Replace a character by another in a file

Question

I'd like to modify some characters of a file in-place, without having to copy the entire content of the file in another, or overwrite the existing one. However, it doesn't seem possible to just replace a character by another:

>>> f = open("foo", "a+")  # file does not exist
>>> f.write("a")
1
>>> f.seek(0)
0
>>> f.write("b")
1
>>> f.seek(0)
0
>>> f.read()
'ab'

Here I'd have expected "a" to be replaced by "b", so that the content of the file would be just "b", but this is not the case. Is there a way to do this?

Does the answer need to be pure python? Would you be open to using system commands? — Dan Grahn
– Dan Grahn, Commented Jun 28, 2013 at 15:58
@screenmutt: I'd prefer a Python way, but if there's no way to do this in Python, yes, I'm open to do this with system commands. — michaelmeyer
– michaelmeyer, Commented Jun 28, 2013 at 16:01

Guillaume · Accepted Answer · 2013-06-28 16:10:34Z

3

That's because of the mode you're using, in append mode, the file pointer is moved to the end of file before write, you should open your file in w+ mode:

f = open("foo", "w+")  # file does not exist
f.write("samething")
f.seek(1)
f.write("o")
f.seek(0)
print f.read() # prints "something"

If you want to do that on an existing file without truncating it, you should open it in r+ mode for reading and writing.

edited Jun 28, 2013 at 16:10

answered Jun 28, 2013 at 16:01

Guillaume

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2 Comments

michaelmeyer Over a year ago

In fact, the file will not be empty, and I don't want to overwrite it.

Guillaume Over a year ago

Yeah, in fact you should be using r+ to open it read/write and not truncating

Ashwini Chaudhary · Accepted Answer · 2013-06-28 16:04:38Z

1

Truncate the file using file.truncate first:

>>> f = open("foo", "a+") 
>>> f.write('a')
>>> f.truncate(0)  #truncates the file to 0 bytes
>>> f.write('b')
>>> f.seek(0)
>>> f.read()
'b'

Otherwise open the file in w+mode as suggested by @Guillaume.

edited Jun 28, 2013 at 16:04

answered Jun 28, 2013 at 15:59

Ashwini Chaudhary

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2 Comments

michaelmeyer Over a year ago

This actually removes the content of the file.

Stephan Over a year ago

only way to overwrite in append mode @doukremt

rajpy · Accepted Answer · 2013-06-28 16:14:22Z

0

import fileinput

for line in fileinput.input('abc', inplace=True):
    line = line.replace('t', 'ed')
    print line,

This doesn't replace character by character, instead it scans through each line replacing required character and writes the modified line.

For example: file 'abc' contains:

i want 
to replace
character

After executing, output would be:

i waned
edo replace
characeder

Will it help you? Hope so..

answered Jun 28, 2013 at 16:14

rajpy

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Comments

Community · Accepted Answer · 2017-05-23 10:25:32Z

0

I believe that you may be able to modify the example from this answer.

https://stackoverflow.com/a/290494/1669208

import fileinput

for line in fileinput.input("test.txt", inplace=True):
    print line.replace(char1, char2),

edited May 23, 2017 at 10:25

CommunityBot

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answered Jun 28, 2013 at 16:15

Dan Grahn

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