How do I change the only the first word of every line in a text file?

Question

So I have actually worked out how to change the first word but the problem is its not just changing the first word only. So i have a file such as :

2 0.268278 0.356394 0.027123 0.044025
4 0.241745 0.528302 0.035377 0.035639
4 0.257075 0.491614 0.037736 0.033543
5 0.275354 0.307128 0.060142 0.062893
0 0.283019 0.551363 0.115566 0.572327
0 0.043042 0.292453 0.076651 0.159329

and i want to change it to:

3 0.268278 0.356394 0.027123 0.044025
5 0.241745 0.528302 0.035377 0.035639
5 0.257075 0.491614 0.037736 0.033543
9 0.275354 0.307128 0.060142 0.062893
0 0.283019 0.551363 0.115566 0.572327
0 0.043042 0.292453 0.076651 0.159329

So my code is:

    source = 'C:/Users/asmita.nandi/Downloads/Kitchen_black&white/Annotations1'
    for root, dirs, filenames in os.walk(source):
    for f in filenames:
        this_file = open(os.path.join(source, f), "r")
        this_files_data = this_file.readlines()
        this_file.close()
    # opens all txt file in directory
        this_file = open(os.path.join(source, f), "w")
        for line in this_files_data:
            s=line[0]
            if line[0] in "1" :
                this_file.write(line.replace(s,'2'))
            if line[0] in "2" :
                this_file.write(line.replace(s,'3'))
            if line[0] in "0":
                this_file.write(line)
            if line[0] in "3":
                this_file.write(line.replace(s,'4'))
            if line[0] in "4":
                this_file.write(line.replace(s,'5'))
            if line[0] in "5":
                this_file.write(line.replace(s,'9'))
           
       
        this_file.close()

So what my code is doing is not only changing the first word but also any other instance of the number in the line. So something like this happens instead:

3 0.359080 0.359539 0.037133 0.060797
9 0.369104 0.289119 0.070799 0.079472
9 0.678696 0.143606 0.099429 0.090147
5 0.307193 0.590157 0.038915 0.052511
0 0.392099 0.620545 0.173349 0.746331
0 0.653892 0.479036 0.135613 0.752621

What am I doing wrong?

Answer 1

When you are calling replace() it replaces every instance of that character in the string. Instead, just isolate the first character using [0] so that it will only replace there. e.g:

this_file.write(line[0].replace(s,'2'))

EDIT: I see that someone commented the same thing as i was typing mine

Answer 2

try this

import os
os.chdir(r"  **write Folder Path here** ")                                                                                                                       
for paths,folders,files in os.walk(os.getcwd()):
        for file in files:
            if file.endswith("txt"):
                reading_file = open(file,"r")
                new_str = ""
                for line in reading_file:
                    new_line=(line[0].replace(line[0],'0')+line[1:])
                    new_str += new_line
                writing_file = open(file,"w")
                writing_file.write(new_str)
                writing_file.close()