Replace a line in a text file using python

Question

I want to replace some of the contents of my file. I have this code:

username= input("enter your new username")
password= input("enter the new password")

file = open("testfile.txt", "r")
replaced_content = ""
for line in file:
    line = line.strip()
    new_line = line.replace("username:", "username: " + username)
    replaced_content = replaced_content + new_line + "\n"

file.close()
write_file = open("testfile.txt", "w")
write_file.write(replaced_content)
write_file.close()

Here, testfile.txt contains:

username:
password:

The problem is when I input the replacement text, it's being added rather than replaced. For example, when I enter a username, I want to replace the line "username:" by "username: admin"; but when I run the code repeatedly, it gets added repeatedly, thus:

username: admin admin
password:

If my username is already in the text file I want to replace it with an other one and not adding the new to the other. How can I make this work? (I try to not import packages or other things like that in my code.)

"I try to not import packages or other thing like that in my script" Why? The standard library exists for a reason. — Karl Knechtel, Aug 07 '21 at 16:13
"The problem is when I enter what I want to replace with, it's adding it not replacing." Well, yes, that's what your code says to do. `line.replace("username:", "username: " + username)`. It looks for the `"username:"` text and then creates *the result of adding the name to that* and does the replacement. Of course it will put the `"username:"` back in. It seems that the `"username:"` text *isn't actually what you want to replace, but instead *the rest of the line after it*, right? It is only possible to solve programming problems when you *clearly understand what you want to do*. — Karl Knechtel, Aug 07 '21 at 16:15
I edited the question to ask the actual question more clearly, fix some minor grammatical issues, and remove off-topic commentary. Stack Overflow is not a discussion forum and thus [we just want the question, not any social niceties](https://meta.stackoverflow.com/questions/260776/should-i-remove-fluff-when-editing-questions). — Karl Knechtel, Aug 07 '21 at 16:22
Does this answer your question? [How to search and replace text in a file?](https://stackoverflow.com/questions/17140886/how-to-search-and-replace-text-in-a-file) — pu239, Aug 07 '21 at 16:26

score 0 · Accepted Answer · answered Aug 07 '21 at 16:00

0

Check if the line equal "username:" and only do the replacement then. In this code it will replace the username: in a line "username: admin" with "username: " + username giving you the extra admin at the end

answered Aug 07 '21 at 16:00

Andrew

146
5

By the way check out the fileinput module that comes with python which is a handy way of processing files and can do in place filtering – Andrew Aug 07 '21 at 16:05

score 0 · Answer 2 · answered Aug 07 '21 at 16:08

The issue is that you find a "username:" in the line and replace it with "username: " + username. So if you had a line like "username: admin", it would simply replace the username as asked, and it would become "username: admin admin".

Try changing the

new_line = line.replace("username:", "username: " + username)

to

new_line = "username: " + username if line.count("username:") > 0 else line

score 0 · Answer 3 · answered Aug 07 '21 at 16:30

Try this (untested, please report of any errors found)v

username= input("enter your new username")
password= input("enter the new password")
new_l=[username, password]

write_file = open("testfile.txt", "r+")
lines=write_file.readlines()

for i,j in zip(lines, new_l):
        write_file.write(i.strip('\n')+j)
        write_file.write('\n')
write_file.close()

Replace a line in a text file using python

3 Answers3