good question

Question

if we use 'in' in while loop it becomes a infinite loop but in for loop it becomes as length of list why??

Answer 1

In is a keyword used to check, whether a certain item exists in a list (or other similar objects) and it evaluates a boolean: true, if the searched item exists in the list, otherwise false.

Examples:
9 in [9, 4, 5] ---> True
2 in [1, 3, 4] ---> False
8 in [7, 8, 8] ---> True

Range objects work much like lists and you are also allowed to check whether an item exists in a range object:

9 in range(10) ---> True
10 in range(10) ---> False
1 in range(0, 10, 2) ---> False

And a similar boolean was evaluated by the while loop.

Using while loop:
while i in range(10):
print(i)

Would still result in error, because variable i isn't defined,

but I'm assuming you tried something like this:

for i in range(10):
print(i)

while i in range(10):
print(i)

Where i was defined by the for loop and i ended up with value 9, which you used in the while loop expression and which would equal:

while 9 in range(10):
print(i)

and because 9 in range(10) is always True, the loop became infinite.