interaction of nonlocal and except leading to incorrect behavior #68509
Labels
interpreter-core
(Objects, Python, Grammar, and Parser dirs)
type-bug
An unexpected behavior, bug, or error
Comments
|
To reproduce in Python 3.4.2: def f():
x = None
def g():
nonlocal x
try:
raise Exception()
except Exception as x:
pass
g()
# ↓ UnboundLocalError: local variable 'x' referenced before assignment
print("x", x)
f()Compare this to: def f():
x = None
def g():
nonlocal x
with open("/dev/null") as x:
pass
g()
print("x", x)
f()(which prints: x <_io.TextIOWrapper name='/dev/null' mode='r' encoding='UTF-8'>) |
whitequark
mannequin
added
interpreter-core
|
The first example seems to behave as I would expect. The UnboundLocalError is raised by the print() call, because the “x” variable has been deleted by the exception handler. Equivalent code without using “nonlocal”: >>> def f():
... x = None
... try:
... raise Exception()
... except Exception as x:
... pass
... print("x", x) # UnboundLocal due to exception handler
...
>>> f()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 7, in f
UnboundLocalError: local variable 'x' referenced before assignmentIn both cases, I think this is correct behaviour. See <https://docs.python.org/3.4/reference/compound_stmts.html#except\>, which says “When an exception has been assigned using ‘as target’, it is cleared at the end of the except clause.” |
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Indeed, if you replace the except clause with a 'del x', you get the same UnboundLocalError. This is working as designed. |
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