Python in ljust, rjust, center and zfill usage resolve

Alignment of the string in the output:
S.ljust (width, [FillChar])
　　　　　　　　　　　　　　　　　　　　　　# output width characters, S left-aligned, and the gap filled with FillChar, default spaces.
S.rjust (width, [fillchar]) # right justified
S.center (width, [fillchar]) # aligned intermediate
S.zfill (width) # output width characters, and aligned on the right, make up the shortage with 0

>>>s = 'this is a string.'
>>>s.ljust(50,"*")
'this is a string.*********************************'
>>>s.rjust(50, "*")
'*********************************this is a string.'
>>>s.center(50, "*")
'****************this is a string.*****************'
>>>s.zfill(50)
'000000000000000000000000000000000this is a string.'
>>>17as (a)

Original string constant length.