Read and overwrite a file in Python

Question

Currently I'm using this:

f = open(filename, 'r+')
text = f.read()
text = re.sub('foobar', 'bar', text)
f.seek(0)
f.write(text)
f.close()

But the problem is that the old file is larger than the new file. So I end up with a new file that has a part of the old file on the end of it.

Answer 1

The fileinput module has an inplace mode for writing changes to the file you are processing without using temporary files etc. The module nicely encapsulates the common operation of looping over the lines in a list of files, via an object which transparently keeps track of the file name, line number etc if you should want to inspect them inside the loop.

from fileinput import FileInput
for line in FileInput("file", inplace=1):
    line = line.replace("foobar", "bar")
    print(line)

Answer 2

Probably it would be easier and neater to close the file after text = re.sub('foobar', 'bar', text), re-open it for writing (thus clearing old contents), and write your updated text to it.

Answer 3

I find it easier to remember to just read it and then write it.

For example:

with open('file') as f:
    data = f.read()
with open('file', 'w') as f:
    f.write('hello')

Answer 4

Honestly you can take a look at this class that I built which does basic file operations. The write method overwrites and append keeps old data.

class IO:
    def read(self, filename):
        toRead = open(filename, "rb")

        out = toRead.read()
        toRead.close()
        
        return out
    
    def write(self, filename, data):
        toWrite = open(filename, "wb")

        out = toWrite.write(data)
        toWrite.close()

    def append(self, filename, data):
        append = self.read(filename)
        self.write(filename, append+data)
        

Answer 5

To anyone who wants to read and overwrite by line, refer to this answer. https://stackoverflow.com/a/71285415/11442980

filename = input("Enter filename: ")
with open(filename, 'r+') as file:
    lines = file.readlines()
    file.seek(0)
    for line in lines:
        value = int(line)
        file.write(str(value + 1))
    file.truncate()

Answer 6

Try writing it in a new file..

f = open(filename, 'r+')
f2= open(filename2,'a+')
text = f.read()
text = re.sub('foobar', 'bar', text)
f.seek(0)
f.close()
f2.write(text)
fw.close()