expression.py 文件源码

def __setitem__(self, key, value):
        if key in self:

            # this warning is primarily to catch select() statements
            # which have conflicting column names in their exported
            # columns collection

            existing = self[key]
            if not existing.shares_lineage(value):
                util.warn('Column %r on table %r being replaced by '
                          '%r, which has the same key.  Consider '
                          'use_labels for select() statements.' % (key,
                          getattr(existing, 'table', None), value))
            self._all_cols.remove(existing)
            # pop out memoized proxy_set as this
            # operation may very well be occurring
            # in a _make_proxy operation
            ColumnElement.proxy_set._reset(value)
        self._all_cols.add(value)
        self._data[key] = value