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Here's an example (copied from msg142063) of what the traceback is without the patch:
>>> from functools import lru_cache
>>> @lru_cache()
... def func(arg): raise ValueError()
... 
>>> func(3)
Traceback (most recent call last):
 File "/home/wolf/dev/py/3.2/Lib/functools.py", line 176, in wrapper
 result = cache[key]
KeyError: (3,)
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/wolf/dev/py/3.2/Lib/functools.py", line 180, in wrapper
 result = user_function(*args, **kwds)
 File "<stdin>", line 2, in func
ValueError
The patch gets rid of the first traceback (the one before "During handling...").
I should also mention that my second point might not be valid if the cache hits are mostly successful. I haven't done any specific benchmark, and I don't know how fast is 'key in dict' compared to raising an exception. If it's e.g. 10 times faster, it should make lru_cache faster when the hits:miss ratio is lower than 10:1.