How do I go two characters left from the current position in file in Python?

Question

I am doing a project and in it I have to frequently edit a file from the position somewhere in the center. I want to go to the 2 characters left to the position from where the word 'rules' starts.

I have tried something like this:

with open("file.txt", "r+") as fh:
    content = fh.read()
    index = content.seek("rules") - 2

But it takes me to the two lines up instead of taking to two characters back.

This is how the file look:

rule1
rule2
rule3
rule4
.
.
.
.
rule100

rules are very good.

So, basically it is the last line of the file which starts with 'rules' and I want to go at the end of the line that is starting with 'rule100' and write a new line that will be something 'rule101' and save the file.

Do you have to read the whole file or are you just searching for the *rules* line? — Banana, May 13 '20 at 08:44
I have to search for the 'rules', go back two characters and then write something and then save the file. — Khubaib Khawar, May 13 '20 at 08:48
Basically, there is only one line that starts with 'rules' in the file. I want to go two characters back from the starting point of that line and then write something. — Khubaib Khawar, May 13 '20 at 08:49

Denis Tsoi · Accepted Answer · 2020-05-13T09:45:49.087

1

We'll firstly split the file into lines. - then we'll find the index where "rules" from the list.
From there - we'll insert the new rule we wish to insert.

Then join this list as updated_content.

We then seek to the beginning of the file and write it with updated_content

with open("text.txt", "r+") as fh:
    content = fh.read().splitlines()

    rulesIndex = content.index("rules are very good.")
    content.insert(rulesIndex - 1, "rule101")

    updated_content = "\n".join(content)

    fh.seek(0)
    fh.write(updated_content)

before

rule1
rule2
rule3
rule4
.
.
.
.
rule100

rules are very good.

after

rule1
rule2
rule3
rule4
.
.
.
.
rule100
rule101

rules are very good.

edited May 13 '20 at 09:45

answered May 13 '20 at 08:45

Denis Tsoi

9,428
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It gives me the `ValueError: substring not found` – Khubaib Khawar May 13 '20 at 08:48
Do you already have the position of rules? (This isn’t indicated in the question. – Denis Tsoi May 13 '20 at 08:49
No, the file opens and it starts from the beginning of the file. I don't have the position of the 'rules' already. – Khubaib Khawar May 13 '20 at 08:50
getting `AttributeError: 'str' object has no attribute 'seek'` – Khubaib Khawar May 13 '20 at 08:59

score 1 · Answer 2 · answered May 13 '20 at 09:39

1

If you only add content to the file you could use append. Then you won't need a special line and search for it. Just do:

with open("file.txt", "a") as fh:
    fh.append("rule101")

answered May 13 '20 at 09:39

Banana

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How do I go two characters left from the current position in file in Python?

2 Answers2