This behavior (that the OSError has the ValueError as context) should be tested and documented: 


>>> try:
...   raise TypeError(1)
... except:
...   raise ValueError(2)
... finally:
...   raise OSError(3)
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
TypeError: 1

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
ValueError: 2

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 6, in <module>
OSError: 3