How do you set a string of bytes from an environment variable in Python?

The easiest option is to simply set it as binary data in your shell. This uses ANSI string quoting and avoids the need for any sort of conversion on the Python side.

export FOO=$'\x1b\xba\x94(\xae\xd0\xb2\xa6\xf2f\xf6\x1fI\xed\xbao$\xc6D\x08\xba\x81\x96v'

NB: this type of string is not part of the POSIX specification at this time, but is in the process of being added. Support is present in almost all major shells, including Bash, ksh, and zsh. Ensure your shell supports it before relying on its use.

With zwer's answer I tried the following

first from bash (this is the same binary literal given by ybakos)

export FOO='\x1b\xba\x94(\xae\xd0\xb2\xa6\xf2f\xf6\x1fI\xed\xbao$\xc6D\x08\xba\x81\x96v'

then I launched the python shell (I have python 3.5.2)

>>> import os
>>> # ybakos's original binary literal
>>> foo =  b'\x1b\xba\x94(\xae\xd0\xb2\xa6\xf2f\xf6\x1fI\xed\xbao$\xc6D\x08\xba\x81\x96v'
>>> # ewer's python 3.x solution
>>> FOO = bytes(os.environ["FOO"], "utf-8").decode('unicode_escape')
>>> foo == FOO
False
>>> ^D

The last line of foo == FOO should return true, so the solution does not appear to work correctly.

I noticed that there is an os.envirnb dictionary, but I couldn't figure out to set an Environment Variable to a binary literal, so I tried the following alternative which uses base64 encoding to get an ASCII version of the binary literal.

First launch python shell

>>> import os
>>> import base64
>>> foo = os.urandom(24)
>>> foo
b'{\xd9q\x90\x8b\xba\xecv\xb3\xcb\x1e<\xd7\xba\xf1\xb4\x99\xf056\x90U\x16\xae'
>>> foo_base64 = base64.b64encode(foo)
>>> foo_base64
b'e9lxkIu67Hazyx4817rxtJnwNTaQVRau'
>>> ^D

Then in the bash shell

export FOO_BASE64='e9lxkIu67Hazyx4817rxtJnwNTaQVRau'

Then back in the python shell

>>> import os
>>> import base64
>>> # the original binary value from the first python shell session
>>> foo = b'{\xd9q\x90\x8b\xba\xecv\xb3\xcb\x1e<\xd7\xba\xf1\xb4\x99\xf056\x90U\x16\xae'
>>> dec_foo = base64.b64decode(bytes(os.environ.get('FOO_BASE64'), "utf-8"))
>>> # the values match!
>>> foo == dec_foo
True
>>> ^D

The last line shows that the 2 results are the same!!

What we are doing, is first getting a binary value from os.urandom() and Base64 encoding it. We then use the Base64 encoded value to set the environment variable. Note: base64.b64encode() returns a binary value, but it will only contain printable ASCII characters.

Then in our program we read in the Base64 encode string value from the environment variable, convert the string into it's binary form, and finally Base64 decode it back to its original value.