python之如何在python中使用切片运算符提取中间部分以外的字母

我试图理解以下示例的所有可能性。

  +---+---+---+---+---+---+---+---+---+ 
  | C | O | M | P | U | T | E | R | S | 
  +---+---+---+---+---+---+---+---+---+ 
  0   1   2   3   4   5   6   7   8   9    
 -9  -8  -7  -6  -5  -4  -3  -2  -1  
 
  COMPUTERS[ 4 : 7 ]     =  UTE 
  COMPUTERS[ 2 : 5 : 2 ] =  MU 
  COMPUTERS[-5 : 1 :-1 ] =  UPM 
  COMPUTERS[ 4 ]         =  U 
  COMPUTERS[-4 :-6 :-1 ] =  TU 
  COMPUTERS[ 2 :-3 : 1 ] =  MPUT 
  COMPUTERS[ 2 :-3 :-1 ] =   
  COMPUTERS[   :   :-1 ] =  SRETUPMOC 
  COMPUTERS[-5 :   ]     =  UTERS 
  COMPUTERS[-5 : 0 :-1 ] =  UPMO 
  COMPUTERS[-5 :   :-1 ] =  UPMOC 
  COMPUTERS[-1 : 1 :-2 ] =  SEUM 

但我没有得到除中间三个以外的字母如何取

EXPECTED OUTPUT 
Ex: 
COMPUTERS =  COMERS (only first three and last three not the middle three letters (PUT)) 

谁能告诉我如何从开头和结尾(:3 和 6:) 取字母

请您参考如下方法:

那不是一片，那是两片:

word[:3] + word[-3:]