FEAT: Fast TB. Try to detect when one of the file we are trying to highlight is too big, and fallback on pre-8.0 traceback code, that avoids stack_data. You can configure the limit with: >>> from IPython.core import ultratb >>> ultratb.FAST_THRESHOLD = 200 We are trying to _guess_ whether the traceback we will render is in a file that will require to parse more than FAST_THRESHOLD lines, though it is actually difficult to get this value correctly, so in some case, the slow path may be used, even if files are longer than FAST_THRESHOLD, especially if the crashing code is at the beginning of the file.