Description:

When a variable is redefined in the same cell, tab-completions still show attributes of the previous type instead of the new one.

Reproduce

In [1]: a = 1

In [2]: a = []
        a.<TAB>  # shows completions like as_integer_ratio, real, etc.

Expected behavior:

Completions should reflect the current type of the variable — in this case, list attributes such as append, extend, etc.

Cause:

In the _attr_matches function, if the object is already defined, we do not consider the lines above in the current cell.