# Buggy code
def main(
    self,
    args=None,
    prog_name=None,
    complete_var=None,
    standalone_mode=True,
    **extra,
):
    """This is the way to invoke a script with all the bells and
    whistles as a command line application.  This will always terminate
    the application after a call.  If this is not wanted, ``SystemExit``
    needs to be caught.

    This method is also available by directly calling the instance of
    a :class:`Command`.

    .. versionadded:: 3.0
       Added the `standalone_mode` flag to control the standalone mode.

    :param args: the arguments that should be used for parsing.  If not
                 provided, ``sys.argv[1:]`` is used.
    :param prog_name: the program name that should be used.  By default
                      the program name is constructed by taking the file
                      name from ``sys.argv[0]``.
    :param complete_var: the environment variable that controls the
                         bash completion support.  The default is
                         ``"_<prog_name>_COMPLETE"`` with prog_name in
                         uppercase.
    :param standalone_mode: the default behavior is to invoke the script
                            in standalone mode.  Click will then
                            handle exceptions and convert them into
                            error messages and the function will never
                            return but shut down the interpreter.  If
                            this is set to `False` they will be
                            propagated to the caller and the return
                            value of this function is the return value
                            of :meth:`invoke`.
    :param extra: extra keyword arguments are forwarded to the context
                  constructor.  See :class:`Context` for more information.
    """
    # Verify that the environment is configured correctly, or reject
    # further execution to avoid a broken script.
    _verify_python_env()

    if args is None:
        args = sys.argv[1:]
    else:
        args = list(args)

    if prog_name is None:
        prog_name = _detect_program_name()

    # Process shell completion requests and exit early.
    self._main_shell_completion(prog_name, complete_var)

    try:
        try:
            with self.make_context(prog_name, args, **extra) as ctx:
                rv = self.invoke(ctx)
                if not standalone_mode:
                    return rv
                # it's not safe to `ctx.exit(rv)` here!
                # note that `rv` may actually contain data like "1" which
                # has obvious effects
                # more subtle case: `rv=[None, None]` can come out of
                # chained commands which all returned `None` -- so it's not
                # even always obvious that `rv` indicates success/failure
                # by its truthiness/falsiness
                ctx.exit()
        except (EOFError, KeyboardInterrupt):
            echo(file=sys.stderr)
            raise Abort()
        except ClickException as e:
            if not standalone_mode:
                raise
            e.show()
            sys.exit(e.exit_code)
        except OSError as e:
            if e.errno == errno.EPIPE:
                sys.stdout = PacifyFlushWrapper(sys.stdout)
                sys.stderr = PacifyFlushWrapper(sys.stderr)
                sys.exit(1)
            else:
                raise
    except Exit as e:
        if standalone_mode:
            sys.exit(e.exit_code)
        else:
            # in non-standalone mode, return the exit code
            # note that this is only reached if `self.invoke` above raises
            # an Exit explicitly -- thus bypassing the check there which
            # would return its result
            # the results of non-standalone execution may therefore be
            # somewhat ambiguous: if there are codepaths which lead to
            # `ctx.exit(1)` and to `return 1`, the caller won't be able to
            # tell the difference between the two
            return e.exit_code
    except Abort:
        if not standalone_mode:
            raise
        echo("Aborted!", file=sys.stderr)
        sys.exit(1)


# Traceback (TypeError)
$ ./testclick subcommand <TAB>Traceback (most recent call last):
File "./testclick", line 23, in <module>
entrypoint(obj={'completions': ['abc', 'def', 'ghi', ]})
File "/home/user/testclick/venv/lib/python3.6/site-packages/click/core.py", line 731, in __call__
return self.main(*args, **kwargs)
File "/home/user/testclick/venv/lib/python3.6/site-packages/click/core.py", line 701, in main
_bashcomplete(self, prog_name, complete_var)
File "/home/user/testclick/venv/lib/python3.6/site-packages/click/core.py", line 46, in _bashcomplete
if bashcomplete(cmd, prog_name, complete_var, complete_instr):
File "/home/user/testclick/venv/lib/python3.6/site-packages/click/_bashcomplete.py", line 216, in bashcomplete
return do_complete(cli, prog_name)
File "/home/user/testclick/venv/lib/python3.6/site-packages/click/_bashcomplete.py", line 205, in do_complete
for item in get_choices(cli, prog_name, args, incomplete):
File "/home/user/testclick/venv/lib/python3.6/site-packages/click/_bashcomplete.py", line 186, in get_choices
ctx, all_args, incomplete, param))
File "/home/user/testclick/venv/lib/python3.6/site-packages/click/_bashcomplete.py", line 124, in get_user_autocompletions
incomplete=incomplete)
File "./testclick", line 7, in _complete
return ctx.obj['completions']
TypeError: 'NoneType' object is not subscriptable

