directly for put/patch/post/get.
-    See WRAPPER_COMMENT in test_helpers.py for details.
-    '''},
-        # Directly fetching Message objects in e.g. views code is often a security bug.
-        {'pattern': '[^r][M]essage.objects.get',
-         'exclude': set(["zerver/tests", "zerver/worker/queue_processors.py"]),
-         'description': 'Please use access_message() to fetch Message objects',
-         },
-        {'pattern': '[S]tream.objects.get',
-         'include_only': set(["zerver/views/"]),
-         'description': 'Please use access_stream_by_*() to fetch Stream objects',
-         },
-        {'pattern': 'get_stream[(]',
-         'include_only': set(["zerver/views/", "zerver/lib/actions.py"]),
-         # messages.py needs to support accessing invite-only streams
-         # that you are no longer subscribed to, so need get_stream.
-         'exclude': set(['zerver/views/messages.py']),
-         'exclude_line': set([
-             # This is a check for whether a stream rename is invalid because it already exists
-             ('zerver/lib/actions.py', 'get_stream(new_name, stream.realm)'),
-             # This one in check_message is kinda terrible, since it's
-             # how most instances are written, but better to exclude something than nothing
-             ('zerver/lib/actions.py', 'stream = get_stream(stream_name, realm)'),
-             ('zerver/lib/actions.py', 'get_stream(signups_stream, admin_realm)'),
-         ]),
-         'description': 'Please use access_stream_by_*() to fetch Stream objects',
-         },
-        {'pattern': '[S]tream.objects.filter',
-         'include_only': set(["zerver/views/"]),
-         'description': 'Please use access_stream_by_*() to fetch Stream objects',
-         },
-        {'pattern': '^from (zerver|analytics|confirmation)',
-         'include_only': set(["/migrations/"]),
-         'exclude': set(['zerver/migrations/0032_verify_all_medium_avatar_images.py',
-                         'zerver/migrations/0041_create_attachments_for_old_messages.py',
-                         'zerver/migrations/0060_move_avatars_to_be_uid_based.py']),
-         'description': "Don't import models or other code in migrations; see docs/schema-migrations.md",
-         },
-        {'pattern': 'datetime[.](now|utcnow)',
-         'include_only': set(["zerver/", "analytics/"]),
-         'description': "Don't use datetime in backend code.\n"
-         "See https://zulip.readthedocs.io/en/latest/code-style.html#naive-datetime-objects",
-         },
-        {'pattern': 'render_to_response\(',
-         'description': "Use render() instead of render_to_response().",
-         'exclude': set(['tools/lint']),
-         },
-        # This rule might give false positives in virtualenv setup files which should be excluded,
-        # and comments which should be rewritten to avoid use of "python2", "python3", etc.
-        {'pattern': 'python[23]',
-         'exclude': set(['tools/lib/provision.py',
-                         'tools/setup/setup_venvs.py',
-                         'scripts/lib/setup_venv.py',
-                         'tools/lint']),
-         'description': 'Explicit python invocations should not include a version'}
-    ]) + whitespace_rules
-    bash_rules = [
-        {'pattern': '#!.*sh [-xe]',
-         'description': 'Fix shebang line with proper call to /usr/bin/env for Bash path, change -x|-e switches'
-                        ' to set -x|set -e'},
-    ] + whitespace_rules[0:1]  # type: RuleList
-    css_rules = cast(RuleList, [
-        {'pattern': '^[^:]*:\S[^:]*;$',
-         'description': "Missing whitespace after : in CSS"},
-        {'pattern': '[a-z]{',
-         'description': "Missing whitespace before '{' in CSS."},
-        {'pattern': 'https://',
-         'description': "Zulip CSS should have no dependencies on external resources"},
-        {'pattern': '^[ ][ ][a-zA-Z0-9]',
-         'description': "Incorrect 2-space indentation in CSS",
-         'exclude': set(['static/third/thirdparty-fonts.css']),
-         'strip': '\n'},
-        {'pattern': '{\w',
-         'description': "Missing whitespace after '{' in CSS (should be newline)."},
-        {'pattern': ' thin[; ]',
