import pandas as pd
import numpy as np
from pandas import DataFrame, Series
df = DataFrame(
    {'integer':[1,2,3,6,7,23,8,3],
     'float':[2,3.4,5,6,2,4.7,4,8],
     'string':['saya',None,'aku','cinta','kamu','a','b','jika']}
)
df

df_new = DataFrame(
{'string':['a','b'],
 'str_value':[2,4]}
)
df_new

df_merge_inner = pd.merge(df,df_new,on='string',how='inner')
print df_merge_inner

df_merge_outer = pd.merge(df,df_new,on='string',how='outer')
print df_merge_outer

df_merge_left = pd.merge(df,df_new,on='string',how='left')
df_merge_left

df_merge_right = pd.merge(df,df_new,on='string',how='right')
df_merge_right