source code:

# -*- coding: utf-8 -*-
import random
def generate_verification_code():
    ''' randomly generated a 6 bit verification code '''
    code_list = []
    for i in range(10): # 0-9 number
        code_list.append(str(i))
    for i in range(65, 91): # A-Z
        code_list.append(chr(i))
    for i in range(97, 123): # a-z
        code_list.append(chr(i))
    myslice = random.sample(code_list, 6)  
    verification_code = ''.join(myslice) # list to string
    # print code_list
    # print type(myslice)
    return verification_code
def generate_verification_code2():
    ''' randomly generated a 6 bit verification code '''
    code_list = []
    for i in range(2):
        random_num = random.randint(0, 9) # The number of randomly generated 0-9
       # using random.randint () function to generate a random integer a, such that 65<=a<=90
       # corresponding from "A" to "Z" of the ASCII code
        a = random.randint(65, 90)
        b = random.randint(97, 122)
        random_uppercase_letter = chr(a)
        random_lowercase_letter = chr(b)
        code_list.append(str(random_num))
        code_list.append(random_uppercase_letter)
        code_list.append(random_lowercase_letter)
    verification_code = ''.join(code_list)
    return verification_code
if __name__ == '__main__':
    code = generate_verification_code()
    code2 = generate_verification_code2()
    print code
    print code2

Test execution results:

    fF3UzK 
    1Db2Aa