cowsAndBulls.py

#rschmidt@libert.xyz
#http://www.practicepython.org/exercise/2014/07/05/18-cows-and-bulls.html

import random


def start():

	num = input('Enter a 3 digit number: ')
	return num

def game():
	
	randNum = random.randrange(100,900)
	att = 0
	
	while True:
		num = start()
		cowbull = CowBull(num,randNum)
	
		if cowbull['cow'] == 3:
			print ('You Win, the number was: ',randNum, 'attempts: ', att)
			break
		
		else:
			print ( 'cows',cowbull['cow'],'bulls',cowbull['bull'])
			att = att + 1

			
def CowBull(num,randnum):

	cowbull ={'cow':0,'bull':0}
	randStr = str(randnum)

	for i in range(len(num)):

		if randStr[i] == num[i]:
			cowbull['cow'] = cowbull['cow'] + 1
	
		elif num[i] in randStr:
			cowbull['bull'] = cowbull['bull'] + 1


	return cowbull
		
		
if __name__ == '__main__':
	game()

Instead of:
randNum = random.randrange(100,900)
try:
randNum = str(random.randrange(100,900))
This will allow you to remove:
randStr = str(randnum)
Also you should be more explicit in your call:
num = start()
Make it:
num = str(start())
I like polymorphism and encourage it, but if you plan to use implicit then you need to error test as sometimes though you should receive a str from input, you receive an int.

Agreed with @dclay1983. Otherwise, looks great!

instead of :
cowbull['cow'] = cowbull['cow'] + 1
use:
cowbull['cow'] += 1

do the same with bull