Scope of caught exception instance in Python 2 and

问题:

Since in Python variables are accessible outside of their loops and try-except blocks, I naively thought that this code snippet below would work fine because e would be accessible:

try:
    int('s')
except ValueError as e:
    pass
print(e)

In Python 2 (2.7 tested), it does work 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?

回答1:

I later found an answer as PEP 3110 explains that in Python 3 the caught name is removed at the end of the except suite to enable more efficient garbage collection. There is also recommended syntax if you wish to avoid this occurring:

Situations where it is necessary to keep an exception instance around past the end of the except suite can be easily translated like so

try:
    ...
except E as N:
    ...
...

becomes

try:
    ...
except E as N:
    n = N
    ...
…

This way, when N is deleted at the end of the block, n will persist and can be used as normal.