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. ?

• jonrsharpe: You very carefully encode the characters as hex - why are you expecting to see anything else?
• Martijn Pieters: What output did you expect then? I'm not sure you understood what binary mode does, or even what Python uses \xhh notation for (and it is just syntax, a way to produce a value, not the value itself).
• aMa: @jonrsharpe, I want to write "\x61\x62\x63\x64\x65\x66\x67" as binary to test.bin (not as ascii string). How can I do that? And finally the test.bin should be a binary file.
• Martijn Pieters: @aMa: all files are binary. Opening a file in text mode only enables special handling of newlines (and on Windows, causes 0x0a being interpreted as the end of the file). As such, just write 'abcd'. On Python 3, you'd need to encode text to bytes with str.encode('ascii').
• Martijn Pieters: @aMa: However, a binary file is not a sequence of hexadecimals! Some hex editors may display contents as hex, but that's just represetation, not the actual value in the file.

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:

>>> 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 \r\n instead.

Answer 2

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("\x61\x62\x63\x64")