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.

\x61\x62\x63\x64\x65\x66\x67

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("\x61\x62\x63\x64\x65\x66\x67")
file.close() 

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

Answer 1

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:

>>> '\x61'
'a'
>>> 'a'
'a'
>>> 'a' == '\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 string:

somestring = 'abcd'

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

There is nothing magical about binary files; 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 \r\n instead.

You certainly do not have to produce hexadecimal escapes to write binary data.

On Python 3 strings are Unicode data and cannot just be written to a file without encoding, but on Python the str type is already encoded bytes. So on Python 3 you'd use:

somestring = 'abcd'

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

or you'd use a byte string literal; b'abcd'.