Is there a formatted byte string literal in Python 3.6+?

Question

I\'m looking for a formatted byte string literal. Specifically, something equivalent to

name = \"Hello\"
bytes(f\"Some format string {name}\")

Answer 1

No. The idea is explicitly dismissed in the PEP:

For the same reason that we don't support bytes.format(), you may not combine 'f' with 'b' string literals. The primary problem is that an object's __format__() method may return Unicode data that is not compatible with a bytes string.

Binary f-strings would first require a solution for bytes.format(). This idea has been proposed in the past, most recently in PEP 461. The discussions of such a feature usually suggest either

• adding a method such as __bformat__() so an object can control how it is converted to bytes, or

• having bytes.format() not be as general purpose or extensible as str.format().

Both of these remain as options in the future, if such functionality is desired.

Answer 2

From python 3.6.2 this percent formatting for bytes works for some use cases:

print(b"Some stuff %a. Some other stuff" % my_byte_or_unicode_string)

But as AXO commented:

This is not the same. %a (or %r) will give the representation of the string, not the string iteself. For example b'%a' % b'bytes' will give b"b'bytes'", not b'bytes'.

Which may or may not matter depending on if you need to just present the formatted byte_or_unicode_string in a UI or if you potentially need to do further manipulation.

Answer 3

In 3.6+ you can do:

>>> a = 123
>>> f'{a}'.encode()
b'123'

Answer 4

This was one of the bigger changes made from python 2 to python3. They handle unicode and strings differently.

This s how you'd convert to bytes.

string = "some string format"
string.encode()
print(string)

This is how you'd decode to string.

string.decode()

I had a better appreciation for the difference between Python 2 versus 3 change to unicode through this coursera lecture by Charles Severence. You can watch the entire 17 minute video or fast forward to somewhere around 10:30 if you want to get to the differences between python 2 and 3 and how they handle characters and specifically unicode.

I understand your actual question is how you could format a string that has both strings and bytes.

inBytes = b"testing"
inString = 'Hello'
type(inString) #This will yield <class 'str'>
type(inBytes) #this will yield <class 'bytes'>

Here you could see that I have a string a variable and a bytes variable.

This is how you would combine a byte and string into one string.

formattedString=(inString + ' ' + inBytes.encode())

Answer 5

I managed to format a literal in order to send it through a socket:

msgIndex = 0
while(IsSomeCondition()):
  msgIndex += 1
  sock.sendto(
    b"msg number %s" % bytes(str(msgIndex), encoding="utf8"),
    (receiverIP, receiverPort)
  )
# --> b'msg number 1'
# --> b'msg number 2'
# --> b'msg number 3'

I'm using Python 3.8.5.