In Python 3, the range() function generates a sequence of numbers on the fly, rather than creating a list of all the numbers in the sequence upfront. This means that when you use the in operator to check if a number is in the range, Python only has to generate the numbers in the range up to the number you're checking, rather than generating the entire range.

For example, when you run the following code:

1000000000000000 in range(1000000000000001)

It's worth noting that this behavior is specific to Python 3. In Python 2, the range() function creates a list of all the numbers in the range upfront, so checking if a number is in the range is slower, especially for large ranges.