Python Magic __getitem__

The last article discussed __call__ Magic methods , Let's talk about __getitem__ The wonder of .

Python Objects in are divided into iteratable objects and non iteratable objects , So what is an iteratable object ？

It can be said simply , have access to for i in obj The traversal objects are all iteratable objects , Out-of-service for i in obj Traversal is the non iterative object .

therefore range(n)、list、tuple、str、dict And so on are iteratable objects , and int、float With general Function object 、 Class object And so on are non iteratable objects .

Iteratable objects are iteratable , It is because of the internal implementation __iter__ or __getitem__ Magic methods

1. If not defined __getitem__

   class MyList:
   def __init__(self,len:int):
   self.list=[i for i in range(len)]
   self.length=len
   def __repr__(self)->str:
   return f"MyList({
   self.length}):{
   self.list}"
   x=MyList(10)
   x[7]
   

   The result is wrong , Show MyList Cannot be accessed by subscript ：

   Traceback (most recent call last):
   File "d:\Github-Projects\Crack_Detection_Daily\He\env.py", line 14, in <module>
   x[7]
   TypeError: 'MyList' object is not subscriptable
   

2. Definition __getitem__ after

   from numpy import iterable
   class MyList:
   def __init__(self, len: int):
   self.list = [i for i in range(len)]
   self.length = len
   def __getitem__(self, index: int) -> int:
   return self.list[index]
   def __repr__(self) -> str:
   return f"MyList({
   self.length}):{
   self.list}"
   x = MyList(10)
   print(x[7])
   print(x.__getitem__(7))
   print(iterable(x))
   print(x[7] is x.__getitem__(7))
   for i in x:
   print(i,end=' ')
   

   notes ：numpy.iterable(x) Can be judged x Whether it is an iterative object

   Output is ：

   7
   7
   True
   True
   0 1 2 3 4 5 6 7 8 9
   

   thus it can be seen , adopt x[7] Equivalent to x.__getitem__(7), That defines the __getitem__ Method can make this element an iteratable object , Can be accessed by subscript .

   More Than This , Generally speaking, as long as it is an iterative object , Both x.__getitem__(y) <==> x[y], Dictionaries are no exception ：

   dic = {
   
   'hjd': 'yyds',
   'zyy': 'goddess'
   }
   print(dic.__getitem__('hjd'))
   print(dic['hjd'])
   print(dic['hjd'] is dic.__getitem__('hjd'))
   

   result ：

   yyds
   yyds
   True