library.py

class Library:
  d1 = {}
  def __init__(self):
    pass
  def add_book(self,book):
    self.d1[book] = True
    
  def check_out_book(self,book):
    if book in self.d1:
      del(self.d1[book])

  def return_book(self,book):
    self.d1[book] = True
  
  def has_book(self,book):
    if book in self.d1:
      return True
    return False

l = Library()
l.add_book("physics")
l.add_book("math")
print l.has_book("physics") #Expected true
print l.has_book("math") #Expected true
l.check_out_book("math")
print l.has_book("math") #Expected false