If you have set sys.excepthook to some function which calls traceback.print_exception, then I would expect to get identical traceback/exception messages. If you run raise SyntaxError("some message"), then print_exception has the extra line File "<string>", line None.

This comes from lines 558-561 of traceback.py:

# It was a syntax error; show exactly where the problem was found.
filename = self.filename or "<string>"
lineno = str(self.lineno) or '?'
yield '  File "{}", line {}\n'.format(filename, lineno)

Is it expected behaviour that these two functions behave differently, or should there be something like:
if self.filename or self.lineno:
etc.

Also, if self.lineno is None, then str(self.lineno) evaluates to "None" and so the ? is never used.

Reproduced on 3.10:

Running Release|Win32 interpreter...
Python 3.10.0a1+ (heads/exceptionGroup-stage1-dirty:928c211ad8, Oct 28 2020, 14:36:37) [MSC v.1916 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> raise SyntaxError("some message")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
SyntaxError: some message

>>> import traceback
>>> import sys
>>> sys.excepthook = traceback.print_exception
>>> raise SyntaxError("some message")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line None
SyntaxError: some message

After debugging the default excepthook code (C version), I see that the difference is because the SyntaxError's lineno is None.

It doesn't mind that the filename is None (it defaults it to <string>) but when it can't get the lineno it gives up here:

If you run this script:

---------------------------------------

import traceback
import sys

se = SyntaxError("wrong!")
se.filename = "myfile.py"
print("========== lineno is None ==========")
print('traceback.print_exception:')
traceback.print_exception(type(se), se, None)
print('---------------------')
print('sys.excepthook:')
sys.excepthook(type(se), se, None)
print('---------------------')

se.lineno = 55

print("========== lineno is 55 ==========")
print('traceback.print_exception:')
traceback.print_exception(type(se), se, None)
print('---------------------')
print('sys.excepthook:')
sys.excepthook(type(se), se, None)
print('---------------------')

The output is:
---------------------------------------
Running Release|x64 interpreter...
========== lineno is None ==========
traceback.print_exception:
File "myfile.py", line None
SyntaxError: wrong!
---------------------
sys.excepthook: <-- file-lineno missing
SyntaxError: wrong! (myfile.py)
---------------------
========== lineno is 55 ==========
traceback.print_exception:
File "myfile.py", line 55
SyntaxError: wrong!
---------------------
sys.excepthook:
File "myfile.py", line 55 <-- file-lineno is there
SyntaxError: wrong!
---------------------
---------------------------------------

New changeset 069560b by Irit Katriel in branch 'master':
bpo-34463: Make python tracebacks identical to C tracebacks for SyntaxErrors without a lineno (GH-23427)
069560b

New changeset 8e5c61a by Miss Islington (bot) in branch '3.9':
bpo-34463: Make python tracebacks identical to C tracebacks for SyntaxErrors without a lineno (GH-23427)
8e5c61a

New changeset 727bed6 by Irit Katriel in branch '3.8':
[3.8] bpo-34463: Make python tracebacks identical to C tracebacks for (bpo-23899)
727bed6