Python Interview Questions 1

Motivation

Recently I had two interviews with python related positions. I believe it’s worthy to record it. I can’t remember everything in detail, so I just try to tell the story in my way. If you find something wrong, please let me know. Thanks!

Questions

1. Please write down the result.

 class A:
     x = 5
     def __init__(self):
         self.y = 1

     class B(A):
     x = 6

     class C(A):
         pass

 A.x = 7
 B.x = 8
 a = A()
 a.y = 2
 b = B()
 b.y = 3
 c = C()

 print(A.x, B.x, C.x)
 print(a.y, b.y, c.y)

Ans:

7 8 7
2 3 1

2. Please write down the result.

 def func(x, dic={}):
     dic[x] = x
     return dic

 res = {}
 for i in range(10):
 res = func(i)

 ans = 0
 for v in res.values():
 ans += v

 print(ans)

Ans:

45

Explanation:

The default value in function occupies a piece of memory when it’s declared. If no argument, it would use the same object to finish the function call so as those x values are inserted into dic.

3. Design a Calculator class and implement the functionality of plus, minus, multiply, and divide. Please try your best to make it handy to use. Hint: Maybe we can use it as a library.

Ans 1: Implement class methods.

 class Calculator():
     @classmethod
     def plus(cls, val1, val2):
         return val1 + val2
        
     @classmethod
     def minus(cls, val1, val2):
         return val1 - val2
        
     @classmethod
     def multiply(cls, val1, val2):
         return val1 * val2
        
     @classmethod
     def divide(cls, val1, val2):
         if val2 == 0:
         raise ZeroDivisionError
         return val1 / val2
    
 ans = Calculator.plus(5, 3)
 ans = Calculator.minus(5, 3)
 ans = Calculator.multiply(5, 3)
 ans = Calculator.divide(5, 3)

Ans 2: Rewrite class operator.

 class Calculator():
     def __init__(self, val):
         self._value = val

     def __add__(self, item):
         return self._value + item._value

     def __sub__(self, item):
         return self._value - item._value

     def __mul__(self, item):
         return self._value * item._value
            
     def __truediv__(self, item):
         if item._value == 0:
         raise ZeroDivisionError
         return self._value / item._value

 ans = Calculator(5) + Calculator(3)
 ans = Calculator(5) - Calculator(3)
 ans = Calculator(5) * Calculator(3)
 ans = Calculator(5) / Calculator(3)

Explanation:

Actually, I came up with these two ideas when I was testing. Not sure which one is better.

4. There is a special class below and we hope the adding value into it only via insert. Assume that the insert function is completed. So please add something to make it raise InsertError correctly.

 class InsertError(Exception):
     pass

 class Dictionary():
     # Need to be added...

 dic = Dictionary()
 dic.insert("Key1", "Value1")

 try:
     dic["Key2"] = "Value2"
 except :
     print("InsertError is caught.") 

 # result: InsertError is caught.

Ans:

 class Dictionary():
     def __setitem__(self, key, value):
         raise InsertError