Can't change working directory

I used 

>import os

>os.cwd('..')

also tried

>import os

>os.cwd("C:/Users/manleyl/PycharmProjects")

and the only ouput I ever get when asking

>os.getcwd()

is

C:\Python34\python.exe C:/Users/manleyl/PycharmProjects/size_project/size_1.py

Process finished with exit code 0