Syntax error display skips the first line of the traceback

[SukramLeinad]

Bug report

Bug description:

Steps to reproduce:

c := 3

Expected is:

Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    c := 3
      ^^
SyntaxError: invalid syntax

But the actual result is:

  File "<python-input-0>", line 1
    c := 3
      ^^
SyntaxError: invalid syntax

While the visual difference is small, it could reflect an actual bug.

CPython versions tested on:

3.13

Operating systems tested on:

Windows

[AlexWaygood]

This seems to be a general problem with all syntax errors. It doesn't seem like it's a recent change, though; I can reproduce the difference in display from other kinds of exceptions with older Python versions as well:

~/dev % python3.11 -c "c := 3"                  
  File "<string>", line 1
    c := 3
      ^^
SyntaxError: invalid syntax
~/dev [1] % python3.11 -c "a"     
Traceback (most recent call last):
  File "<string>", line 1, in <module>
NameError: name 'a' is not defined

[skirpichev]

Is this a bug at all? :) There is no traceback, it's not a runtime error.

[SukramLeinad]

Is this a bug at all? :) There is no traceback, it's not a runtime error.

I would argue that it is a bug for the following reasons:

1. Python has put a lot of work into improving error reporting recently. This bug pretty much negates that, especially for inexperienced users who are very easily confused.
2. CPython is the basis for various Python shells like Pythonwin.exe, which have certain expectations as they reformat errors statements. A case in point is Excessive exception traces with Pythonwin.exe v307 mhammond/pywin32#2393 (comment) where the buggy error statement gets even more corrupted.
3. The problem could be a symptom of an actually more serious underlying bug, so it's worth investigating. For example, I noticed that not only the first line is skipped, but also the module on the second line.

[skirpichev]

especially for inexperienced users who are very easily confused

Wouldn't they be more confused with a line "Traceback ..." where is no actual traceback object?

Actually, it's documented that the header "Traceback (most recent call last)" shouldn't be printed here:
https://docs.python.org/3.13/library/traceback.html#traceback.print_exception

various Python shells like Pythonwin.exe, which have certain expectations as they reformat errors statements

If they expect something else than documented in the Python docs - it's not CPython's fault.

I noticed that not only the first line is skipped, but also the module on the second line.

But there is no module, isn't? Your code has syntax error, it's not compiled.

[skirpichev]

CC @pablogsal

[SukramLeinad]

Wouldn't they be more confused with a line "Traceback ..." where is no actual traceback object?

Sorry, but that is beside my point.

Actually, it's documented that the header [...]

I just noticed that Pythonwin.exe - like CPython - correctly prints the headers for all errors except syntax errors. So I don't think the defect is directly related to the traceback.print_exception method.

But there is no module, isn't? Your code has syntax error, it's not compiled.

That is a good point.

[SukramLeinad]

Ok, got it:
As the code has a syntax error, it is not compiled.
As it is not compiled, it cannot be executed.
As it is not executed, there is no traceback object.
As there is no traceback object, the header “Traceback (most recent call last):” is omitted.

I agree this is not a defect. Sorry for your inconvenience...