Improve the error message for `assert` and `AssertionError` by adding location information

[sobolevn]

Feature or enhancement

Right now assert generate errors that are hard to read, mostly because they lack context.
Example:

>>> assert 1 == 0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError

Pitch

Let's make them better:

>>> assert 1 == 0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    assert 1 == 0
           ^^^^^^
AssertionError

This is a good starting point.

Later this can be enhanced to use more context.

Linked PRs

• gh-105724: Add location information to assert errors #105935

[sunmy2019]

You need to modify Lib/traceback.py and (maybe) make _format_syntax_error more general.

[markshannon]

You need to modify Lib/traceback.py and (maybe) make _format_syntax_error more general.

I don't think that is necessary. Changing the location of the RAISE_VARARGS instruction in the assertion should be sufficient.

[heysujal]

Can I work on this issue ?

[sobolevn]

@heysujal sorry, I am already working on it :(
Please, feel free to take any other free issue. If you want any help, please feel free to ping me.

[sobolevn]

@markshannon @Fidget-Spinner I might be missing something, but looks like assert already supports this. There are two possible scenarios:

1. assert is used in a named file, then it works
2. assert is used in a repl, then it does not work (obviously)

Examples, file called ex.py:

def some():
    b = None
    assert b

some()

Result:

» ./python.exe ex.py
Traceback (most recent call last):
  File "/Users/sobolev/Desktop/cpython/ex.py", line 8, in <module>
    some()
  File "/Users/sobolev/Desktop/cpython/ex.py", line 5, in some
    assert b
           ^
AssertionError

The same happens for CPython tests, if I add some failing assert to test_typing:

./python.exe -m test -v test_typing
======================================================================
FAIL: test_basics (test.test_typing.LiteralStringTests.test_basics)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/sobolev/Desktop/cpython/Lib/test/test_typing.py", line 328, in test_basics
    assert False
           ^^^^^
AssertionError

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

But, REPL does not work for obvious reason: it cannot allow to read source lines. Example (the same code as in the first example):

>>> def some():
...     b = None
...     assert b
... 
>>> some()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in some
AssertionError

So, I am not sure what the task is.

[markshannon]

What version of Python are you testing against?
I'm not seeing that behavior and the code suggests that the location is the whole line, not the expression.
https://github.com/python/cpython/blob/main/Python/compile.c#L3893

[sobolevn]

I am using the main version.

[sobolevn]

@markshannon never mind, I was running my own fork with the changes I made 🤦

[markshannon]

Good to know that your changes work 🙂

[markshannon]

We've all done it. I hate to think how much time I've wasted debugging the wrong branch in the past.