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

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

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

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.

Can I work on this issue ?

@heysujal

@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.

@markshannon

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

assert is used in a named file, then it works
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.

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

I am using the main version.

@markshannon

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

Good to know that your changes work 🙂

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

pythongh-105724: Add location information to `assert` errors

We don't want to increase the cost of execution if the assertion passes, which limits what transformations we can apply.
We can improve a few cases, though.

Boolean operation.

assert cond1 and cond2 is compiled as:

if not cond1 or not cond2:
    raise AssertionError # location `cond1 and cond2`

We could compile it as:

if not cond1:
    raise AssertionError # location `cond1`
if not cond2:
    raise AssertionError # location `cond2`

which is a bit bulkier, but is as fast as it executes the same VM instructions when the assert passes.

Comparison of local variable to constant or local variable

assert var1 == var2
Since we can recompute var1 or var2 without side effects, we can embed the values in the error message:

if not var1 == var2:
    _msg = f"{var1} != {var2}"
    raise AssertionError(_msg) # location `var1 == var2`

This is complicated somewhat if str(var1) raises.
We can add an instrinsic to handle the odd cases.

if not var1 == var2:
    instrinsic_assert_eq(var1, var2)

I can surely work on this after the initial PR is merged 👍

gh-105724: Add location information to `assert` errors (GH-105935)

@sobolevn

I can surely work on this after the initial PR is merged 👍

@sobolevn Still stuff to do here, or can we close this issue?

There are still tasks to complete in this issue.
Thanks for your work on closing issues! 👍

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Improve the error message for `assert` and `AssertionError` by adding location information #105724

Feature or enhancement

Pitch

Linked PRs

Boolean operation.

Comparison of local variable to constant or local variable

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Improve the error message for assert and AssertionError by adding location information #105724

Description

Feature or enhancement

Pitch

Linked PRs

Activity

sunmy2019 commented on Jun 13, 2023

markshannon commented on Jun 13, 2023

heysujal commented on Jun 14, 2023

sobolevn commented on Jun 14, 2023

sobolevn commented on Jun 14, 2023

markshannon commented on Jun 14, 2023

sobolevn commented on Jun 14, 2023

sobolevn commented on Jun 14, 2023

markshannon commented on Jun 14, 2023

markshannon commented on Jun 14, 2023

markshannon commented on Jun 20, 2023

Boolean operation.

Comparison of local variable to constant or local variable

sobolevn commented on Jun 20, 2023

hugovk commented on Nov 10, 2023

sobolevn commented on Nov 10, 2023

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Improve the error message for `assert` and `AssertionError` by adding location information #105724