Writing an ASCII string as binary in python

Question

I have a ASCII string = “abcdefghijk”. I want to write this to a binary file in binary format using python.

I tried following:

str  = "abcdefghijk"
fp = file("test.bin", "wb")
hexStr = "".join( (("\x%s") % (x.encode("hex"))) for x in str)
fp.write(hexStr)
fp.close()

However, when I open the test.bin I see the following in ascii format instead of binary.

x61x62x63x64x65x66x67

I understand it because for two slashes here (“\x%s”). How could I resolve this issue? Thanks in advance.

Update :

Following gives me the expected result:

file = open("test.bin", "wb")
file.write("x61x62x63x64x65x66x67")
file.close()

But how do I achieve this with “abcdef” ASCII string. ?

Asked By: aMa

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Source

Answer 1

I think you don’t necessarily understand what binary/ascii is … all files are binary in the sense that its just bits. ascii is just a representation of some bits… 99.9999 % of file editors will display your bits as ascii if they can , and if there is no other encoding declared in the file itself …

fp.write("abcd")

is exactly equivelent to

fp.write("x61x62x63x64")

Answered By: Joran Beasley

Answer 2

You misunderstood what xhh does in Python strings. Using x notation in Python strings is just syntax to produce certain codepoints.

You can use 'x61' to produce a string, or you can use 'a'; both are just two ways of saying give me a string with a character with hexadecimal value 61, e.g. the a ASCII character:

>>> b'x61'
'a'
>>> b'a'
'a'
>>> b'a' == b'x61'
True

The xhh syntax then, is not the value; there is no and no x and no 6 and 1 character in the final result.

You should just write your bytestring:

somestring = b'abcd'

with open("test.bin", "wb") as file:
    file.write(somestring.encode())

Note that I used bytestrings (b'...') in my code examples here. ‘Regular’ strings are Unicode data and cannot just be written to a binary file without encoding. The x.. same escaping syntax works in normal literal string syntax too, but then you need to encode your string to bytes when writing:

somestring = 'x61bcd'  # value: 'abcd'

with open("test.bin", "wb") as file:
    file.write(somestring.encode('ascii'))

You certainly do not have to produce hexadecimal escapes to write binary data. Just because some binary file viewers represent data in a file as hexadecimal doesn’t mean that the data is written in hexadecimal form!

Originally, this answer was written with Python 2 in mind, where the distinction between a binary and regular text file was less pronounced. There, the only difference with a file opened in text mode is that a binary file will not automatically translate n newlines to the line separator standard for your platform; e.g. on Windows writing n produces rn instead.

Answered By: Martijn Pieters

Writing an ASCII string as binary in python

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