Python 链表打印问题

问：

我正在做 leetcode 206 并想打印一些中间结果。我正在使用以下代码。如果我不检查节点是否为无，它会打印但给我错误。

AttributeError: 'NoneType' object has no attribute 'val'
    print(curr.val)
Line 16 in reverseList (Solution.py)
    ret = Solution().reverseList(param_1)
Line 51 in _driver (Solution.py)
    _driver()
Line 61 in <module> (Solution.py)

但是，如果我检查一下，它会打印出没有错误。有人知道为什么会这样吗？

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution(object):
    def reverseList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        curr = head     
        prev = None

        
        print(curr.val) # this one prints but with error
        if curr!=None:
            print(curr.val) # this one prints without error

        while curr:
            
            temp = curr.next          
            curr.next = prev          
            prev = curr           
            curr = temp
            
            
        return prev

我希望它既有错误，又没有错误。但是什么导致了这种差异呢？