Writing 8 bit values to a binary file in python v3.3

Question

Please let me know the best way to write 8bit values to a file in python. The values can be between 0 and 255. I open the file as follows:

f = open('temp', 'wb')

Assume the value I am writing is an int between 0 and 255 assigned to a variable, e.g.

x = 13

This does not work:

f.write(x)

..as you experts know does not work. python complains about not being able to write ints to buffer interface. As a work around I am writing it as a hex digit. Thus:

f.write(hex(x))

..and that works but is not only space inefficient but clearly not the right python way. Please help. Thanks.

Have a look at: docs.python.org/2/library/functions.html#chr — Jon Clements
– Jon Clements, Commented Aug 13, 2013 at 6:47

Jim Garrison · Accepted Answer · 2013-08-13 07:06:10Z

3

Try explicitly creating a bytes object:

f.write(bytes([x]))

You can also output a series of bytes as follows:

f.write(bytes([65, 66, 67]))

answered Aug 13, 2013 at 7:06

Jim Garrison

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1 Comment

B Brown Over a year ago

Many thanks Jim! I really appreciate the example. Much more helpful this way.

joente · Accepted Answer · 2013-08-13 07:23:23Z

1

As an alternative you can use the struct module...

import struct
x = 13
with open('temp', 'wb') as f:
    f.write(struct.pack('>I', x))  # Big-endian, unsigned int

To read x from the file...

with open('temp', 'rb') as f:
    x, = struct.unpack(">I", f.read())

edited Aug 13, 2013 at 7:23

answered Aug 13, 2013 at 7:15

joente

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Comments

Mario Rossi · Accepted Answer · 2013-08-13 07:12:04Z

0

You just need to write an item of bytes data type, not integer.

answered Aug 13, 2013 at 7:12

Mario Rossi

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