General
It provides a standard interface to extract, format and print stack traces of Python programs.
It mimics the behavior of the Python interpreter when it prints a stack trace.
The module uses traceback objects (of type types.TracebackType), which are assigned to the __traceback__ field of BaseException
instances.
Helpful functions
Formal functions:
Print up stack trace entries from traceback object tb:
traceback.print_tb(tb, limit=None, file=None)
– if limit is positive: starting from the caller’s frame
– Otherwise, print the last abs(limit) entries.
– If limit is None, all entries are printed.
Print exception information and stack trace entries from traceback object tb:
traceback.print_exception(exc, /, [value, tb, ]limit=None, file=None, chain=True)
This differs from print_tb() in the following ways:
– if tb is not None, it prints a header Traceback (most recent call last).
– it prints the exception type and value after the stack trace
Variant of the traceback.print_tb() function that starts from the invocation point:
traceback.print_stack(f=None, limit=None, file=None)
Shorthand functions:
Shorthand for print_exception(sys.exception(), limit, file, chain):
traceback.print_exc(limit=None, file=None, chain=True)
Like print_exc(limit) but returns a string instead of printing to a file:
traceback.format_exc(limit=None, chain=True)
Examples
Print tracebacks or exceptions
source code:
import traceback from time import sleep def print_tb(): print(f'print_tb()', flush=True) try: invalid = 'a'[1] except Exception as e: traceback.print_tb(e.__traceback__) def print_exception(): print(f'print_exception()', flush=True) try: invalid = 'a'[1] except Exception as e: traceback.print_exception(e) def print_exc_shorthand_for_print_exception(): print(f'print_exc_shorthand_for_print_exception()', flush=True) try: invalid = 'a'[1] except Exception as e: traceback.print_exc() def print_exception_with_format_exc(): print(f'print_exception_with_format_exc()', flush=True) try: invalid = 'a'[1] except Exception as e: print(traceback.format_exc()) if __name__ == '__main__': print_exception() sleep(0.5) print(f'----------', flush=True) # shorthand for print_exception(sys.exception()) print_exc_shorthand_for_print_exception() sleep(0.5) print(f'----------', flush=True) print_exception_with_format_exc() sleep(0.5) print(f'----------', flush=True) print_tb() sleep(0.5) print(f'----------', flush=True) |
Output:
print_exception() Traceback (most recent call last): File "C:\programmation\workspace-python\python_blog_examples\traceback\print_tb_example.py", line 16, in print_exception invalid = 'a'[1] IndexError: string index out of range ---------- print_exc_shorthand_for_print_exception() Traceback (most recent call last): File "C:\programmation\workspace-python\python_blog_examples\traceback\print_tb_example.py", line 24, in print_exc_shorthand_for_print_exception invalid = 'a'[1] IndexError: string index out of range ---------- print_exception_with_format_exc() Traceback (most recent call last): File "C:\programmation\workspace-python\python_blog_examples\traceback\print_tb_example.py", line 32, in print_exception_with_format_exc invalid = 'a'[1] IndexError: string index out of range ---------- print_tb() File "C:\programmation\workspace-python\python_blog_examples\traceback\print_tb_example.py", line 8, in print_tb invalid = 'a'[1] ---------- Process finished with exit code 0 |
Print the stacks of all running threads thanks to print_stack()
Source code:
import sys import threading import traceback from threading import Thread from time import sleep def a(): i: int = 0 while True: i += 1 sleep(0.25) def b(): sleep(5) Thread(daemon=True, name='a', target=lambda: a()).start() Thread(daemon=True, name='b', target=lambda: b()).start() for th in threading.enumerate(): print(th, flush=True) traceback.print_stack(sys._current_frames()[th.ident]) # The sleep is required to get an output as expected, otherwise the print_stack() outputs are # merged. sleep(0.5) print('---------------------', flush=True) sleep(0.5) sys.exit() |
Output:
C:\Users\david\AppData\Local\Programs\Python\Python310\python.exe C:/programmation/workspace-python/python_blog_examples/traceback/print_stack_of_all_threads.py <_MainThread(MainThread, started 20232)> File "C:\programmation\workspace-python\python_blog_examples\traceback\print_stack_of_all_threads.py", line 23, in <module> traceback.print_stack(sys._current_frames()[th.ident]) --------------------- <Thread(a, started daemon 3712)> File "C:\Users\david\AppData\Local\Programs\Python\Python310\lib\threading.py", line 973, in _bootstrap self._bootstrap_inner() File "C:\Users\david\AppData\Local\Programs\Python\Python310\lib\threading.py", line 1016, in _bootstrap_inner self.run() File "C:\Users\david\AppData\Local\Programs\Python\Python310\lib\threading.py", line 953, in run self._target(*self._args, **self._kwargs) File "C:\programmation\workspace-python\python_blog_examples\traceback\print_stack_of_all_threads.py", line 19, in <lambda> Thread(daemon=True, name='a', target=lambda: a()).start() File "C:\programmation\workspace-python\python_blog_examples\traceback\print_stack_of_all_threads.py", line 12, in a sleep(0.25) --------------------- <Thread(b, started daemon 18128)> File "C:\Users\david\AppData\Local\Programs\Python\Python310\lib\threading.py", line 973, in _bootstrap self._bootstrap_inner() File "C:\Users\david\AppData\Local\Programs\Python\Python310\lib\threading.py", line 1016, in _bootstrap_inner self.run() File "C:\Users\david\AppData\Local\Programs\Python\Python310\lib\threading.py", line 953, in run self._target(*self._args, **self._kwargs) File "C:\programmation\workspace-python\python_blog_examples\traceback\print_stack_of_all_threads.py", line 20, in <lambda> Thread(daemon=True, name='b', target=lambda: b()).start() File "C:\programmation\workspace-python\python_blog_examples\traceback\print_stack_of_all_threads.py", line 16, in b sleep(5) --------------------- Process finished with exit code 0 |