How to write 8 bit unsigned integer to a file using python?

Question

I want to write 8 bit unsigned integers to file. In c++ we can do it using fprintf in which the format is:

%[flags][width][.precision][length]specifier

Is there any way I can do it using python? I tried finding the size of the file by asigning value of x =1 to print only 9 integer values, I didn't understand how the file size was 35 bytes.

f = open('myfile','w')
while x>0:
for i in range(0,9):
    a[i] = random.randint(0,255)
    f.write("%d" % a[i])
    f.write(" ")
f.write('\n')
x = x-1
f.close()

Your code writes the textual, decimal representation of numbers in the range 0-255. Your prose indicates that you want to write the non-textual binary representation. Do you understand the difference? If so, which do you actually want? — Robᵩ
– Robᵩ, Commented Apr 1, 2016 at 17:41
why shouldn't it be? you're getting some random integer from 0->255, and since you're writing out to a text file, you're getting the string representation of that number, 1 is one character/byte, 10 is two characters/bytes, 100 is 3 characters/bytes, and youre randint() just happened to generate numbers that come to 24 chars/bytes (+ 10 spaces + newline) — Marc B
– Marc B, Commented Apr 1, 2016 at 17:43
@MarcB I understand what you saying but I don't want to write string representation of the number, how do I write the number? — morrie-student
– morrie-student, Commented Apr 1, 2016 at 17:58

Robᵩ · Accepted Answer · 2016-04-01 18:03:33Z

2

The following code writes nine 8-bit unsigned integers a file, using 1-byte-per-word binary representation.

import struct
import random

f = open('myfile','wb')
for i in range(0,9):
    a = random.randint(0,255)
    f.write(struct.pack("=B", a))
f.close()

The important features of this program are:

it uses mode 'wb', not 'w' to open the output file.
it uses struct.pack to create the binary data.

edited Apr 1, 2016 at 18:03

answered Apr 1, 2016 at 17:45

Robᵩ

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1 Comment

Lightness Races in Orbit Over a year ago

By "this code", for a minute I thought you meant the OP's and was really confused. Perhaps clarify.

martineau · Accepted Answer · 2016-04-01 18:06:34Z

0

You need to open the file in binary mode and convert the unsigned integer value into a one character string. Here's a simple way of doing it:

import random

with open('myfile', 'wb') as f:
    for _ in range(0, 9):
        a = random.randint(0, 255)
        f.write(chr(a))

answered Apr 1, 2016 at 18:06

martineau

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