aosabook/500lines · error · ServerException

Unknown object '{0}'

Error message

Unknown object '{0}'

What it means

Raised by case_always_fail.act in the 03-handlers server (minimal case-dispatch build). As the unconditional last case, it fires for any existing object that is not a regular file (this build has no directory handling), e.g. a directory request.

Source

Thrown at web-server/code/03-handlers/server.py:40

class case_existing_file(object):
    '''File exists.'''

    def test(self, handler):
        return os.path.isfile(handler.full_path)

    def act(self, handler):
        handler.handle_file(handler.full_path)

#-------------------------------------------------------------------------------

class case_always_fail(object):
    '''Base case if nothing else worked.'''

    def test(self, handler):
        return True

    def act(self, handler):
        raise ServerException("Unknown object '{0}'".format(handler.path))

#-------------------------------------------------------------------------------

class RequestHandler(BaseHTTPServer.BaseHTTPRequestHandler):
    '''
    If the requested path maps to a file, that file is served.
    If anything goes wrong, an error page is constructed.
    '''

    Cases = [case_no_file(),
             case_existing_file(),
             case_always_fail()]

    # How to display an error.
    Error_Page = """\
        <html>
        <body>
        <h1>Error accessing {path}</h1>

View on GitHub (pinned to fba689d101)

Solutions

  1. Request a regular file rather than a directory.
  2. Add a directory/index case to the Cases list before case_always_fail.
  3. Keep only regular files under the served root.

Example fix

// before
# GET /images/ (directory, no handler) -> case_always_fail
// after
# add case_directory_index_file / case_directory_no_index_file, or request a file
Defensive patterns

Strategy: fallback

Validate before calling

# add a directory case before case_always_fail so dirs are not 'Unknown object'
class case_directory_index_file(object):
    def test(self, handler):
        return os.path.isdir(handler.full_path)
    def act(self, handler):
        handler.handle_dir(handler.full_path)

# Cases = [case_no_file(), case_existing_file(), case_directory_index_file(), case_always_fail()]

Try / catch

try:
    for case in self.Cases:
        if case.test(self):
            case.act(self); break
except ServerException:
    self.handle_error(msg)

Prevention

When it happens

Trigger: A request whose full_path exists but is not a regular file (os.path.isfile False), so case_existing_file.test fails and the always-fail case is reached.

Common situations: Requesting a directory URL with no directory case registered; special files in the served tree; symlinks isofile rejects.

Related errors

AI-assisted analysis of aosabook/500lines@fba689d101 (2026-08-13). Data as JSON: /api/errors/c98164adfb0436b9. Report an issue: GitHub.