Fix exception causes all over the codebase

[cool-RR]

I recently went over Matplotlib, Pandas and NumPy, fixing a small mistake in the way that Python 3's exception chaining is used. If you're interested, I can do it here too. I've done it on just one file right now.

The mistake is this: In some parts of the code, an exception is being caught and replaced with a more user-friendly error. In these cases the syntax raise new_error from old_error needs to be used.

Python 3's exception chaining means it shows not only the traceback of the current exception, but that of the original exception (and possibly more.) This is regardless of raise from. The usage of raise from tells Python to put a more accurate message between the tracebacks. Instead of this:

During handling of the above exception, another exception occurred:

You'll get this:

The above exception was the direct cause of the following exception:

The first is inaccurate, because it signifies a bug in the exception-handling code itself, which is a separate situation than wrapping an exception.

Let me know what you think!

[coveralls]

Coverage remained the same at 90.69% when pulling 1072cf8 on cool-RR:patch-1 into 201daa6 on PyCQA:master.

[Pierre-Sassoulas]

I agree with the changes, but are those really the only occurrence of this in the whole pylint's codebase?

[cool-RR]

I'm happy to hear that :)

Below is a list of places where this mistake might be happening. After we're done with this PR, I'll try to find time to do the rest. Note that there might be false positives here:

pylint/checkers/base.py: 400
pylint/checkers/spelling.py: 48
pylint/checkers/typecheck.py: 541
pylint/checkers/utils.py: 537
pylint/checkers/utils.py: 569
pylint/checkers/utils.py: 595
pylint/config/option.py: 90
pylint/lint/pylinter.py: 513
pylint/pyreverse/vcgutils.py: 202
tests/functional/i/invalid_exceptions_raised.py: 91
tests/functional/i/invalid_exceptions_raised.py: 105
tests/functional/m/misplaced_bare_raise.py: 83
tests/functional/n/no_else_raise.py: 110
tests/functional/r/regression_3416_unused_argument_raise.py: 12
tests/functional/s/stop_iteration_inside_generator.py: 72
tests/functional/t/try_except_raise.py: 11
tests/functional/t/try_except_raise.py: 43
tests/functional/t/try_except_raise.py: 52

Also: Would you be open to adding a PyLint rule that catches this mistake in people's code?

[Pierre-Sassoulas]

Let's do all the similar fixes directly in this PR. No need to change the functional tests, everything in tests/functional/ is not supposed to be clean code that we maintain. Quite the opposite it's supposed to trigger problems :)

Also, yes, I think that could become a pylint's convention message.

[cool-RR]

Changed this PR to include all instances of this problem in the codebase.

[Pierre-Sassoulas]

Thank you for taking the time to change this :)

[cool-RR]

My pleasure :)

[cool-RR]

I just made a blog post about this change here: https://blog.ram.rachum.com/post/621791438475296768/improving-python-exception-chaining-with