-         'description': "thin CSS attribute is under-specified, please use 1px."},
-        {'pattern': ' medium[; ]',
-         'description': "medium CSS attribute is under-specified, please use pixels."},
-        {'pattern': ' thick[; ]',
-         'description': "thick CSS attribute is under-specified, please use pixels."},
-    ]) + whitespace_rules  # type: RuleList
-    prose_style_rules = [
-        {'pattern': '[^\/\#\-\"]([jJ]avascript)',  # exclude usage in hrefs/divs
-         'description': "javascript should be spelled JavaScript"},
-        {'pattern': '[^\/\-\.\"\'\_\=\>]([gG]ithub)[^\.\-\_\"\<]',  # exclude usage in hrefs/divs
-         'description': "github should be spelled GitHub"},
-        {'pattern': '[oO]rganisation',  # exclude usage in hrefs/divs
-         'description': "Organization is spelled with a z"},
-        {'pattern': '!!! warning',
-         'description': "!!! warning is invalid; it's spelled '!!! warn'"},
-    ]  # type: RuleList
-    html_rules = whitespace_rules + prose_style_rules + [
-        {'pattern': 'placeholder="[^{]',
-         'description': "`placeholder` value should be translatable.",
-         'exclude_line': [('templates/zerver/register.html', 'placeholder="acme"'),
-                          ('templates/zerver/register.html', 'placeholder="Acme"'),
-                          ('static/templates/settings/realm-domains-modal.handlebars',
-                           '<td><input type="text" class="new-realm-domain" placeholder="acme.com"></input></td>')],
-         'exclude': set(["static/templates/settings/emoji-settings-admin.handlebars",
-                         "static/templates/settings/realm-filter-settings-admin.handlebars",
-                         "static/templates/settings/bot-settings.handlebars"])},
-        {'pattern': "placeholder='[^{]",
-         'description': "`placeholder` value should be translatable."},
-        {'pattern': 'script src="http',
-         'description': "Don't directly load dependencies from CDNs.  See docs/front-end-build-process.md"},
-        {'pattern': "title='[^{]",
-         'description': "`title` value should be translatable."},
-        {'pattern': 'title="[^{\:]',
-         'exclude_line': set([
-             ('templates/zerver/markdown_help.html',
-              '<td><img alt=":heart:" class="emoji" src="/static/generated/emoji/images/emoji/heart.png" title=":heart:" /></td>')
-         ]),
-         'exclude': set(["templates/zerver/emails"]),
-         'description': "`title` value should be translatable."},
-        {'pattern': '\Walt=["\'][^{"\']',
-         'description': "alt argument should be enclosed by _() or it should be an empty string.",
-         'exclude': set(['tools/lint',
-                         'static/templates/settings/display-settings.handlebars',
-                         'templates/zerver/keyboard_shortcuts.html',
-                         'templates/zerver/markdown_help.html']),
-         },
-        {'pattern': '\Walt=["\']{{ ?["\']',
-         'description': "alt argument should be enclosed by _().",
-         'exclude': set(['tools/lint']),
-         },
-    ]  # type: RuleList
-    handlebars_rules = html_rules + [
-        {'pattern': "[<]script",
-         'description': "Do not use inline <script> tags here; put JavaScript in static/js instead."},
-        {'pattern': "{{t '.*' }}[\.\?!]",
-         'description': "Period should be part of the translatable string."},
-        {'pattern': '{{t ".*" }}[\.\?!]',
-         'description': "Period should be part of the translatable string."},
-        {'pattern': "{{/tr}}[\.\?!]",
-         'description': "Period should be part of the translatable string."},
-    ]
-    jinja2_rules = html_rules + [
-        {'pattern': "{% endtrans %}[\.\?!]",
-         'description': "Period should be part of the translatable string."},
-        {'pattern': "{{ _(.+) }}[\.\?!]",
-         'description': "Period should be part of the translatable string."},
-    ]
-    json_rules = []  # type: RuleList # fix newlines at ends of files
-    # It is okay that json_rules is empty, because the empty list
-    # ensures we'll still check JSON files for whitespace.
