How to open a file for both reading and writing?

Question

Is there a way to open a file for both reading and writing?

As a workaround, I open the file for writing, close it, then open it again for reading. But is there a way to open a file for both reading and writing?

what problem you are solving? maybe there is a better solution than writing/reading a file, e.g. `mmap` — Roman Bodnarchuk, Jul 11 '11 at 10:08
Could you give us your code so we will be able to answer you. You can also try to take a look: http://docs.python.org/tutorial/inputoutput.html#reading-and-writing-files . However i have tried to use r+b and it works. Also is there any benefit to use one file descriptor in diff functions? — Artsiom Rudzenka, Jul 11 '11 at 10:09
@RomanBodnarchuk `mmap` is a great idea, but what if you have to deal with concurrency? Is there a way to reserve access? — Dr_Zaszuś, Feb 20 '20 at 14:27

score 429 · Answer 1 · answered Apr 12 '13 at 16:04

429

Here's how you read a file, and then write to it (overwriting any existing data), without closing and reopening:

with open(filename, "r+") as f:
    data = f.read()
    f.seek(0)
    f.write(output)
    f.truncate()

answered Apr 12 '13 at 16:04

Flimm

136,138
45
251
267

70

use `a+` to cover end-case that the file does not exist (will be created) – Jossef Harush Kadouri Oct 21 '15 at 19:26
35

seek() and truncate() are both critical! – smwikipedia Feb 17 '16 at 09:49
@Flimm adding len(data) to seek, reads and appends a new string, but it also seems to remove the last character of the text file. – multigoodverse May 01 '16 at 10:41
I just opened a new question regarding reading and appending: http://stackoverflow.com/questions/36965852/how-do-i-read-and-append-to-text-file-in-one-pass – multigoodverse May 01 '16 at 10:58
It doesn't seem like 'r+' overwrites the file, but rather appends. Is there a way to have to overwrite? – Lithimlin May 01 '18 at 15:50
1

@Lithimlin `r+` allows both reads and writes. You can overwrite the file by following the instructions in the answer. – Flimm May 01 '18 at 19:56
16

@JossefHarush Note that the documentation for `a` states 'on some Unix systems, means that all writes append to the end of the file regardless of the current seek position'. In this case the `f.seek(0)` won't work as expected. I just fell foul of this on Linux. – Graeme Jun 25 '18 at 16:02
@Graeme - great input, haven't bumped into this behavior. what solved your case? `r+`? – Jossef Harush Kadouri Jun 26 '18 at 06:17
@JossefHarush Plain `open()` isn't enough if it might not exist. You need to an `fd=os.open(..., os.O_RDWR|os.O_CREAT)` to ensure creation, then `os.fdopen(fd, 'r+')`. See https://stackoverflow.com/a/10352231/2801913 – Graeme Jun 26 '18 at 19:05
25

It's better you explain why `seek` and `truncate` is used here. Most of the readers come from google and do copy-paste. – Shiplu Mokaddim Mar 13 '19 at 10:58
44

After you have read the file, the file pointer(fp) has moved forward, so you need to set it to the beginning. That's that `seek(0)` does: it places the fp to position `0` (*i.e.* the beginning). `truncate()` truncate the file to the provided number of bytes, *i.e.* removes all of the file content after the specified number of bytes. Imagine that your file has the string `Hello, world` and you write `Bye`. If you don't `truncate()` the content at the end will be `Byelo, world`, since you never deleted the text that existed in the file. `truncate()` truncates the file to the current fp. – Illya Gerasymchuk Jul 14 '19 at 12:18
7

Is this faster than first open for just reading 'r', close, and then open for wrtiting 'w'? – Dr_Zaszuś Feb 20 '20 at 14:25
Could we use `w+` as well? – alper Apr 02 '22 at 13:26
1

@alper No, you can't use `w+` as this truncates file as part of opening so reading it is not possible as there is no content anymore. – Piotr Dobrogost Jul 31 '22 at 10:57

score 322 · Answer 2 · edited Aug 18 '22 at 07:00

322

Summarize the I/O behaviors:

Mode	r	r+	w	w+	a	a+
Read	+	+		+		+
Write		+	+	+	+	+
Create			+	+	+	+
Cover			+	+
Point in the beginning	+	+	+	+
Point in the end					+	+

Decision tree for the table above:

edited Aug 18 '22 at 07:00

BSMP

4,596
8
33
44

answered Nov 19 '19 at 01:14

AbstProcDo

19,953
19
81
138

11

What software did you use to make the tree diagram? – Flux Jan 21 '20 at 10:27
1

I would be interested too - Dia? – nerdoc Jan 27 '20 at 21:14
18

What does "truncate" mean here? – Conor James Thomas Warford Hen Oct 18 '20 at 22:33
9

@ConorJamesThomasWarfordHen in this context "truncate" means "truncate the file to a length of 0". I.e. remove all previous contents of the file and start with an empty file at the time of opening. – blubberdiblub Apr 04 '21 at 20:37
13

What does "cover" mean here? – user3804598 Nov 26 '21 at 11:31
@Flux You could use [draw.io](https://app.diagrams.net/) – Rune S. H. Dec 01 '21 at 12:17
Bump: What does "cover" mean here? – Smart Manoj Oct 17 '22 at 02:15
@user3804598 I believe that cover here means that it's going to write over whatever was in the file – qwerty_url Dec 23 '22 at 17:05

score 52 · Answer 3 · edited Apr 25 '16 at 07:56

52

r+ is the canonical mode for reading and writing at the same time. This is not different from using the fopen() system call since file() / open() is just a tiny wrapper around this operating system call.

edited Apr 25 '16 at 07:56

Delgan

18,571
11
90
141

answered Jul 11 '11 at 10:09

3

it append file content, not write from beginning – TomSawyer Feb 26 '20 at 07:53
fopen is a library call, not a system call. – spawn Feb 01 '21 at 22:38
`open` is a system call, `fopen` is not a system call. It's `fopen` that's a wrapper around `open`. – altermetax Mar 10 '22 at 11:13

score 25 · Answer 4 · answered Jul 11 '11 at 10:16

25

I have tried something like this and it works as expected:

f = open("c:\\log.log", 'r+b')
f.write("\x5F\x9D\x3E")
f.read(100)
f.close()

Where:

f.read(size) - To read a file’s contents, call f.read(size), which reads some quantity of data and returns it as a string.

And:

f.write(string) writes the contents of string to the file, returning None.

Also if you open Python tutorial about reading and writing files you will find that:

'r+' opens the file for both reading and writing.

On Windows, 'b' appended to the mode opens the file in binary mode, so there are also modes like 'rb', 'wb', and 'r+b'.

answered Jul 11 '11 at 10:16

Artsiom Rudzenka

27,895
4
34
52

7

Also reading then writing works equally well using 'r+b' mode, but you have to use f.seek(0) between f.read() and f.write() to place the cursor back at the beginning of the file. – gaborous Nov 14 '12 at 18:05
3

Note that if the data you're writing isn't longer that the data already there, it will not get truncated. Use the `truncate` method to stop this. – Flimm Apr 12 '13 at 15:27

How to open a file for both reading and writing?

4 Answers4

Linked

Related