Writing an ASCII string as binary in python

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.