strange difference between list() and tuple() using iter()

[Skaperen]

when using iter() then to get the items from the iterator using list() or tuple() there is a strange difference. here is the demo:

Output:
lt2a/phil /home/phil 86> py
Python 3.6.9 (default, Oct  8 2020, 12:12:24) 
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> a=['foo','bar','boo','far','baz']
>>> a
['foo', 'bar', 'boo', 'far', 'baz']
>>> list(x for x in a)
['foo', 'bar', 'boo', 'far', 'baz']
>>> tuple(x for x in a)
('foo', 'bar', 'boo', 'far', 'baz')
>>> list(iter(x for x in a))
['foo', 'bar', 'boo', 'far', 'baz']
>>> tuple(iter(x for x in a))
('foo', 'bar', 'boo', 'far', 'baz')
>>> b=iter(x for x in a)
>>> list(b)
['foo', 'bar', 'boo', 'far', 'baz']
>>> tuple(b)
()
>>> 

so, tuple() varies from list().

even more strange, the variance of tuple() depends on if the iterator was stored in a variable (b in this case). is there an explanation of why it differs like this or is this a bug? can someone try this in 3.7 or 3.8?

[buran]

>>> a=['foo','bar','boo','far','baz']
>>> b=iter(x for x in a)
>>> type(b)
<class 'generator'>
>>> type(x for x in a)
<class 'generator'>
>>> tuple(b)
('foo', 'bar', 'boo', 'far', 'baz')
>>> list(b)
[]

x for x in a is a generator expression as well as b is generator. When you use it for first time you consume and exhaust it. That is why on second use you get empty container. You can see that if you first pass it to tuple() you get tuple, and on second use - empty list. Of course if it is not assign to a variable, you work with new generator every time, so that is "the variance" in behavior.

by the way, the use if iter() is not needed in this case (i.e. on generator expression). You can use next() on generator.

>>> next(x for x in a)
'foo'
>>> spam=(x for x in a)
>>> next(spam)
'foo'
>>> next(spam)
'bar'
>>> next(spam)
'boo'
>>> next(spam)
'far'
>>> next(spam)
'baz'
>>> next(spam)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration
>>> list(spam)
[]

at the end the generator is exhausted and you get StopIteration error.