How to use the vdirsyncer.exceptions.PreconditionFailed function in vdirsyncer

def test_delete_nonexisting(self, s, get_item):
        with pytest.raises(exceptions.PreconditionFailed):
            s.delete('1', '"123"')

r = func(method, url, data=data, headers=headers, auth=auth, verify=verify)

    # See https://github.com/kennethreitz/requests/issues/2042
    content_type = r.headers.get('Content-Type', '')
    if not latin1_fallback and \
       'charset' not in content_type and \
       content_type.startswith('text/'):
        logger.debug('Removing latin1 fallback')
        r.encoding = None

    logger.debug(r.status_code)
    logger.debug(r.headers)
    logger.debug(r.content)

    if r.status_code == 412:
        raise exceptions.PreconditionFailed(r.reason)
    if r.status_code == 404:
        raise exceptions.NotFoundError(r.reason)

    r.raise_for_status()
    return r

r = func(method, url, **kwargs)

    # See https://github.com/kennethreitz/requests/issues/2042
    content_type = r.headers.get('Content-Type', '')
    if not latin1_fallback and \
       'charset' not in content_type and \
       content_type.startswith('text/'):
        logger.debug('Removing latin1 fallback')
        r.encoding = None

    logger.debug(r.status_code)
    logger.debug(r.headers)
    logger.debug(r.content)

    if r.status_code == 412:
        raise exceptions.PreconditionFailed(r.reason)
    if r.status_code in (404, 410):
        raise exceptions.NotFoundError(r.reason)

    r.raise_for_status()
    return r

def _write(self):
        if self._last_etag is not None and \
           self._last_etag != get_etag_from_file(self.path):
            raise exceptions.PreconditionFailed((
                'Some other program modified the file {!r}. Re-run the '
                'synchronization and make sure absolutely no other program is '
                'writing into the same file.'
            ).format(self.path))
        text = join_collection(
            item.raw for item, etag in self._items.values()
        )
        try:
            with atomic_write(self.path, mode='wb', overwrite=True) as f:
                f.write(text.encode(self.encoding))
        finally:
            self._items = None
            self._last_etag = None

class PreconditionFailed(Error):
    '''
      - The item doesn't exist although it should
      - The item exists although it shouldn't
      - The etags don't match.

    Due to CalDAV we can't actually say which error it is.
    This error may indicate race conditions.
    '''


class NotFoundError(PreconditionFailed):
    '''Item not found'''


class AlreadyExistingError(PreconditionFailed):
    '''Item already exists.'''
    existing_href = None


class WrongEtagError(PreconditionFailed):
    '''Wrong etag'''


class ReadOnlyError(Error):
    '''Storage is read-only.'''


class InvalidResponse(Error, ValueError):
    '''The backend returned an invalid result.'''

pair_name = None


class PreconditionFailed(Error):
    '''
      - The item doesn't exist although it should
      - The item exists although it shouldn't
      - The etags don't match.

    Due to CalDAV we can't actually say which error it is.
    This error may indicate race conditions.
    '''


class NotFoundError(PreconditionFailed):
    '''Item not found'''


class AlreadyExistingError(PreconditionFailed):
    '''Item already exists.'''
    existing_href = None


class WrongEtagError(PreconditionFailed):
    '''Wrong etag'''


class ReadOnlyError(Error):
    '''Storage is read-only.'''

def has(self, href):
        '''Check if an item exists by its href.

        :returns: True or False
        '''
        try:
            self.get(href)
        except exceptions.PreconditionFailed:
            return False
        else:
            return True