# Buggy code
def _create_etag(path, filter, recursive, lm=None):
    if lm is None:
        lm = _create_lastmodified(path, recursive)

    if lm is None:
        return None

    hash = hashlib.sha1()

    def hash_update(value):
        value = value.encode("utf-8")
        hash.update(value)

    hash_update(str(lm))
    hash_update(str(filter))
    hash_update(str(recursive))

    if path.endswith("/files") or path.endswith("/files/sdcard"):
        # include sd data in etag
        hash_update(repr(sorted(printer.get_sd_files(), key=lambda x: x[0])))

    hash_update(_DATA_FORMAT_VERSION)  # increment version if we change the API format

    return hash.hexdigest()


# Traceback (TypeError)
2019-10-08 14:14:51,399 - octoprint.server.preemptive_cache - INFO - Preemptively caching / (ui _default) for {'base_url': 'http://192.168.0.113:5000/', 'path': '/', 'query_string': 'l10n=en'}
[2019-10-08 14:14:51,556] ERROR in app: Exception on / [GET]
Traceback (most recent call last):
File "c:\octoprint\venv\lib\site-packages\flask\app.py", line 1982, in wsgi_app
response = self.full_dispatch_request()
File "c:\octoprint\venv\lib\site-packages\flask\app.py", line 1614, in full_dispatch_request
rv = self.handle_user_exception(e)
File "c:\octoprint\venv\lib\site-packages\flask\app.py", line 1517, in handle_user_exception
reraise(exc_type, exc_value, tb)
File "c:\octoprint\venv\lib\site-packages\flask\_compat.py", line 33, in reraise
raise value
File "c:\octoprint\venv\lib\site-packages\flask\app.py", line 1612, in full_dispatch_request
rv = self.dispatch_request()
File "c:\octoprint\venv\lib\site-packages\flask\app.py", line 1598, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
File "c:\octoprint\venv\lib\site-packages\octoprint\server\views.py", line 181, in index
fetch_template_data(refresh=force_refresh)
File "c:\octoprint\venv\lib\site-packages\octoprint\server\views.py", line 755, in fetch_template_data
sorted_missing = sorted(missing_in_order, key=key_func)
TypeError: '<' not supported between instances of 'str' and 'NoneType'
[2019-10-08 14:14:51,743] ERROR in app: Exception on / [GET]
Traceback (most recent call last):
File "c:\octoprint\venv\lib\site-packages\flask\app.py", line 1982, in wsgi_app
response = self.full_dispatch_request()
File "c:\octoprint\venv\lib\site-packages\flask\app.py", line 1614, in full_dispatch_request
rv = self.handle_user_exception(e)
File "c:\octoprint\venv\lib\site-packages\flask\app.py", line 1517, in handle_user_exception
reraise(exc_type, exc_value, tb)
File "c:\octoprint\venv\lib\site-packages\flask\_compat.py", line 33, in reraise
raise value
File "c:\octoprint\venv\lib\site-packages\flask\app.py", line 1612, in full_dispatch_request
rv = self.dispatch_request()
File "c:\octoprint\venv\lib\site-packages\flask\app.py", line 1598, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
File "c:\octoprint\venv\lib\site-packages\octoprint\server\views.py", line 181, in index
fetch_template_data(refresh=force_refresh)
File "c:\octoprint\venv\lib\site-packages\octoprint\server\views.py", line 755, in fetch_template_data
sorted_missing = sorted(missing_in_order, key=key_func)
TypeError: '<' not supported between instances of 'str' and 'NoneType'
[2019-10-08 14:14:51,884] ERROR in app: Exception on / [GET]
Traceback (most recent call last):
File "c:\octoprint\venv\lib\site-packages\flask\app.py", line 1982, in wsgi_app
response = self.full_dispatch_request()
File "c:\octoprint\venv\lib\site-packages\flask\app.py", line 1614, in full_dispatch_request
rv = self.handle_user_exception(e)
File "c:\octoprint\venv\lib\site-packages\flask\app.py", line 1517, in handle_user_exception
reraise(exc_type, exc_value, tb)
File "c:\octoprint\venv\lib\site-packages\flask\_compat.py", line 33, in reraise
raise value
File "c:\octoprint\venv\lib\site-packages\flask\app.py", line 1612, in full_dispatch_request
rv = self.dispatch_request()
File "c:\octoprint\venv\lib\site-packages\flask\app.py", line 1598, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
File "c:\octoprint\venv\lib\site-packages\octoprint\server\views.py", line 181, in index
fetch_template_data(refresh=force_refresh)
File "c:\octoprint\venv\lib\site-packages\octoprint\server\views.py", line 755, in fetch_template_data
sorted_missing = sorted(missing_in_order, key=key_func)
TypeError: '<' not supported between instances of 'str' and 'NoneType'

# Fixed code
def _create_etag(path, filter, recursive, lm=None):
    if lm is None:
        lm = _create_lastmodified(path, recursive)

    if lm is None:
        return None

    hash = hashlib.sha1()

    def hash_update(value):
        value = value.encode("utf-8")
        hash.update(value)

    hash_update(str(lm))
    hash_update(str(filter))
    hash_update(str(recursive))

    if path.endswith("/files") or path.endswith("/files/sdcard"):
        # include sd data in etag
        hash_update(repr(sorted(printer.get_sd_files(), key=lambda x: sv(x[0]))))

    hash_update(_DATA_FORMAT_VERSION)  # increment version if we change the API format

    return hash.hexdigest()