-    markdown_rules = markdown_whitespace_rules + prose_style_rules + [
-        {'pattern': '\[(?P<url>[^\]]+)\]\((?P=url)\)',
-         'description': 'Linkified markdown URLs should use cleaner <http://example.com> syntax.'}
-    ]
-    help_markdown_rules = markdown_rules + [
-        {'pattern': '[a-z][.][A-Z]',
-         'description': "Likely missing space after end of sentence"},
-        {'pattern': '[rR]ealm',
-         'description': "Realms are referred to as Organizations in user-facing docs."},
-    ]
-    txt_rules = whitespace_rules
-
-    def check_custom_checks_py():
-        # type: () -> bool
-        failed = False
-
-        for fn in by_lang['py']:
-            if custom_check_file(fn, python_rules, max_length=140):
-                failed = True
-        return failed
-
-    def check_custom_checks_nonpy():
-        # type: () -> bool
-        failed = False
-
-        for fn in by_lang['js']:
-            if custom_check_file(fn, js_rules):
-                failed = True
-
-        for fn in by_lang['sh']:
-            if custom_check_file(fn, bash_rules):
-                failed = True
-
-        for fn in by_lang['css']:
-            if custom_check_file(fn, css_rules):
-                failed = True
-
-        for fn in by_lang['handlebars']:
-            if custom_check_file(fn, handlebars_rules):
-                failed = True
-
-        for fn in by_lang['html']:
-            if custom_check_file(fn, jinja2_rules):
-                failed = True
-
-        for fn in by_lang['json']:
-            if custom_check_file(fn, json_rules):
-                failed = True
-
-        markdown_docs_length_exclude = {
-            "api/bots/converter/readme.md",
-            "docs/bots-guide.md",
-            "docs/dev-env-first-time-contributors.md",
-            "docs/webhook-walkthrough.md",
-            "docs/life-of-a-request.md",
-            "docs/logging.md",
-            "docs/migration-renumbering.md",
-            "docs/readme-symlink.md",
-            "README.md",
-            "zerver/webhooks/helloworld/doc.md",
-            "zerver/webhooks/solano/doc.md",
-            "zerver/webhooks/trello/doc.md",
-            "zerver/webhooks/papertrail/doc.md",
-        }
-        for fn in by_lang['md']:
-            max_length = None
-            if fn not in markdown_docs_length_exclude:
-                max_length = 120
-            rules = markdown_rules
-            if fn.startswith("templates/zerver/help"):
-                rules = help_markdown_rules
-            if custom_check_file(fn, rules, max_length=max_length):
-                failed = True
-
-        for fn in by_lang['txt'] + by_lang['text']:
-            if custom_check_file(fn, txt_rules):
-                failed = True
-
-        for fn in by_lang['yaml']:
-            if custom_check_file(fn, txt_rules):
-                failed = True
-
-        return failed
-
-    return (check_custom_checks_py, check_custom_checks_nonpy)
-
 def run():
     # type: () -> None
     parser = optparse.OptionParser()
@@ -686,6 +231,8 @@ def run():
     root_dir = os.path.dirname(tools_dir)
     sys.path.insert(0, root_dir)
 
+    from tools.linter_lib.custom_check import build_custom_checkers
+
     from tools.lib.test_script import (
         get_provisioning_status,
     )
diff --git a/tools/linter_lib/__init__.py b/tools/linter_lib/__init__.py
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/tools/linter_lib/custom_check.py b/tools/linter_lib/custom_check.py
new file mode 100644
index 0000000000..ac5b00e90b
--- /dev/null
+++ b/tools/linter_lib/custom_check.py
@@ -0,0 +1,452 @@
+from __future__ import print_function
+from __future__ import absolute_import
+
+import os
+import re
+import sys
+import traceback
+
+from typing import cast, Any, Callable, Dict, List, Optional, Tuple
+
+def build_custom_checkers(by_lang):
+    # type: (Dict[str, List[str]]) -> Tuple[Callable[[], bool], Callable[[], bool]]
+    RuleList = List[Dict[str, Any]]
+
+    def custom_check_file(fn, rules, skip_rules=None, max_length=None):
+        # type: (str, RuleList, Optional[Any], Optional[int]) -> bool
+        failed = False
+        lineFlag = False
+        for i, line in enumerate(open(fn)):
+            line_newline_stripped = line.strip('\n')
+            line_fully_stripped = line_newline_stripped.strip()
