int to str in list elements..

[Abandoned]

Hi..
I have a list as a=[1, 2, 3 .... ] (4 million elements)
and
b=",".join(a)
than
TypeError: sequence item 0: expected string, int found
I want to change list to  a=['1','2','3'] but i don't want to use FOR
because my list very very big.
I'm sorry my bad english.
King regards