替换python字典中的dataframe行元素字符串

import pandas as pd

data = {'col1': ['string1', 'string2', 'string3'],
        'col2': ['string4', 'string5', 'string6']}
df = pd.DataFrame(data)

def replace_string(value):
    if value == 'string1':
        return 'new_string1'
    elif value == 'string2':
        return 'new_string2'
    else:
        return value

df = df.applymap(replace_string)