How to open a file for both reading and writing?

Question

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()

Summarize the I/O behaviors

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

and the decision branch

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.

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'.