vim python: substitute 'spaces' indent to 'tabs'?

[Rocky Zhou]

I am accustomed to vi my pthon scripts with 'tab' indent. But when I
copy some code to my script, the indent may be 'spaces'. So I wanna a
way to substitute those 'spaces' to be 'tabs' conveniently.

For example, I:
expand -t4 test.py >/tmp/test2.py
vi /tmp/test2.py

Then in vim, the indents should be:
/^\( \{4\}\)*/
but I don't know how to substitute this to that:
/^\(\t\{\4\}\)*/

Is there any way?

Thanks.