How can I modify a Python traceback object when raising an exception?

Starting with Python 3.7, you can instantiate a new traceback object and use the .with_traceback() method when throwing. Here's some demo code using either sys._getframe(1) (or a more robust alternative) that raises an AssertionError while making your debugger believe the error occurred in myassert(False): sys._getframe(1) omits the top stack frame.

What I should add is that while this looks fine in the debugger, the console behavior unveils what this is really doing:

Traceback (most recent call last):
  File ".\test.py", line 35, in <module>
    myassert_false()
  File ".\test.py", line 31, in myassert_false
    myassert(False)
  File ".\test.py", line 26, in myassert
    raise AssertionError().with_traceback(back_tb)
  File ".\test.py", line 31, in myassert_false
    myassert(False)
AssertionError

Rather than removing the top of the stack, I have added a duplicate of the second-to-last frame.

Anyway, I focus on how the debugger behaves, and it seems this one works correctly:

"""Modify traceback on exception.

See also https://github.com/python/cpython/commit/e46a8a
"""

import sys
import types


def myassert(condition):
    """Throw AssertionError with modified traceback if condition is False."""
    if condition:
        return

    # This function ... is not guaranteed to exist in all implementations of Python.
    # https://docs.python.org/3/library/sys.html#sys._getframe
    # back_frame = sys._getframe(1)
    try:
        raise AssertionError
    except AssertionError:
        traceback = sys.exc_info()[2]
        back_frame = traceback.tb_frame.f_back

    back_tb = types.TracebackType(tb_next=None,
                                  tb_frame=back_frame,
                                  tb_lasti=back_frame.f_lasti,
                                  tb_lineno=back_frame.f_lineno)
    raise AssertionError().with_traceback(back_tb)


def myassert_false():
    """Test myassert(). Debugger should point at the next line."""
    myassert(False)


if __name__ == "__main__":
    myassert_false()

For python3, here's my answer. Please read the comments for an explanation:

def pop_exception_traceback(exception,n=1):
    #Takes an exception, mutates it, then returns it
    #Often when writing my repl, tracebacks will contain an annoying level of function calls (including the 'exec' that ran the code)
    #This function pops 'n' levels off of the stack trace generated by exception
    #For example, if print_stack_trace(exception) originally printed:
    #   Traceback (most recent call last):
    #   File "<string>", line 2, in <module>
    #   File "<string>", line 2, in f
    #   File "<string>", line 2, in g
    #   File "<string>", line 2, in h
    #   File "<string>", line 2, in j
    #   File "<string>", line 2, in k
    #Then print_stack_trace(pop_exception_traceback(exception),3) would print: 
    #   File "<string>", line 2, in <module>
    #   File "<string>", line 2, in j
    #   File "<string>", line 2, in k
    #(It popped the first 3 levels, aka f g and h off the traceback)
    for _ in range(n):
        exception.__traceback__=exception.__traceback__.tb_next
    return exception

This code might be of interest for you.

It takes a traceback and removes the first file, which should not be shown. Then it simulates the Python behavior:

Traceback (most recent call last):

will only be shown if the traceback contains more than one file. This looks exactly as if my extra frame was not there.

Here my code, assuming there is a string text:

try:
    exec(text)
except:
    # we want to format the exception as if no frame was on top.
    exp, val, tb = sys.exc_info()
    listing = traceback.format_exception(exp, val, tb)
    # remove the entry for the first frame
    del listing[1]
    files = [line for line in listing if line.startswith("  File")]
    if len(files) == 1:
        # only one file, remove the header.
        del listing[0]
    print("".join(listing), file=sys.stderr)
    sys.exit(1)