users = ["admin", "root1", "John_Doe", "Alice", "Bob42"]

badChar=False

for user in users:
    for ch in user:
        if ('a' <= ch <='z') or ('A' <= ch <='Z') or ch == ' ':
            badChar=False
        else:
            badChar=True
            break
    if badChar == False and len(user) <= 20:
        print(user + "  is Valid")
    else:
        print(user + " Error contain special character, space or digit !")