A Python 3.4 module to illustrate a difference between exception handling in generators and context managers.

test_yield.py

"""
This module illustrates a difference in exception handling between
generator functions and context managers.

This script should be run with **Python 3.4.2** or higher (which is what
it was originally run with).

To witness what is illustrated in this module, run the following two
commands from the command-line.

1) Generator example

In this example, notice that the generator does *not* handle the
ValueError, despite its try-except.

    $ python -c "import test_yield; test_yield.test_gen()"
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/Users/chris_g4/Dev/elections/open-rcv/test_yield.py", line 13, in test_gen
        raise ValueError("val")
    ValueError: val

2) Context manager example

In this example, notice that the context manager *does* handle
the ValueError.

    $ python -c "import test_yield; test_yield.test_manager()"
    Traceback (most recent call last):
      File "/Users/chris_g4/Dev/elections/open-rcv/test_yield.py", line 19, in manager
        yield 1
      File "/Users/chris_g4/Dev/elections/open-rcv/test_yield.py", line 24, in test_manager
        raise ValueError("val")
    ValueError: val

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/Users/chris_g4/Dev/elections/open-rcv/test_yield.py", line 24, in test_manager
        raise ValueError("val")
      File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/contextlib.py", line 77, in __exit__
        self.gen.throw(type, value, traceback)
      File "/Users/chris_g4/Dev/elections/open-rcv/test_yield.py", line 21, in manager
        raise TypeError("foo")
    TypeError: foo

"""

from contextlib import contextmanager

def test_gen():
    def gen():
        try:
            yield 1
        except ValueError:
            raise TypeError("foo")

    for value in gen():
        # The generator does not handle this error despite the try-except.
        raise ValueError("val")

def test_manager():
    @contextmanager
    def manager():
        try:
            yield 1
        except ValueError:
            raise TypeError("foo")

    with manager():
        # For the context manager, the error is handled.
        raise ValueError("val")