My small script on value alteration in columns of a data not working I have a data set which has "Speed" as one of the columns (features). The column contains both zero and non-zero values. I want to randomly set 10% of the non-zero values to zeros. This will change the corresponding "class" label to zeros. I mean any value  set to zero, its corresponding class value will be zero as well. I have done this but it is give me errors. Though due to error, I cannot tell it will give me the update/result I want. 

file_path = 'Processed_data/data1.csv'  
df = pd.read_csv(file_path)  
per_change = 0.1  
attr = 'Speed'  
target = 'Class'  
df_spd = df[df['Speed'] &gt; 0.]

num_rows_to_change = int(df.shape[0] * per_change)  
num_with_zero_initial = df[df[attr] == 0].shape[0]  
assert df_spd.shape[0] &gt; num_rows_to_change, \  
    'Number of rows with non-zero speed is less than 10% of the original   dataset.'  
df_update = df_spd.sample(num_rows_to_change)  
df_update[attr] = 0.  
df_update[target] = 0.  
df.update(df_update)  
update_list = df_update.index.tolist()  
num_with_zero_final = df[df['Speed'] == 0].shape[0]  
assert num_with_zero_final == num_with_zero_initial + num_rows_to_change, \  
    'Number of rows needed to change not equal to number of rows changed.'  
df.to_csv('changed.csv')
FYI, I did not go through your code as it is fairly straight forward assuming I had understood it right.

&gt;&gt; import pandas as pd
&gt;&gt; import random
&gt;&gt; df = pd.DataFrame({'a': np.random.rand(10), b: np.random.rand(10)})
&gt;&gt; print(df)
          a         b
0  0.127409  0.508811
1  0.345239  0.674797
2  0.824521  0.381567
3  0.893538  0.062142
4  0.307070  0.769546
5  0.872883  0.175192
6  0.046671  0.592971
7  0.799977  0.632761
8  0.932829  0.456906
9  0.188867  0.470296
&gt;&gt; idx = random.sample(df.index, int(len(df)*0.2)) # random indices from dataframe selection, 20% of them are selected based on 0.2
&gt;&gt; print(idx)
[6, 0]
&gt;&gt; df[df.index.isin(idx)] = [0, 0]
&gt;&gt; df
          a         b
0  0.000000  0.000000
1  0.345239  0.674797
2  0.824521  0.381567
3  0.893538  0.062142
4  0.307070  0.769546
5  0.872883  0.175192
6  0.000000  0.000000
7  0.799977  0.632761
8  0.932829  0.456906
9  0.188867  0.470296


Hope it helps.