Hi all, This is a discussion mail about improving the traceback line number quality in PyPy3. Some context:
CPython has started supporting negative line number offset from one bytecode to the next in its lnotab encoding, see here: https://bugs.python.org/issue26107 https://github.com/python/cpython/commit/f3914eb16d2#diff-cb296cc5109f5640ff3f6d7198a6abee (Please don't ask me why not a single test was added for this new feature.) However, the support is so far only theoretically possible, the Python 3.7/3.8 bytecode compiler does not seem to make use of the feature. Negative line number jumps occur quite regularly with continuation lines, eg like this: def foobar(a, b): return "" + (1, a, b, 2 ) If you call this function, the traceback will contain this line: File "x.py", line 5, in foobar 2 TypeError: can only concatenate str (not "tuple") to str Which is obviously misleading. A slightly more realistic example are function decorators. There, the execution order is regularly "backwards": first the function definition is executed, then the individual decorators are run. There is currently a hack to sometimes leave the line number on the line of the first decorator, leading to confusing tracebacks. Example: def works(f): return f def broken(f): assert 0 @works @broken @works def g(): pass Running this, you'll get the following traceback: Traceback (most recent call last): File "y.py", line 9, in <module> @works File "y.py", line 5, in broken assert 0 AssertionError Pointing you again to the wrong line. We could fix this problem rather easily by removing the code that prevents the bytecode compiler from emitting negative line number offsets. Should I implement this? Advantages: - A lot less confusing tracebacks in real-world cases Disadvantages: - Different behavior from CPython (but arguably more correct) - Tracing tools could potentially be confused - Potentially larger lnotab strings due to more jumping around Cheers, Carl Friedrich _______________________________________________ pypy-dev mailing list pypy-dev@python.org https://mail.python.org/mailman/listinfo/pypy-dev
