del in Python functions deleting variable before the line?

Asked Oct 22 '17 at 21:54

Active Oct 22 '17 at 21:59

Viewed 32 times

Consider the following code:

foo=None
def bar():
    print(foo, flush=True) # UnboundLocalError
    print('MARK')
    del foo

print(foo)
bar()
#print(foo)

Basically I have a function that calls del in it. However, when I run the program, I get UnboundLocalError for that 'foo' referenced before assignment. If I comment out the del line, the exception is gone. I suppose the del should be called exactly on the line it is on, so that the variable is invalidated only after that. But it doesn't seem to be the case.

Is this expected behavior? (Specifically, I'm using Python 3.6.2)

edited Oct 22 '17 at 21:59

asked Oct 22 '17 at 21:54

YiFei

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What are you even trying to do here? – melpomene Oct 22 '17 at 21:56
@melpomene Trying to delete a global variable from inside a function. – YiFei Oct 22 '17 at 21:57
Doesn't that need a `global foo` declaration in the function? – melpomene Oct 22 '17 at 21:58
@melpomene, Yes you're right. But why does that triggers a error on the print line? – YiFei Oct 22 '17 at 21:58
You either need a global foo or you need to define foo in your function – OmO Walker Oct 22 '17 at 21:58
1

Presumably Python counts `del foo` as a modification of `foo`, so it implicitly creates `foo` as a local variable. Which you then try to print before it was set. – melpomene Oct 22 '17 at 21:59
@melpomene Oh, that makes sense. Thanks. – YiFei Oct 22 '17 at 22:00

del in Python functions deleting variable before the line?

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