Write a python function to check whether a string has atleast one letter and one number.
def check_String(str): 
    flag_l = False
    flag_n = False
    for i in str: 
        if i.isalpha(): 
            flag_l = True  
        if i.isdigit(): 
            flag_n = True
    return flag_l and flag_n 
<s>[INST]<<SYS>>Given a task and its code, generate test cases for the code.<</SYS>>Write a python function to check whether a string has atleast one letter and one number.
def check_String(str): 
    flag_l = False
    flag_n = False
    for i in str: 
        if i.isalpha(): 
            flag_l = True  
        if i.isdigit(): 
            flag_n = True
    return flag_l and flag_n [/INST]assert check_String('thishasboth29') == True
assert check_String('python') == False
assert check_String ('string') == False</s>