[EMAIL PROTECTED] wrote:
> On Apr 2, 3:57 pm, Mel <[EMAIL PROTECTED]> wrote:
>> [EMAIL PROTECTED] wrote:
>>
>> I'd just like to test my
>>
>>> understanding of this. Suppose I create the following generator
>>> object:
>>> g = getNextScalar(1, 2, (3, 4), 5)
>>> when the iterator reaches the tuple argument (3, 4) then, according to
>>> Steve and George, the * in *arg causes this tuple to be expanded into
>>> positional arguments, and it makes sense to do it this way. But what
>>> happens when getNextScalar(arg) is used instead?
>> Try it:
>>
>> Python 2.5.1 (r251:54863, Mar 7 2008, 04:10:12)
>> [GCC 4.1.3 20070929 (prerelease) (Ubuntu 4.1.2-16ubuntu2)] on linux2
>> Type "help", "copyright", "credits" or "license" for more information.
>> >>> def a (arg):
>> ... print arg
>> ...
>> >>> def astar (*arg):
>> ... print arg
>> ...
>> >>> a(3,4)
>> Traceback (most recent call last):
>> File "<stdin>", line 1, in <module>
>> TypeError: a() takes exactly 1 argument (2 given)
>> >>> astar(3,4)
>> (3, 4)
>> >>> a((3,4))
>> (3, 4)
>> >>> astar((3,4))
>> ((3, 4),)
>> >>>
>>
>> Mel.
>
> Well, I understand that (unless I missed the point you're trying to
> make). But with respect to the example I quoted:
>
> def getNextScalar(*args):
> for arg in args:
> if(isinstance(arg, tuple)):
> for f in getNextScalar(arg): # should use *arg
> yield f
> else:
> yield arg
> where the function is declared as def getNextScalar(*arg), but is
> called using getNextScalar(arg), with arg being a tuple: here the
> generator is being passed a single argument, so there's no TypeError
> as in your example. However, it fails - as I understand it - because
> the function keeps passing the same tuple (being unable to access the
> elements inside it) and goes into an infinite loop:
>>>> # works for this example, but not very useful:
>>>> g = getNextScalar(1, 2, 3, 4)
>>>> for i in g:
> print i
>
>
> 1
> 2
> 3
> 4
>
> # third argument is a tuple:
>>>> g = getNextScalar(1, 2, (3, 4))
>>>> for i in g:
> print i
>
>
> 1
> 2
>
> Traceback (most recent call last):
> File "<pyshell#108>", line 1, in <module>
> for i in g:
> File "<pyshell#94>", line 4, in getNextScalar
> for f in getNextScalar(arg):
> File "<pyshell#94>", line 4, in getNextScalar
> for f in getNextScalar(arg):
> File "<pyshell#94>", line 4, in getNextScalar
> for f in getNextScalar(arg):
> ...
Yeah. I feel as though I haven't put the idea clearly. When a
function is defined as `def astar (a*)`, then the parameters that it's
called with, however many they are, are packed up into one tuple.
Illustrated by those print statements.
On the calling side, `a (*(3,4))` will unpack the (3,4) tuple and
treat the contents as ordinary positional parameters of a. The call
`a(*(3,4))` and the call `a(3,4)` are completely equivalent as far as
the function a can see.
This explanation is going nowhere fast. Anyway, if you define
getNextScalar to pack its positional arguments into a tuple that you
can iterate over, THEN if you want to feed it a tuple of values to
iterate over you have to star-unpack the tuple when you call. The
buggy behaviour is continually calling getNextScalar with a single
positional parameter (3,4), packing that single parameter into a
1-tuple ((3,4),) , iterating over the 1-tuple to extract (3,4) and
calling getNextScalar again -- ad dump.
Mel.
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