Python Classes

Course notes for ESC190 Computer Algorithms and Data Structures

Python Classes

Structures in C are similar to classes in Python (Classes in Python are actually much much more powerful, but that’s not the point in this section). We can define a Student class in Python as follows,

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class Student:
  def __init__(self, name, gpa):
      self.name = name
      self.gpa = gpa
  
  def __str__(self):
      return self.name + ": " + str(self.gpa)
if __name__ == "__main__":
  s = Student('Fred', 3.7)
  print(s.name, s.gpa)

Here, __str__ is a built-in function, and we can call it via either:

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s.__str__()
Student.__str__(s)

Here, the first line is just shorthand notation for the second line. What makes this built-in function useful is that we can call print(s) and it will treat this as print(s.__str__()) instead.

We can also write a function to compare two classes, so that we can use operators such as <, >, and ==. For example,

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class Student:
  def __init__(self, name, gpa):
      self.name = name
      self.gpa = gpa
  
  def __str__(self):
      return self.name + ": " + str(self.gpa)
  
  def __lt__(self, other):
      # Return True if self has lower gpa than other
      # If GPAs are tied, compare names

      if self.gpa < other.gpa:
        return True
      elif self.gpa == other.gpa:
        return self.name < other.name
      else:
        return False
if __name__ == "__main__":
  fred = Student('Fred', 3.7)
  bob = Student('Bob', 2.0)
  print(Student.__lt__(fred, bob))
  print(fred < bob)

Recall that this is very similar to the compare function we wrote for qsort. Because we can compare different students, we can also sort them using the default sorted function in Python. For example,

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L = [Student("Fred", 3.7"), Student("Bob", 3.7), Student("Alice", 3.9)]
L.sort()
print(L) # [Bob: 3.7, Fred: 3.7, Alice: 3.9]

will allow us to sort and print out the list of students in a very convenient way (using both the custom print and compare functions we wrote)! Note that this is a more flexible method of sorting classes in Python. The “traditional” way (for simple comparison functions), we will write a function to get a numerical value from the class and sort using that key. For example,

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def get_name(s):
  return s.name
L.sort(key=get_name)

will sort the list of students in L alphabetically. Alternatively, we can use anonymous functions (lambda functions) which we can write in one line to save space. For example,

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L.sort(key=lambda s: s.name)

achieves the same purpose.

However, it is difficult (but not impossible) to modify the “traditional” way and come up with a way to map a more complicated comparison function to a numerical value. Note: comparison functions can be as complex as we want, but they need to be transitive in order to get meaningful results.