#!/usr/bin/env python3 # -*- coding: utf-8 -*- # "Error Handling" - a chapter of "The Debugging Book" # Web site: https://www.debuggingbook.org/html/ExpectError.html # Last change: 2025-01-16 10:55:32+01:00 # # Copyright (c) 2021-2025 CISPA Helmholtz Center for Information Security # Copyright (c) 2018-2020 Saarland University, authors, and contributors # # Permission is hereby granted, free of charge, to any person obtaining a # copy of this software and associated documentation files (the # "Software"), to deal in the Software without restriction, including # without limitation the rights to use, copy, modify, merge, publish, # distribute, sublicense, and/or sell copies of the Software, and to # permit persons to whom the Software is furnished to do so, subject to # the following conditions: # # The above copyright notice and this permission notice shall be included # in all copies or substantial portions of the Software. # # THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS # OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF # MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. # IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY # CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, # TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE # SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. r''' The Debugging Book - Error Handling This file can be _executed_ as a script, running all experiments: $ python ExpectError.py or _imported_ as a package, providing classes, functions, and constants: >>> from debuggingbook.ExpectError import but before you do so, _read_ it and _interact_ with it at: https://www.debuggingbook.org/html/ExpectError.html The `ExpectError` class allows you to catch and report exceptions, yet resume execution. This is useful in notebooks, as they would normally interrupt execution as soon as an exception is raised. Its typical usage is in conjunction with a `with` clause: >>> with ExpectError(): >>> x = 1 / 0 Traceback (most recent call last): File "/var/folders/n2/xd9445p97rb3xh7m1dfx8_4h0006ts/T/ipykernel_30978/2664980466.py", line 2, in x = 1 / 0 ~~^~~ ZeroDivisionError: division by zero (expected) The `ExpectTimeout` class allows you to interrupt execution after the specified time. This is useful for interrupting code that might otherwise run forever. >>> with ExpectTimeout(5): >>> long_running_test() Start 0 seconds have passed 1 seconds have passed 2 seconds have passed 3 seconds have passed Traceback (most recent call last): File "/var/folders/n2/xd9445p97rb3xh7m1dfx8_4h0006ts/T/ipykernel_30978/1223755941.py", line 2, in long_running_test() File "/var/folders/n2/xd9445p97rb3xh7m1dfx8_4h0006ts/T/ipykernel_30978/3930412460.py", line 4, in long_running_test time.sleep(1) File "Timeout.ipynb", line 43, in timeout_handler raise TimeoutError() TimeoutError (expected) The exception and the associated traceback are printed as error messages. If you do not want that, use these keyword options: * `print_traceback` (default True) can be set to `False` to avoid the traceback being printed * `mute` (default False) can be set to `True` to completely avoid any output. For more details, source, and documentation, see "The Debugging Book - Error Handling" at https://www.debuggingbook.org/html/ExpectError.html ''' # Allow to use 'from . import ' when run as script (cf. PEP 366) if __name__ == '__main__' and __package__ is None: __package__ = 'debuggingbook' # Error Handling # ============== if __name__ == '__main__': print('# Error Handling') ## Synopsis ## -------- if __name__ == '__main__': print('\n## Synopsis') ## Catching Errors ## --------------- if __name__ == '__main__': print('\n## Catching Errors') if __name__ == '__main__': # We use the same fixed seed as the notebook to ensure consistency import random random.seed(2001) import traceback import sys from types import FrameType, TracebackType from typing import Union, Optional, Callable, Any class ExpectError: """Execute a code block expecting (and catching) an error.""" def __init__(self, exc_type: Optional[type] = None, print_traceback: bool = True, mute: bool = False): """ Constructor. Expect an exception of type `exc_type` (`None`: any exception). If `print_traceback` is set (default), print a traceback to stderr. If `mute` is set (default: False), do not print anything. """ self.print_traceback = print_traceback self.mute = mute self.expected_exc_type = exc_type def __enter__(self) -> Any: """Begin of `with` block""" return self def __exit__(self, exc_type: type, exc_value: BaseException, tb: TracebackType) -> Optional[bool]: """End of `with` block""" if exc_type is None: # No exception return if (self.expected_exc_type is not None and exc_type != self.expected_exc_type): raise # Unexpected exception # An exception occurred if self.print_traceback: lines = ''.join( traceback.format_exception( exc_type, exc_value, tb)).strip() else: lines = traceback.format_exception_only( exc_type, exc_value)[-1].strip() if not self.mute: print(lines, "(expected)", file=sys.stderr) return True # Ignore it def fail_test() -> None: # Trigger an exception x = 1 / 0 if __name__ == '__main__': with ExpectError(): fail_test() if __name__ == '__main__': with ExpectError(print_traceback=False): fail_test() if __name__ == '__main__': with ExpectError(ZeroDivisionError): fail_test() if __name__ == '__main__': with ExpectError(): with ExpectError(ZeroDivisionError): some_nonexisting_function() # type: ignore ## Catching Timeouts ## ----------------- if __name__ == '__main__': print('\n## Catching Timeouts') import sys import time from .Timeout import Timeout class ExpectTimeout(Timeout): # type: ignore """Execute a code block expecting (and catching) a timeout.""" def __init__(self, timeout: Union[int, float], print_traceback: bool = True, mute: bool = False): """ Constructor. Interrupt execution after `seconds` seconds. If `print_traceback` is set (default), print a traceback to stderr. If `mute` is set (default: False), do not print anything. """ super().__init__(timeout) self.print_traceback = print_traceback self.mute = mute def __exit__(self, exc_type: type, exc_value: BaseException, tb: TracebackType) -> Optional[bool]: """End of `with` block""" super().__exit__(exc_type, exc_value, tb) if exc_type is None: return # An exception occurred if self.print_traceback: lines = ''.join( traceback.format_exception( exc_type, exc_value, tb)).strip() else: lines = traceback.format_exception_only( exc_type, exc_value)[-1].strip() if not self.mute: print(lines, "(expected)", file=sys.stderr) return True # Ignore exception def long_running_test() -> None: print("Start") for i in range(10): time.sleep(1) print(i, "seconds have passed") print("End") if __name__ == '__main__': with ExpectTimeout(5, print_traceback=False): long_running_test() if __name__ == '__main__': with ExpectTimeout(5, print_traceback=False): with ExpectTimeout(3, print_traceback=False): long_running_test() long_running_test() ## Synopsis ## -------- if __name__ == '__main__': print('\n## Synopsis') if __name__ == '__main__': with ExpectError(): x = 1 / 0 if __name__ == '__main__': with ExpectTimeout(5): long_running_test() ## Lessons Learned ## --------------- if __name__ == '__main__': print('\n## Lessons Learned')