Error Handling¶

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

This notebook needs some understanding on advanced concepts in Python, notably
- classes
- the Python with statement
- tracing
- measuring time
- exceptions

Synopsis¶

To 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 printed
mute (default False) can be set to True to completely avoid any output.

Catching Errors¶

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.

import bookutils.setup

import traceback
import sys

from types import FrameType, TracebackType

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()

with ExpectError(print_traceback=False):
    fail_test()

We can specify the type of the expected exception. This way, if something else happens, we will get notified.

with ExpectError(ZeroDivisionError):
    fail_test()

with ExpectError():
    with ExpectError(ZeroDivisionError):
        some_nonexisting_function()

Catching Timeouts¶

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.

import sys
import time

from Timeout import Timeout

class ExpectTimeout(Timeout):
    """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()

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()

That's it, folks – enjoy!

Lessons Learned¶

With the ExpectError class, it is very easy to handle errors without interrupting notebook execution.

Error Handling¶

Synopsis¶

Catching Errors¶

Catching Timeouts¶

Lessons Learned¶

How to Cite this Work