Confusion about iterator types

Martin von Loewis loewis at informatik.hu-berlin.de
Sun Aug 12 17:06:46 CEST 2001


Kirby Urner <urner at alumni.princeton.edu> writes:

>   >>> mylist = [1,2,3,4]
>   >>> myiter = iter(mylist)
> 
> I then get:
> 
>   >>> type(myiter)
>   <type 'iterator'>
> 
> But 'iterator' by itself has no corresponding type in types.  

It sure does:

>>> import types
>>> type(myiter) is types.SequenceIterType 
1

> If I use the isinstance() method, I can confirm that myiter is a
> SequenceIterType, but shouldn't it just tell me that?

What way of telling do you expect? Something about the name of the
type? Watch this

>>> types.DictionaryType.__name__
'dictionary'
>>> types.FunctionType.__name__
'function'

So in general, the __name__ of a type is only roughly related to the
name to which it is bound in the types module.

> Shouldn't type(foo) return a type name that more closely matches its
> type in types?  Why not have?
>  
>  >>> type(myiter)
>  <type 'sequence-iter'>

I personally see no advantage in having it renamed, if you do, feel
free to submit a feature request at sf.net/projects/python. If you can
point to an actual problem that the current name causes, you should
make it a bug report.

> Also, some clarification re the below would be helpful:
> 
>   >>> def f(n): return n*n
> 
>   >>> callable(f)
>   1
>   >>> k = iter(f,10)
>   >>> type(k)
>   <type 'callable-iterator'>
>   >>> k()
>   Traceback (most recent call last):
>     File "<pyshell#142>", line 1, in ?
>       k()
>   TypeError: object is not callable: <callable-iterator object at 00B6D10C>

It is an iterator that iterates over a callable object; the iterator
itself is not callable. So you are supposed to do

>>> k.next()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: f() takes exactly 1 argument (0 given)

An iterator object has a .next() method to retrieve a value. As you
can see, the iterator tries to call f with no arguments, so the
callable shouldn't expect any arguments, either. A meaningful
application of this is

>>> data = [1,4,10,20]
>>> def fetch():
...   if len(data):
...     result = data[0]
...     data[0:1] = []
...     return result
...   else:
...     return "Done"
... 
>>> k=iter(fetch,"Done")
>>> k.next()
1
>>> k.next()
4
>>> k.next()
10
>>> k.next()
20
>>> k.next()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
StopIteration

Of course, this isn't a very useful application; Guido thought more of
something like

>>> f=open("/etc/passwd")
>>> k=iter(f.readline,"")
>>> k.next()
'root:x:0:1:Super-User:/:/sbin/sh\n'

> And again, there's no 'callable-iterator' type in types.

Sure is, it is called FunctionIterType. callable-iterator would not be
valid identifier...

Regards,
Martin



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