contextualized 1.3

Get tracebacks with context.

If you have ever wrote a Python program that processes large input files you probably have been in a situation where you get an error on certain file and line. You may get a traceback telling you have e.g. a KeyError but still don’t know what line in that text file provoked it.

Then you may start adding print(line) calls every time a line is processed, but that’s quirky, clutters your terminal quickly and you’ll have to eventually remove it, only to later add it when the next bug hits.

This module is an alternative to improve this kind of debugging issue, making it clean and robust, whilst providing a easy to use API.

This module works by providing you with a context object where you can set state variables like file name and line number. When an unhandled exception is raised, this module catches it and prints the value of the state variables along with the traceback.

Example code:

from contextualized import contextualized_tracebacks

def main():
    dcontext.file = 'hello.txt'
    a()

def a():
    dcontext.line = 20
    b()

def b():
    # buggy code
    nonexistent += 1

with contextualized_tracebacks(['file', 'line', 'character']) as dcontext:
    main()

This program will show this once run:

Traceback (most recent call last):
  File "contextualized_example.py", line 16, in <module>
    main()
  File "contextualized_example.py", line 5, in main
    a()
  File "contextualized_example.py", line 9, in a
    b()
  File "contextualized_example.py", line 13, in b
    nonexistent += 1
UnboundLocalError: local variable 'nonexistent' referenced before assignment
The exception was caught on file 'hello.txt', line 20.

The context information is right after the traceback. Notice how only non None fields are shown. They also preserve the order from the contextualized_tracebacks() call.

Overriding the traceback print function

You can override the default traceback print with the print_tb argument. For example, this example will print the context before the traceback.

from contextualized import contextualized_tracebacks

def main():
    dcontext.file = 'hello.txt'
    a()

def a():
    dcontext.line = 20
    b()

def b():
    # buggy code
    nonexistent += 1

def my_print_tb(context, tb_info):
    import traceback
    import sys

    print('An exception was caught on %s.' % context._to_prose(),
          file=sys.stderr)
    traceback.print_exception(*tb_info)

with contextualized_tracebacks(['file', 'line', 'character'],
                               print_tb=my_print_tb) as dcontext:
    main()

This will be the new output:

An exception was caught on file 'hello.txt', line 20.
Traceback (most recent call last):
  File "contextualized_example.py", line 25, in <module>
    main()
  File "contextualized_example.py", line 5, in main
    a()
  File "contextualized_example.py", line 9, in a
    b()
  File "contextualized_example.py", line 13, in b
    nonexistent += 1
UnboundLocalError: local variable 'nonexistent' referenced before assignment

Links

• Project home page (GitHub)

• Contextualized in PyPI

Changelog

• 1.0: Initial release.

• 1.1: SystemExit exceptions no longer are catched, thus triggering normal exit() by the interpreter.

• 1.2: Modified setup.py to install successfully with LANG=C.

• 1.3: Fixed a bug that caused contextualized not to work in Python 2.

License

This module is under the MIT License.

Copyright (c) 2015 Juan Luis Boya García

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.