documentation for raw bytes

[Ofnuts]

If a file is opened in "binary", you get  a bytes object. Otherwise you get a str object:
Assuming a "bytes.dat" file that contains Déjà vu!\n (encoded in UTF-8) the following code:

with open('bytes.dat','rb') as f:
   dat=f.read()
   print(type(dat), len(dat))
   
with open('bytes.dat','r') as f:
   dat=f.read()
   print(type(dat), len(dat))

yields:

Output:
<class 'bytes'> 11
<class 'str'> 9

Of course, sys.stdin is already opened in text mode, so it reads str objects and not bytes. But you can read the binary buffer object sys.stdin is based on. See the note here.