Pandas – dataframe with multi indices to csv

I have a random dataframe with multi index as such:

import numpy as np 
from itertools import product
import pandas as pd

c1 = np.arange(3,5,1)
c2 = np.arange(7,9,1)
c3 = np.arange(0,135,45)

df=  pd.DataFrame(list(product(c1, c2, c3)), columns=['c1', 'c2','c3'])
df['c4'] = df.index

df = df.set_index(['c1', 'c2','c3'])

When I save the dataframe to csv, I get a csv with duplicate values within the MultiIndex c1,c2,c3. I want to have only the unique values of c1, c2 occuring once in the csv file since they all occur successively. How can I mask these values in Pandas before saving it to csv?

>Solution :

You can mask before write to_csv notice here no need set_index

df.c2.mask(df.duplicated(['c1','c2']),'',inplace=True)
df.c1.mask(df.duplicated('c1'),'',inplace=True)
df
Out[415]: 
   c1 c2  c3  c4
0   3  7   0   0
1         45   1
2         90   2
3      8   0   3
4         45   4
5         90   5
6   4  7   0   6
7         45   7
8         90   8
9      8   0   9
10        45  10
11        90  11