The code in this notebook helps with handling errors. Normally, an error in notebook code causes the execution of the code to stop; while an infinite loop in notebook code causes the notebook to run without end. This notebook provides two classes to help address these concerns.
Prerequisites
with statementTo use the code provided in this chapter, write
>>> from debuggingbook.ExpectError import <identifier>
and then make use of the following features.
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_98441/2664980466.py", line 2, in <cell line: 1>
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_98441/1223755941.py", line 2, in <cell line: 1>
long_running_test()
File "/var/folders/n2/xd9445p97rb3xh7m1dfx8_4h0006ts/T/ipykernel_98441/3930412460.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 printedmute (default False) can be set to True to completely avoid any output.The class ExpectError allows to express that some code produces an exception. A typical usage looks as follows:
from ExpectError import ExpectError
with ExpectError():
function_that_is_supposed_to_fail()
If an exception occurs, it is printed on standard error; yet, execution continues.
# ignore
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
Here's an example:
def fail_test() -> None:
# Trigger an exception
x = 1 / 0
with ExpectError():
fail_test()
Traceback (most recent call last):
File "/var/folders/n2/xd9445p97rb3xh7m1dfx8_4h0006ts/T/ipykernel_98441/1235320646.py", line 2, in <cell line: 1>
fail_test()
File "/var/folders/n2/xd9445p97rb3xh7m1dfx8_4h0006ts/T/ipykernel_98441/278441162.py", line 3, in fail_test
x = 1 / 0
ZeroDivisionError: division by zero (expected)
with ExpectError(print_traceback=False):
fail_test()
ZeroDivisionError: division by zero (expected)
We can specify the type of the expected exception. This way, if something else happens, we will get notified.
with ExpectError(ZeroDivisionError):
fail_test()
Traceback (most recent call last):
File "/var/folders/n2/xd9445p97rb3xh7m1dfx8_4h0006ts/T/ipykernel_98441/1259188418.py", line 2, in <cell line: 1>
fail_test()
File "/var/folders/n2/xd9445p97rb3xh7m1dfx8_4h0006ts/T/ipykernel_98441/278441162.py", line 3, in fail_test
x = 1 / 0
ZeroDivisionError: division by zero (expected)
with ExpectError():
with ExpectError(ZeroDivisionError):
some_nonexisting_function() # type: ignore
Traceback (most recent call last):
File "/var/folders/n2/xd9445p97rb3xh7m1dfx8_4h0006ts/T/ipykernel_98441/2242794116.py", line 2, in <cell line: 1>
with ExpectError(ZeroDivisionError):
File "/var/folders/n2/xd9445p97rb3xh7m1dfx8_4h0006ts/T/ipykernel_98441/2242794116.py", line 3, in <cell line: 1>
some_nonexisting_function() # type: ignore
NameError: name 'some_nonexisting_function' is not defined (expected)
The class ExpectTimeout(seconds) allows expressing that some code may run for a long or infinite time; execution is thus interrupted after seconds seconds. A typical usage looks as follows:
from ExpectError import ExpectTimeout
with ExpectTimeout(2) as t:
function_that_is_supposed_to_hang()
If an exception occurs, it is printed on standard error (as with ExpectError); yet, execution continues.
Should there be a need to cancel the timeout within the with block, t.cancel() will do the trick.
The implementation uses sys.settrace(), as this seems to be the most portable way to implement timeouts. It is not very efficient, though. Also, it only works on individual lines of Python code and will not interrupt a long-running system function.
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
Here's an example:
def long_running_test() -> None:
print("Start")
for i in range(10):
time.sleep(1)
print(i, "seconds have passed")
print("End")
with ExpectTimeout(5, print_traceback=False):
long_running_test()
Start 0 seconds have passed 1 seconds have passed 2 seconds have passed 3 seconds have passed
TimeoutError (expected)
Note that it is possible to nest multiple timeouts.
with ExpectTimeout(5, print_traceback=False):
with ExpectTimeout(3, print_traceback=False):
long_running_test()
long_running_test()
Start 0 seconds have passed 1 seconds have passed
TimeoutError (expected)
Start 0 seconds have passed 1 seconds have passed 2 seconds have passed 3 seconds have passed
TimeoutError (expected)
That's it, folks – enjoy!
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_98441/2664980466.py", line 2, in <cell line: 1>
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_98441/1223755941.py", line 2, in <cell line: 1>
long_running_test()
File "/var/folders/n2/xd9445p97rb3xh7m1dfx8_4h0006ts/T/ipykernel_98441/3930412460.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 printedmute (default False) can be set to True to completely avoid any output.ExpectError class, it is very easy to handle errors without interrupting notebook execution.