problem in using generator

[akbarza]

hi
after running the code:

def gen():
    for i in range(3):
        yield i*i
    return

g1=gen()
for i in g1():
    print(i)

the below error will be shown in the output:

Error:
Traceback (most recent call last):
  File "<pyshell#36>", line 1, in <module>
    for i in g1():
TypeError: 'generator' object is not callable

but if i use gen()itself , namely:

for i in gen():
    print(i)

then the output will be:

Output:
0
1
4

or if the code will be changed to:

g1=gen()
for i in g1:
    print(i)

then the last true output is displayed again.
what is the reason for this? plz, explain.
thanks

[Gribouillis]

In the first code, you are invoking gen()() and Python tells you that the object returned by the call gen() is not callable.

[perfringo]

This is not generator specific.

() signals to Python that this is a callable object and should be called. Python tries to do it.

If you call it second time then it depends what first callable returned. To illustrate:

>>> def literal():
...     return 'a'
...
>>> literal()
'a'
>>> literal()()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object is not callable

No good - literal function returned str and it's not callable. TypeError is raised.

However, if function returns another function (which is callable) then it works:

>>> def num():
...     return 42
...
>>> def return_callable():
...     return num
...
>>> return_callable()()
42
>>> a = return_callable()
>>> a()
42