Issue

This Content is from Stack Overflow. Question asked by Eftal Gezer

Assume I have a file content.txt, with the content Ex nihilo nihil fit. I am willing to replace it with Ex nihilo nihil est. The code is:

with open("content.txt", "r+") as f:
    content = f.read()
    content = content.replace("fit", "est")
    print(content)
    f.write(content)
    f.close()

After that, the content of the file becomes:

Ex nihilo nihil fit
Ex nihilo nihil est

Why? The only thing I need is Ex nihilo nihil est.

Solution

r+ opens the file and puts a pointer at the first byte. When you use f.read() the pointer moves to the end of the file, so when you try to write it starts at the end of the file. To move back to the start (so you can override), use f.seek(0):

with open("text.txt", "r+") as f:
    content = f.read()
    content = content.replace("fit", "est")
    print(content)
    f.seek(0)
    f.truncate(0)
    f.write(content)
    f.close()

This Question was asked in StackOverflow by Eftal Gezer and Answered by actuallyatiger It is licensed under the terms of CC BY-SA 2.5. - CC BY-SA 3.0. - CC BY-SA 4.0.