+            skip = False
+            lineFlag = True
+            for rule in skip_rules or []:
+                if re.match(rule, line):
+                    skip = True
+            if line_fully_stripped.endswith('  # nolint'):
+                continue
+            if skip:
+                continue
+            for rule in rules:
+                exclude_list = rule.get('exclude', set())
+                if fn in exclude_list or os.path.dirname(fn) in exclude_list:
+                    continue
+                exclude_list = rule.get('exclude_line', set())
+                if (fn, line_fully_stripped) in exclude_list:
+                    continue
+                if rule.get("include_only"):
+                    found = False
+                    for item in rule.get("include_only", set()):
+                        if item in fn:
+                            found = True
+                    if not found:
+                        continue
+                try:
+                    line_to_check = line_fully_stripped
+                    if rule.get('strip') is not None:
+                        if rule['strip'] == '\n':
+                            line_to_check = line_newline_stripped
+                        else:
+                            raise Exception("Invalid strip rule")
+                    if re.search(rule['pattern'], line_to_check):
+                        sys.stdout.write(rule['description'] + ' at %s line %s:\n' % (fn, i+1))
+                        print(line)
+                        failed = True
+                except Exception:
+                    print("Exception with %s at %s line %s" % (rule['pattern'], fn, i+1))
+                    traceback.print_exc()
+            if isinstance(line, bytes):
+                line_length = len(line.decode("utf-8"))
+            else:
+                line_length = len(line)
+            if (max_length is not None and line_length > max_length and
+                '# type' not in line and 'test' not in fn and 'example' not in fn and
+                not re.match("\[[ A-Za-z0-9_:,&()-]*\]: http.*", line) and
+                    "#ignorelongline" not in line and 'migrations' not in fn):
+                print("Line too long (%s) at %s line %s: %s" % (len(line), fn, i+1, line_newline_stripped))
+                failed = True
+            lastLine = line
+        if lineFlag and '\n' not in lastLine:
+            print("No newline at the end of file.  Fix with `sed -i '$a\\' %s`" % (fn,))
+            failed = True
+        return failed
+
+    whitespace_rules = [
+        # This linter should be first since bash_rules depends on it.
+        {'pattern': '\s+$',
+         'strip': '\n',
+         'description': 'Fix trailing whitespace'},
+        {'pattern': '\t',
+         'strip': '\n',
+         'exclude': set(['zerver/lib/bugdown/codehilite.py',
+                         'tools/travis/success-http-headers.txt']),
+         'description': 'Fix tab-based whitespace'},
+    ]  # type: RuleList
+    markdown_whitespace_rules = list([rule for rule in whitespace_rules if rule['pattern'] != '\s+$']) + [
+        # Two spaces trailing a line with other content is okay--it's a markdown line break.
+        # This rule finds one space trailing a non-space, three or more trailing spaces, and
+        # spaces on an empty line.
+        {'pattern': '((?<!\s)\s$)|(\s\s\s+$)|(^\s+$)',
+         'strip': '\n',
+         'description': 'Fix trailing whitespace'},
+        {'pattern': '^#+[A-Za-z0-9]',
+         'strip': '\n',
+         'description': 'Missing space after # in heading'},
+    ]  # type: RuleList
+    js_rules = cast(RuleList, [
+        {'pattern': '[^_]function\(',
+         'description': 'The keyword "function" should be followed by a space'},
+        {'pattern': '.*blueslip.warning\(.*',
+         'description': 'The module blueslip has no function warning, try using blueslip.warn'},
+        {'pattern': '[)]{$',
+         'description': 'Missing space between ) and {'},
+        {'pattern': '["\']json/',
+         'description': 'Relative URL for JSON route not supported by i18n'},
+        # This rule is constructed with + to avoid triggering on itself
+        {'pattern': " =" + '[^ =>~"]',
+         'description': 'Missing whitespace after "="'},
