# coding: utf8

user_name = input("Enter your name: ")


def print_name(user_name):
    print("Привет " + user_name)


print_name()

/usr/bin/python3.4 /home/silentstorm/PycharmProjects/my_first_app/test.py
Enter your name: Silent
Traceback (most recent call last):
  File "/home/silentstorm/PycharmProjects/my_first_app/test.py", line 10, in <module>
    print_name()
TypeError: print_name() missing 1 required positional argument: 'user_name'

Process finished with exit code 1

print_name(user_name)