Error python

#Error python
from urllib.request import urlretrieve


def descargar(tu_web):

    obtener = urlretrieve("http://"+tu_web)
    web = input(":")
descargar(web)
#error :
Traceback (most recent call last):
  File "C:\busco.py", line 8, in <module>
    descargar(web)
NameError: name 'web' is not defined