这个问题出在哪里了？（Python）

#输入某年某月某日，查出这一天是这一年的第几天
#

import datetime
dtstr = input('Enter the datetime:(202200402):')
dt = datetime.datetime.strptime(dtstr, "%Y%m%d")
another_dtstr = [:4] + '0101'
another_dt = datetime.datetime.strptime(another_dtstr, "%Y%m%d")
print(int((dt - another_dt).days) + 1)

C:\Users\Al\PycharmProjects\venv\Scripts\python.exe C:/Users/Al/PycharmProjects/上机测试.py
File "C:\Users\Al\PycharmProjects\上机测试.py", line 4
another_dtstr = [:4] + '0101'
^
SyntaxError: invalid syntax

Process finished with exit code 1

你这个[:4]没有指定是什么吧

改之后

another_dtstr = dtstr [:4] +'0101'

import datetime
dtstr = input('Enter the datetime:(202200402):')
dt = datetime.datetime.strptime(dtstr,"%Y%m%d")
another_dtstr = dtstr [:4] +'0101'
another_dt = datetime.datetime.strptime(another_dtstr,'%Y%m%d')
print(int((dt-another_dt).days)+ 1)