+        {'pattern': '^[ ]*//[A-Za-z0-9]',
+         'description': 'Missing space after // in comment'},
+        {'pattern': 'if[(]',
+         'description': 'Missing space between if and ('},
+        {'pattern': 'else{$',
+         'description': 'Missing space between else and {'},
+        {'pattern': '^else {$',
+         'description': 'Write JS else statements on same line as }'},
+        {'pattern': '^else if',
+         'description': 'Write JS else statements on same line as }'},
+        {'pattern': 'console[.][a-z]',
+         'exclude': set(['static/js/blueslip.js',
+                         'frontend_tests/zjsunit',
+                         'frontend_tests/casper_lib/common.js',
+                         'frontend_tests/node_tests',
+                         'static/js/debug.js']),
+         'description': 'console.log and similar should not be used in webapp'},
+        {'pattern': 'i18n[.]t',
+         'include_only': set(['static/js/portico']),
+         'description': 'i18n.t is not available in portico pages yet'},
+        {'pattern': '[.]text\(["\'][a-zA-Z]',
+         'description': 'Strings passed to $().text should be wrapped in i18n.t() for internationalization'},
+        {'pattern': 'compose_error\(["\']',
+         'description': 'Argument to compose_error should be a literal string enclosed '
+                        'by i18n.t()'},
+        {'pattern': 'ui.report_success\(',
+         'description': 'Deprecated function, use ui_report.success.'},
+        {'pattern': 'report.success\(["\']',
+         'description': 'Argument to report_success should be a literal string enclosed '
+                        'by i18n.t()'},
+        {'pattern': 'ui.report_error\(',
+         'description': 'Deprecated function, use ui_report.error.'},
+        {'pattern': 'report.error\(["\']',
+         'description': 'Argument to report_error should be a literal string enclosed '
+                        'by i18n.t()'},
+    ]) + whitespace_rules
+    python_rules = cast(RuleList, [
+        {'pattern': '^(?!#)@login_required',
+         'description': '@login_required is unsupported; use @zulip_login_required'},
+        {'pattern': '".*"%\([a-z_].*\)?$',
+         'description': 'Missing space around "%"'},
+        {'pattern': "'.*'%\([a-z_].*\)?$",
+         'exclude': set(['analytics/lib/counts.py',
+                         'analytics/tests/test_counts.py',
+                         ]),
+         'exclude_line': set([
+             ('zerver/views/users.py',
+              "return json_error(_(\"Email '%(email)s' not allowed for realm '%(realm)s'\") %"),
+             ('zproject/settings.py',
+              "'format': '%(asctime)s %(levelname)-8s %(message)s'"),
+         ]),
+         'description': 'Missing space around "%"'},
+        # This rule is constructed with + to avoid triggering on itself
+        {'pattern': " =" + '[^ =>~"]',
+         'description': 'Missing whitespace after "="'},
+        {'pattern': '":\w[^"]*$',
+         'description': 'Missing whitespace after ":"'},
+        {'pattern': "':\w[^']*$",
+         'description': 'Missing whitespace after ":"'},
+        {'pattern': "^\s+[#]\w",
+         'strip': '\n',
+         'description': 'Missing whitespace after "#"'},
+        {'pattern': "assertEquals[(]",
+         'description': 'Use assertEqual, not assertEquals (which is deprecated).'},
+        {'pattern': "== None",
+         'description': 'Use `is None` to check whether something is None'},
+        {'pattern': "type:[(]",
+         'description': 'Missing whitespace after ":" in type annotation'},
+        {'pattern': "# type [(]",
+         'description': 'Missing : after type in type annotation'},
+        {'pattern': "#type",
+         'description': 'Missing whitespace after "#" in type annotation'},
+        {'pattern': 'if[(]',
+         'description': 'Missing space between if and ('},
+        {'pattern': ", [)]",
+         'description': 'Unnecessary whitespace between "," and ")"'},
+        {'pattern': "%  [(]",
+         'description': 'Unnecessary whitespace between "%" and "("'},
+        # This next check could have false positives, but it seems pretty
+        # rare; if we find any, they can be added to the exclude list for
+        # this rule.
