It is inconvenient for programming in python mode.
Although according to python.el,it says I will have 4 indents at once TAB,but actuarially,my tab key makes 10 spaces.
Finally,I use default emacs key binding,C-u space.
In default,C-u is repeat four times following key.For example,C-u a makes string 'aaaa'.
So C-u space result in string '____'.This is right indent for python programming.