Exception instance scope in Python 2 and 3
Since in Python variables are available outside of their loops and blocks try
- except
I naively thought that this piece of code below would work fine because it e
would be available:
try:
int('s')
except ValueError as e:
pass
print(e)
In Python 2 (2.7 tested) it works as I expected and the output is:
invalid literal for int() with base 10: 's'
However, in Python 3, I was surprised that the output is:
NameError: name 'e' is not defined
Why is this?
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Later I found the answer as PEP 3110 explains that in Python 3 the caught name is removed at the end of the set except
to allow for more efficient garbage collection. It is also recommended to use the syntax if you want to avoid this:
Situations in which it is necessary to keep an exception instance around past the end of the except packet can be easily translated like this
try: ... except E as N: ... ...
becomes
try: ... except E as N: n = N ... …
Thus, when N is removed at the end of a block, n will be preserved and can be used as usual.
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