print_order.py

# Suppose we have a class:
#
# public class Foo {
#   public void first() { print("first"); }
#   public void second() { print("second"); }
#   public void third() { print("third"); }
# }
# The same instance of Foo will be passed to three different threads. Thread A will call first(),
# thread B will call second(), and thread C will call third(). Design a mechanism and modify the
# program to ensure that second() is executed after first(), and third() is executed after second().

Example 1:

Input: [1,2,3]
Output: "firstsecondthird"
Explanation: There are three threads being fired asynchronously. The input [1,2,3] means thread A calls first(), thread B calls second(), and thread C calls third(). "firstsecondthird" is the correct output.
Example 2:

Input: [1,3,2]
Output: "firstsecondthird"
Explanation: The input [1,3,2] means thread A calls first(), thread B calls third(), and thread C calls second(). "firstsecondthird" is the correct output.

# ------------------------------------------------------------------------------------------------------

import threading

class Foo:
    def __init__(self):
        self.locks = (threading.Lock(), threading.Lock())
        self.locks[0].acquire()
        self.locks[1].acquire()

    def first(self, printFirst):
        printFirst()
        self.locks[0].release()

    def second(self, printSecond):
        with self.locks[0]:
            printSecond()
            self.locks[1].release()


    def third(self, printThird):
        with self.locks[1]:
            printThird()