why is a generator function sometimes called not a generator? - python-3.x

Question

For the same object definition in the same program I get TypeError: 'G' object is not iterable in one place and not in another. I am wondering why and if there is a workaround?
class G: # generator class
def __init__(self):
self.c = 1
def __next__(self):
yield self.c
g = G()
for i in range(3):
print(next(g))
g = G()
i = 0
for x in g:
print(x)
i += 1
if i >= 3:
break
In the first for loop it works but in the second for loop I get the runtime error message TypeError: 'G' object is not iterable
The exact output is here:
<generator object G.__next__ at 0x000001ECA83E43C0>
<generator object G.__next__ at 0x000001ECA83E43C0>
<generator object G.__next__ at 0x000001ECA83E43C0>
Traceback (most recent call last):
File "C:\Users\15104\AppData\Local\Programs\Python\Python310\lib\code.py", line 90, in runcode
exec(code, self.locals)
File "<input>", line 1, in <module>
File "C:\Program Files\JetBrains\PyCharm 2022.2\plugins\python\helpers\pydev\_pydev_bundle\pydev_umd.py", line 198, in runfile
pydev_imports.execfile(filename, global_vars, local_vars) # execute the script
File "C:\Program Files\JetBrains\PyCharm 2022.2\plugins\python\helpers\pydev\_pydev_imps\_pydev_execfile.py", line 18, in execfile
exec(compile(contents+"\n", file, 'exec'), glob, loc)
File "C:/Users/15104/Dropbox/Programming/Python/try_tesserOCR/try1/try_with1.py", line 16, in <module>
for x in g:
TypeError: 'G' object is not iterable
Although in this simple example one might say "why would anybody do that", this is because it is taken out of a longer program. Taking out the second g = G() doesn't change the behavior.
The only difference I see in the code is that the first loop calls the next() function explicitly and the second one calls it implicitly through the for x in g: statement. However it seems like "for" is perfectly valid way to use an iterable.