convert decimal to string python

You can use the float_format keyword argument of the to_string() method:

df.to_string(columns=['label'], index=False, float_format=lambda x: '{:d}'.format(x))

I think you have some NaN values, so int are converted to float because na type promotions.

So you can read data in column label as str and then it works nice:

import pandas as pd
import numpy as np
import io

temp=u"""lab1;label
5;1
5;2
7;
7;3
"""
#after testing replace io.StringIO(temp) to filename
df = pd.read_csv(io.StringIO(temp), sep=';', dtype={'label':str})
print (df)
  lab1 label
0    5     1
1    5     2
2    7   NaN
3    7     3

df['class_label'] = df.label.fillna('')
print (df)
  lab1 label class_label
0    5     1           1
1    5     2           2
2    7   NaN            
3    7     3           3

print (df.to_string(columns=['class_label'],index=False))
class_label
         1
         2

         3

Using astype(int) will change a float to an int and will drop your .0 as desired.

import pandas as pd
df = pd.DataFrame({'label': [1.0, 2.0, 4.0, 1.0]})
print(df)

    label
0   1.0
1   2.0
2   4.0
3   1.0

df.label = df.label.astype(int)
print(df)

    label
0   1
1   2
2   4
3   1

Here we do not need to convert this to a string. This will be done when exporting to .csv or .txt and will preserve the int.