Python 3.8 supports using a limited set of non-ASCII Unicode characters in identifiers. So, it seems that it is valid to use Σ as a character in an identifier.
However, something is wrong...
Problem
def f(Σ):
print(f'{Σ=}')
f(1)
f(Σ=2)
f(**{'Σ': 3})
The first two calls are fine, but the third fails:
Σ=1
Σ=2
Traceback (most recent call last):
File "sigma.py", line 24, in <module>
f(**{'Σ': 3})
TypeError: f() got an unexpected keyword argument 'Σ'
Analysis
Let's see what is actually going on:
def f2(**kw):
for name, value in kw.items():
print(f'{name}={value} {ord(name)=}')
f2(Σ=2)
f2(**{'Σ': 3})
It prints:
Σ=2 ord(name)=931
Σ=3 ord(name)=120506
I called it with Σ both times, but it was changed to the very similar simpler Σ in the first call.
It seems that an argument named Σ (U+1D6BA) is implicitly renamed to Σ (U+03A3), and in every call to the function, argument Σ is also implicitly renamed to Σ, except if it is passed as **kwargs.
The Questions
Is this a bug? It does not look like it is accidental. Is it documented? Is there a set of true characters and a list of alias characters available somewhere?