+        {'pattern': ' % [a-zA-Z0-9_.]*\)?$',
+         'exclude_line': set([
+             ('tools/tests/test_template_parser.py', '{% foo'),
+         ]),
+         'description': 'Used % comprehension without a tuple'},
+        {'pattern': '.*%s.* % \([a-zA-Z0-9_.]*\)$',
+         'description': 'Used % comprehension without a tuple'},
+        {'pattern': 'django.utils.translation',
+         'include_only': set(['test']),
+         'description': 'Test strings should not be tagged for translationx'},
+        {'pattern': 'json_success\({}\)',
+         'description': 'Use json_success() to return nothing'},
+        # To avoid json_error(_variable) and json_error(_(variable))
+        {'pattern': '\Wjson_error\(_\(?\w+\)',
+         'exclude': set(['zerver/tests']),
+         'description': 'Argument to json_error should be a literal string enclosed by _()'},
+        {'pattern': '\Wjson_error\([\'"].+[),]$',
+         'exclude': set(['zerver/tests']),
+         'exclude_line': set([
+             # We don't want this string tagged for translation.
+             ('zerver/views/compatibility.py', 'return json_error("Client is too old")'),
+         ]),
+         'description': 'Argument to json_error should a literal string enclosed by _()'},
+        # To avoid JsonableError(_variable) and JsonableError(_(variable))
+        {'pattern': '\WJsonableError\(_\(?\w.+\)',
+         'exclude': set(['zerver/tests']),
+         'description': 'Argument to JsonableError should be a literal string enclosed by _()'},
+        {'pattern': '\WJsonableError\(["\'].+\)',
+         'exclude': set(['zerver/tests']),
+         'description': 'Argument to JsonableError should be a literal string enclosed by _()'},
+        {'pattern': '([a-zA-Z0-9_]+)=REQ\([\'"]\\1[\'"]',
+         'description': 'REQ\'s first argument already defaults to parameter name'},
+        {'pattern': 'self\.client\.(get|post|patch|put|delete)',
+         'exclude': set(['zilencer/tests.py']),
+         'description': \
+         '''Do not call self.client directly for put/patch/post/get.
+    See WRAPPER_COMMENT in test_helpers.py for details.
+    '''},
+        # Directly fetching Message objects in e.g. views code is often a security bug.
+        {'pattern': '[^r][M]essage.objects.get',
+         'exclude': set(["zerver/tests", "zerver/worker/queue_processors.py"]),
+         'description': 'Please use access_message() to fetch Message objects',
+         },
+        {'pattern': '[S]tream.objects.get',
+         'include_only': set(["zerver/views/"]),
+         'description': 'Please use access_stream_by_*() to fetch Stream objects',
+         },
+        {'pattern': 'get_stream[(]',
+         'include_only': set(["zerver/views/", "zerver/lib/actions.py"]),
+         # messages.py needs to support accessing invite-only streams
+         # that you are no longer subscribed to, so need get_stream.
+         'exclude': set(['zerver/views/messages.py']),
+         'exclude_line': set([
+             # This is a check for whether a stream rename is invalid because it already exists
+             ('zerver/lib/actions.py', 'get_stream(new_name, stream.realm)'),
+             # This one in check_message is kinda terrible, since it's
+             # how most instances are written, but better to exclude something than nothing
+             ('zerver/lib/actions.py', 'stream = get_stream(stream_name, realm)'),
+             ('zerver/lib/actions.py', 'get_stream(signups_stream, admin_realm)'),
+         ]),
+         'description': 'Please use access_stream_by_*() to fetch Stream objects',
+         },
+        {'pattern': '[S]tream.objects.filter',
+         'include_only': set(["zerver/views/"]),
+         'description': 'Please use access_stream_by_*() to fetch Stream objects',
+         },
+        {'pattern': '^from (zerver|analytics|confirmation)',
+         'include_only': set(["/migrations/"]),
+         'exclude': set(['zerver/migrations/0032_verify_all_medium_avatar_images.py',
+                         'zerver/migrations/0041_create_attachments_for_old_messages.py',
+                         'zerver/migrations/0060_move_avatars_to_be_uid_based.py']),
+         'description': "Don't import models or other code in migrations; see docs/schema-migrations.md",
+         },
+        {'pattern': 'datetime[.](now|utcnow)',
+         'include_only': set(["zerver/", "analytics/"]),
+         'description': "Don't use datetime in backend code.\n"
+         "See https://zulip.readthedocs.io/en/latest/code-style.html#naive-datetime-objects",
+         },
+        {'pattern': 'render_to_response\(',
+         'description': "Use render() instead of render_to_response().",
+         },
+        # This rule might give false positives in virtualenv setup files which should be excluded,
+        # and comments which should be rewritten to avoid use of "python2", "python3", etc.
