How can we pass bytes as the key of keyword arguments to functions?

Question

Let us have the following example:

def fun(**args):
    print(str(args))

dic = {'name': 'Pulkit'}
fun(**dic)

This code works fine and I have the following output:

{'name': 'Pulkit'}

Now lets pass the value as bytes:

dic_vb = {'name': b'Pulkit'}
fun(**dic_vb)

This also works fine and have the following output:

{'name': b'Pulkit'}

But things change when I try to have the key as bytes:

dic_kb = {b'name': 'Pulkit'}

This results in TypeError saying:

TypeError: fun() keywords must be strings

Is there any way we can pass bytes as keyword arguments. I also checked CPython code at repo which deals with keyword arguments and seems like we can't pass. Is there any workaround or do I need to make sure unicodes are passed.

I am dealing with a codebase where we have lot of such instances on Python 2 and need to be ported to Python 3. So is only way possible is to convert all of the keyword arguments to unicodes?

Answer 1

No, keywords need to be valid identifiers, it is the same reason why you cannot provide dictionaries that have numbers as keys to a function expecting **kwargs:

>>> dic = {1: 'Pulkit'}
>>> fun(**dic)
TypeErrorTraceback (most recent call last)
<ipython-input-54-83c29ed0f08e> in <module>()
      3 
      4 dic = {1: 'Pulkit'}
----> 5 fun(**dic)

TypeError: fun() keywords must be strings

That is also what you saw in the source, with the negation (!) of PyUnicode_Check acting as the enforcer.

It is required; you need to provide keys that are able to act as names. To see what is and what is not allowed, just try assigning it to a name in the interactive interpreter and see what errs:

>>> b'my_list' = [...]  # err
>>> 23 = [21, 22]       # err

A viable (and sole, if decode is not allowed) option is passing the dictionary as just another argument and then accessing the values through with normal dictionary look-ups.

Of course, if decoding the bytes to strings before invoking the function is a viable alternative I would suggest you pursue that option:

>>> dic = {b'name': 'Pulkit'}
>>> fun(**{b.decode(): v for b, v in dic.items()})