Python求第n个斐波那契数报错：'int' object is not subscriptable

Hey there! I’ve run into this exact error a handful of times when tinkering with Fibonacci implementations—let’s break down what’s going on and fix it.

What the Error Actually Means #

That error pops up when you try to use square bracket notation ([index]) on an integer. Think about it: you can’t do 5[2] because integers don’t support index access like lists or strings do. In your Fibonacci code, you’ve almost certainly accidentally turned a variable that should be subscriptable (like a list) into an integer, then tried to index it.

Common Culprits in Fibonacci Code #

Let’s look at two typical mistakes that trigger this error:

1. Recursive Implementation Mix-Up

You might have written something like this, where you incorrectly try to index the integer result of your recursive call:

def fib(n):
    if n <= 1:
        return n
    fib_total = fib(n-1) + fib(n-2)
    return fib_total[n]  # Oops! fib_total is an int, not a list

The recursive calls return integers, so fib_total becomes a plain number—trying to index it with [n] breaks everything.

2. Iterative Implementation Variable Overwrite

If you’re using an iterative approach, you might have replaced your list variable with an integer mid-loop:

def fib(n):
    fib_seq = [0, 1]
    for i in range(2, n+1):
        fib_seq = fib_seq[i-1] + fib_seq[i-2]  # Now fib_seq is an int, not a list!
    return fib_seq[n]

After the first iteration, fib_seq gets set to a sum (an integer), so any later attempt to index it throws the error.

Corrected Working Code Examples #

Here are two solid implementations to fix this:

Recursive Version (Simple)

def fib(n):
    if n <= 1:
        return n
    # No indexing needed—just return the sum of the previous two values
    return fib(n-1) + fib(n-2)

Iterative Version (More Efficient for Large n)

def fib(n):
    if n <= 1:
        return n
    a, b = 0, 1
    for _ in range(2, n+1):
        a, b = b, a + b  # Update values without overwriting a list
    return b

Quick Debugging Tip #

If you want to confirm the issue firsthand, add a print(type(your_variable)) right before the line that throws the error. For example, if the error hits on return fib_seq[n], print print(type(fib_seq))—it’ll show <class 'int'>, which tells you exactly where you turned your subscriptable object into an integer.

内容的提问来源于stack exchange，提问作者Aron Tesfay