+        {'pattern': 'python[23]',
+         'exclude': set(['tools/lib/provision.py',
+                         'tools/setup/setup_venvs.py',
+                         'scripts/lib/setup_venv.py']),
+         'description': 'Explicit python invocations should not include a version'}
+    ]) + whitespace_rules
+    bash_rules = [
+        {'pattern': '#!.*sh [-xe]',
+         'description': 'Fix shebang line with proper call to /usr/bin/env for Bash path, change -x|-e switches'
+                        ' to set -x|set -e'},
+    ] + whitespace_rules[0:1]  # type: RuleList
+    css_rules = cast(RuleList, [
+        {'pattern': '^[^:]*:\S[^:]*;$',
+         'description': "Missing whitespace after : in CSS"},
+        {'pattern': '[a-z]{',
+         'description': "Missing whitespace before '{' in CSS."},
+        {'pattern': 'https://',
+         'description': "Zulip CSS should have no dependencies on external resources"},
+        {'pattern': '^[ ][ ][a-zA-Z0-9]',
+         'description': "Incorrect 2-space indentation in CSS",
+         'exclude': set(['static/third/thirdparty-fonts.css']),
+         'strip': '\n'},
+        {'pattern': '{\w',
+         'description': "Missing whitespace after '{' in CSS (should be newline)."},
+        {'pattern': ' thin[; ]',
+         'description': "thin CSS attribute is under-specified, please use 1px."},
+        {'pattern': ' medium[; ]',
+         'description': "medium CSS attribute is under-specified, please use pixels."},
+        {'pattern': ' thick[; ]',
+         'description': "thick CSS attribute is under-specified, please use pixels."},
+    ]) + whitespace_rules  # type: RuleList
+    prose_style_rules = [
+        {'pattern': '[^\/\#\-\"]([jJ]avascript)',  # exclude usage in hrefs/divs
+         'description': "javascript should be spelled JavaScript"},
+        {'pattern': '[^\/\-\.\"\'\_\=\>]([gG]ithub)[^\.\-\_\"\<]',  # exclude usage in hrefs/divs
+         'description': "github should be spelled GitHub"},
+        {'pattern': '[oO]rganisation',  # exclude usage in hrefs/divs
+         'description': "Organization is spelled with a z"},
+        {'pattern': '!!! warning',
+         'description': "!!! warning is invalid; it's spelled '!!! warn'"},
+    ]  # type: RuleList
+    html_rules = whitespace_rules + prose_style_rules + [
+        {'pattern': 'placeholder="[^{]',
+         'description': "`placeholder` value should be translatable.",
+         'exclude_line': [('templates/zerver/register.html', 'placeholder="acme"'),
+                          ('templates/zerver/register.html', 'placeholder="Acme"'),
+                          ('static/templates/settings/realm-domains-modal.handlebars',
+                           '<td><input type="text" class="new-realm-domain" placeholder="acme.com"></input></td>')],
+         'exclude': set(["static/templates/settings/emoji-settings-admin.handlebars",
+                         "static/templates/settings/realm-filter-settings-admin.handlebars",
+                         "static/templates/settings/bot-settings.handlebars"])},
+        {'pattern': "placeholder='[^{]",
+         'description': "`placeholder` value should be translatable."},
+        {'pattern': 'script src="http',
+         'description': "Don't directly load dependencies from CDNs.  See docs/front-end-build-process.md"},
+        {'pattern': "title='[^{]",
+         'description': "`title` value should be translatable."},
+        {'pattern': 'title="[^{\:]',
+         'exclude_line': set([
+             ('templates/zerver/markdown_help.html',
+              '<td><img alt=":heart:" class="emoji" src="/static/generated/emoji/images/emoji/heart.png" title=":heart:" /></td>')
+         ]),
+         'exclude': set(["templates/zerver/emails"]),
+         'description': "`title` value should be translatable."},
+        {'pattern': '\Walt=["\'][^{"\']',
+         'description': "alt argument should be enclosed by _() or it should be an empty string.",
+         'exclude': set(['static/templates/settings/display-settings.handlebars',
+                         'templates/zerver/keyboard_shortcuts.html',
+                         'templates/zerver/markdown_help.html']),
+         },
+        {'pattern': '\Walt=["\']{{ ?["\']',
+         'description': "alt argument should be enclosed by _().",
+         },
+    ]  # type: RuleList
+    handlebars_rules = html_rules + [
+        {'pattern': "[<]script",
+         'description': "Do not use inline <script> tags here; put JavaScript in static/js instead."},
+        {'pattern': "{{t '.*' }}[\.\?!]",
+         'description': "Period should be part of the translatable string."},
+        {'pattern': '{{t ".*" }}[\.\?!]',
+         'description': "Period should be part of the translatable string."},
+        {'pattern': "{{/tr}}[\.\?!]",
+         'description': "Period should be part of the translatable string."},
+    ]
+    jinja2_rules = html_rules + [
+        {'pattern': "{% endtrans %}[\.\?!]",
+         'description': "Period should be part of the translatable string."},
+        {'pattern': "{{ _(.+) }}[\.\?!]",
+         'description': "Period should be part of the translatable string."},
+    ]
+    json_rules = []  # type: RuleList # fix newlines at ends of files
+    # It is okay that json_rules is empty, because the empty list
+    # ensures we'll still check JSON files for whitespace.