# Fixed code
def main(
    self,
    args=None,
    prog_name=None,
    complete_var=None,
    standalone_mode=True,
    **extra,
):
    """This is the way to invoke a script with all the bells and
    whistles as a command line application.  This will always terminate
    the application after a call.  If this is not wanted, ``SystemExit``
    needs to be caught.

    This method is also available by directly calling the instance of
    a :class:`Command`.

    .. versionadded:: 3.0
       Added the `standalone_mode` flag to control the standalone mode.

    :param args: the arguments that should be used for parsing.  If not
                 provided, ``sys.argv[1:]`` is used.
    :param prog_name: the program name that should be used.  By default
                      the program name is constructed by taking the file
                      name from ``sys.argv[0]``.
    :param complete_var: the environment variable that controls the
                         bash completion support.  The default is
                         ``"_<prog_name>_COMPLETE"`` with prog_name in
                         uppercase.
    :param standalone_mode: the default behavior is to invoke the script
                            in standalone mode.  Click will then
                            handle exceptions and convert them into
                            error messages and the function will never
                            return but shut down the interpreter.  If
                            this is set to `False` they will be
                            propagated to the caller and the return
                            value of this function is the return value
                            of :meth:`invoke`.
    :param extra: extra keyword arguments are forwarded to the context
                  constructor.  See :class:`Context` for more information.
    """
    # Verify that the environment is configured correctly, or reject
    # further execution to avoid a broken script.
    _verify_python_env()

    if args is None:
        args = sys.argv[1:]
    else:
        args = list(args)

    if prog_name is None:
        prog_name = _detect_program_name()

    # Process shell completion requests and exit early.
    self._main_shell_completion(extra, prog_name, complete_var)

    try:
        try:
            with self.make_context(prog_name, args, **extra) as ctx:
                rv = self.invoke(ctx)
                if not standalone_mode:
                    return rv
                # it's not safe to `ctx.exit(rv)` here!
                # note that `rv` may actually contain data like "1" which
                # has obvious effects
                # more subtle case: `rv=[None, None]` can come out of
                # chained commands which all returned `None` -- so it's not
                # even always obvious that `rv` indicates success/failure
                # by its truthiness/falsiness
                ctx.exit()
        except (EOFError, KeyboardInterrupt):
            echo(file=sys.stderr)
            raise Abort()
        except ClickException as e:
            if not standalone_mode:
                raise
            e.show()
            sys.exit(e.exit_code)
        except OSError as e:
            if e.errno == errno.EPIPE:
                sys.stdout = PacifyFlushWrapper(sys.stdout)
                sys.stderr = PacifyFlushWrapper(sys.stderr)
                sys.exit(1)
            else:
                raise
    except Exit as e:
        if standalone_mode:
            sys.exit(e.exit_code)
        else:
            # in non-standalone mode, return the exit code
            # note that this is only reached if `self.invoke` above raises
            # an Exit explicitly -- thus bypassing the check there which
            # would return its result
            # the results of non-standalone execution may therefore be
            # somewhat ambiguous: if there are codepaths which lead to
            # `ctx.exit(1)` and to `return 1`, the caller won't be able to
            # tell the difference between the two
            return e.exit_code
    except Abort:
        if not standalone_mode:
            raise
        echo("Aborted!", file=sys.stderr)
        sys.exit(1)