是否可以禁用跳轉到列表的末尾？

Question

在我正在編寫的程序中，通過使用負數來禁用跳轉到列表末尾的功能將非常方便。我的意思是，我想這樣的：

list = [1, 2, 3, 4] 
number = list[-1] 
print(number) 

不輸出4，而是例如給我像它會一個IndexError如果我試圖訪問列表中的第5項。

編程新手，所以不確定是否可以這樣做，但它會爲我節省很多代碼。當然，我可以解決這個問題，但如果這個選項存在，它會很整潔！

Answer 1

if index<0: 
    value = l[0] 
else: 
    value = l[index] 

而且名單是一個Python關鍵字，所以就像你在你的例子有不使用它。

Answer 2

我不能完全確定什麼是你想通過這樣做來完成，但你可以嘗試覆蓋__getitem__和__setitem__像這樣：

class NewList(list): 
    def __getitem__(self, key): 
     if isinstance(key, int) and key < 0: 
      raise IndexError("Cannot index to values lower than 0.") 
     return super().__getitem__(key) 

    def __setitem__(self, key, value): 
     if isinstance(key, int) and key < 0: 
      raise IndexError("Cannot index to values lower than 0.") 
     return super().__setitem__(key, value) 

list = NewList 

請記住這樣做是不是一個很好的主意，可能會導致內部問題，所以我不會建議它。

Answer 3

負索引lst[-1]與lst[len(lst)-1]基本相同，這是正索引。所以你想要做的事情沒有意義，因爲這些地方的物品仍然可以訪問。

但你可以做到這一點無論如何是這樣的：

class CustomList(list): 
    def __getitem__(self, key): 
     if isinstance(key, slice): 
      if key.start < 0 or key.stop < 0: 
       raise TypeError("Negative indexes are not allowed") 
      else: 
       super(self.__class__, self).__getitem__(key) 
     elif isinstance(key, int): 
      if key < 0: 
       raise TypeError("Negative indexes are not allowed") 
      else: 
       super(self.__class__, self).__getitem__(key) 
     else: 
      raise TypeError("Index must be a slice object or an integer") 

    def __delitem__(self, key): 
     if isinstance(key, slice): 
      if key.start < 0 or key.stop < 0: 
       raise TypeError("Negative indexes are not allowed") 
      else: 
       super(self.__class__, self).__delitem__(key) 
     elif isinstance(key, int): 
      if key < 0: 
       raise TypeError("Negative indexes are not allowed") 
      else: 
       super(self.__class__, self).__delitem__(key) 
     else: 
      raise TypeError("Index must be a slice object or an integer") 

    def __setitem__(self, key, value): 
     if isinstance(key, slice): 
      if key.start < 0 or key.stop < 0: 
       raise TypeError("Negative indexes are not allowed") 
      else: 
       super(self.__class__, self).__setitem__(key, value) 
     elif isinstance(key, int): 
      if key < 0: 
       raise TypeError("Negative indexes are not allowed") 
      else: 
       super(self.__class__, self).__setitem__(key, value) 
     else: 
      raise TypeError("Index must be a slice object or an integer") 

alist = [1, 2, 3, 4] 
print(alist[-1]) 
print(alist[-3:-1]) 
blist = CustomList([1, 2, 3, 4]) 
print(blist[1]) 
print(blist[1:3]) 
blist[1] = 10 
print(blist) 
blist[1:3] = [20, 30] 
print(blist) 
try: 
    print(blist[-1]) 
except TypeError as ex: 
    print(ex) 
try: 
    print(blist[-3:-1]) 
except TypeError as ex: 
    print(ex) 
try: 
    blist[-1] = 100 
except TypeError as ex: 
    print(ex) 
    print(blist) 
try: 
    blist[-3:-1] = [200, 300] 
except TypeError as ex: 
    print(ex) 
    print(blist) 
try: 
    del blist[-1] 
except TypeError as ex: 
    print(ex) 
    print(blist)  
del blist[len(blist)-1] 
print(blist) 

這會給下面的輸出：

4 
[2, 3] 
None 
None 
[1, 10, 3, 4] 
[1, 20, 30, 4] 
Negative indexes are not allowed 
Negative indexes are not allowed 
Negative indexes are not allowed 
[1, 20, 30, 4] 
Negative indexes are not allowed 
[1, 20, 30, 4] 
Negative indexes are not allowed 
[1, 20, 30, 4] 
[1, 20, 30]