+    markdown_rules = markdown_whitespace_rules + prose_style_rules + [
+        {'pattern': '\[(?P<url>[^\]]+)\]\((?P=url)\)',
+         'description': 'Linkified markdown URLs should use cleaner <http://example.com> syntax.'}
+    ]
+    help_markdown_rules = markdown_rules + [
+        {'pattern': '[a-z][.][A-Z]',
+         'description': "Likely missing space after end of sentence"},
+        {'pattern': '[rR]ealm',
+         'description': "Realms are referred to as Organizations in user-facing docs."},
+    ]
+    txt_rules = whitespace_rules
+
+    def check_custom_checks_py():
+        # type: () -> bool
+        failed = False
+
+        for fn in by_lang['py']:
+            if 'custom_check.py' in fn:
+                continue
+            if custom_check_file(fn, python_rules, max_length=140):
+                failed = True
+        return failed
+
+    def check_custom_checks_nonpy():
+        # type: () -> bool
+        failed = False
+
+        for fn in by_lang['js']:
+            if custom_check_file(fn, js_rules):
+                failed = True
+
+        for fn in by_lang['sh']:
+            if custom_check_file(fn, bash_rules):
+                failed = True
+
+        for fn in by_lang['css']:
+            if custom_check_file(fn, css_rules):
+                failed = True
+
+        for fn in by_lang['handlebars']:
+            if custom_check_file(fn, handlebars_rules):
+                failed = True
+
+        for fn in by_lang['html']:
+            if custom_check_file(fn, jinja2_rules):
+                failed = True
+
+        for fn in by_lang['json']:
+            if custom_check_file(fn, json_rules):
+                failed = True
+
+        markdown_docs_length_exclude = {
+            "api/bots/converter/readme.md",
+            "docs/bots-guide.md",
+            "docs/dev-env-first-time-contributors.md",
+            "docs/webhook-walkthrough.md",
+            "docs/life-of-a-request.md",
+            "docs/logging.md",
+            "docs/migration-renumbering.md",
+            "docs/readme-symlink.md",
+            "README.md",
+            "zerver/webhooks/helloworld/doc.md",
+            "zerver/webhooks/solano/doc.md",
+            "zerver/webhooks/trello/doc.md",
+            "zerver/webhooks/papertrail/doc.md",
+        }
+        for fn in by_lang['md']:
+            max_length = None
+            if fn not in markdown_docs_length_exclude:
+                max_length = 120
+            rules = markdown_rules
+            if fn.startswith("templates/zerver/help"):
+                rules = help_markdown_rules
+            if custom_check_file(fn, rules, max_length=max_length):
+                failed = True
+
+        for fn in by_lang['txt'] + by_lang['text']:
+            if custom_check_file(fn, txt_rules):
+                failed = True
+
+        for fn in by_lang['yaml']:
+            if custom_check_file(fn, txt_rules):
+                failed = True
+
+        return failed
+
+    return (check_custom_checks_py, check_custom_checks_nonpy)