为何我这样写会报错？

Python 3.9.5 (tags/v3.9.5:0a7dcbd, May  3 2021, 17:27:52) [MSC v.1928 64 bit (AMD64)] on win32
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>>> a=[]
>>> a[0]=1
Traceback (most recent call last):
  File "<pyshell#1>", line 1, in <module>
    a[0]=1
IndexError: list assignment index out of range
>>>