Long Int literal - Invalid Syntax?

Question

The Python tutorial book I'm using is slightly outdated, but I've decided to continue using it with the latest version of Python to practice debugging. Sometimes there are a few things in the book's code that I learn have changed in the updated Python, and I'm not sure if this is one of them.

While fixing a program so that it can print longer factorial values, it uses a long int to solve the problem. The original code is as follows:

#factorial.py
#   Program to compute the factorial of a number
#   Illustrates for loop with an accumulator

def main():
    n = input("Please enter a whole number: ")
    fact = 1
    for factor in range(int(n), 0, -1):
        fact = fact * factor

    print("The factorial of ", n, " is ", fact)

main()

The long int version is as follows:

#factorial.py
#   Program to compute the factorial of a number
#   Illustrates for loop with an accumulator

def main():
    n = input("Please enter a whole number: ")
    fact = 1L
    for factor in range(int(n), 0, -1):
        fact = fact * factor

    print("The factorial of ", n, " is ", fact)

main()

But running the long int version of the program in the Python shell generates the following error:

>>> import factorial2
Traceback (most recent call last):
  File "<pyshell#3>", line 1, in <module>
    import factorial2
  File "C:\Python34\factorial2.py", line 7
    fact = 1L
            ^
SyntaxError: invalid syntax

Solution 1

Just drop the L; all integers in Python 3 are long. What was long in Python 2 is now the standard int type in Python 3.

The original code doesn't have to use a long integer either; Python 2 switches to the long type transparently as needed anyway.

Note that all Python 2 support is shortly ending (no more updates after 2020/01/01), so at this point in time you'd be much better of switching tutorials and invest your time in learning Python 3. For beginner programmers I recommend Think Python, 2nd edition as it is fully updated for Python 3 and freely available online. Or pick any of the other Stack Overflow Python chatroom recommended books and tutorials

If you must stick to your current tutorial, you could install a Python 2.7 interpreter instead, and work your way through the book without having to learn how to port Python 2 to Python 3 code first. However, you'd then also have to learn how transition from Python 2 to Python 3 in addition.

Solution 2

You just need remove L

fact = 1

Python 3.X integers support unlimited size in contrast to Python 2.X that has a separate type for long integers.