On 2017-10-17 03:07, Steve D'Aprano wrote:
> On Tue, 17 Oct 2017 08:57 am, Thomas Jollans wrote:
>
>> On 16/10/17 21:12, Stefan Ram wrote:
>>> r...@zedat.fu-berlin.de (Stefan Ram) writes:
>>>> »x = None« observably has not the same effect as »del x«:
>>>
>>> Paradoxically, /deleting/ a local variable which did not
>>> ever exist, has the effect of creating a ghost of that
>>> local variable which then will hide a global variable:
>
> [...]
>>> . Case 2: The »del x« creates a ghost »local variable 'x'«
>>> which now will hide the global variable »x«:
>>>
>>> |>>> x = 9
>>> |>>> def f():
>>> |... del x
>>> |... print( x )
>>> |...
>>> |>>> f()
>>> |UnboundLocalError: local variable 'x' referenced before assignment
>>>
>>> .
>>
>> That's not what happens. The UnboundLocalError is in the del statement,
>> not the print call:
>
> That's exactly what happens! Stefan merely explains it poorly. There's no
> mysterious "ghost", it is just that x is treated as a local variable instead
> of a global, and you can't delete the local before it is bound to.
>
> `del x` is treated as a form of assignment, just like `x = 123`, and makes `x`
> a local variable. So in this code:
You're right of course.
While in Stefan's example code the error *was* in the del statement, and
trying to delete something that's not either present in the local
namespace or explicitly "global" *will* fail, del *does* make a variable
local.
>>> x = 1
>>> def f():
... try:
... del x
... except:
... print('del exception caught')
... print(x)
...
>>> f()
del exception caught
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 6, in f
UnboundLocalError: local variable 'x' referenced before assignment
>>> def g():
... global x
... del x
...
>>> g()
>>> x
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined
>>>
>
> def f():
> print(x)
>
> there is no assignment to x, so it is treated as a global. But if we write:
>
> def f():
> x = 123
> print(x)
>
> the Python compiler sees the assignment to x and makes x a local variable.
> Note that the rules for this are deliberately very, very simple, so this will
> make x a local:
>
> def f():
> print(x) # x is local
> x = 123
>
> and even this will too:
>
> def f():
> if False:
> x = 123 # dead code
> print(x) # but enough to make x a local
>
>
> You can replace the line "x = 123" with "del x" in any of those functions, and
> you will get the same effect: x will be treated as a local.
>
>
>
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