df.column_A.str.extract('(?P<one>\\d+)/(?P<len>\\d+)', expand=True).astype(int
    ).apply(lambda x: ['1'] * x.one + ['0'] * (x.len - x.one), axis=1).apply(''
    .join)
Pandas Dataframe: How to parse integers into string of 0s and 1s?