Type checking functions

1. type(object)

   print(type("Goodbye")) # <class 'str'>
   print(type("hello") == type("Goodbye"))  # True
   print(type("hello") == type(5))  # False
   

2. isinstance(object, class)

   class C:
     pass
   class B(C):
     pass
   class A(B):
     pass
   
   a = A()
   b = B()
   c = C()
   
   print(isinstance(a, A))  # True
   print(isinstance(a, B))  # True
   print(isinstance(a, C))  # True
   print(isinstance(c, A))  # False
   

3. issubclass(classA, classB)

   print(issubclass(A, A))  # True
   print(issubclass(A, B))  # True
   print(issubclass(A, C))  # True
   print(issubclass(C, A))  # False
   

Duck Typing

Python在多數情況下，我們不用檢查接下來的操作需要什麼資料類型，只要看起來可以執行就會執行。

Example:

# The sorted function is using the principle of duck typing
# This is a Duck Typing style design

print(sorted(['foo','bar','fox']))  # ['bar', 'foo', 'fox']
print(sorted({'foo','bar','fox'}))  # ['bar', 'foo', 'fox']
print(sorted({'key1':'foo', 'key2':'bar', 'key3':'fox'}))  # ['key1', 'key2', 'key3']
print(sorted('foobarfox'))  # ['a', 'b', 'f', 'f', 'o', 'o', 'o', 'r', 'x']

print(sorted(99644))  # TypeError: 'int' object is not iterable

try:
  print(sorted(99644))
except:
  print("Error happened while sorted")

def calculate(x, y, z):
  return (x + y) * z

print(calculate(3, 2, 2))  # 10
print(calculate('foo', 'bar', 3))  # foobarfoobarfoobar
print(calculate([1, 3], [4, 2], 3))  # [1, 3, 4, 2, 1, 3, 4, 2, 1, 3, 4, 2]

try:
  print(calculate([1, 3], 5, 5))
except:
  print('we got error')

TypedList

class TypedList:
    def __init__(self, example_element, initial_list):
        self.type = type(example_element)
        if not isinstance(initial_list, list):
            raise TypeError("Second argument of TypedLisst must be a list.")
        for element in initial_list:
            self.__check(element)
        self.elements = initial_list[:] # copy by value

    def __check(self, element):
        if type(element) != self.type:
            raise TypeError(
                "Attempted to add an element of incorrect type to a TypedList.")
    
    def __setitem__(self, i, element):
        self.__check(element)
        self.elements[i] = element
    
    def __getitem__(self, i):
        return self.elements[i]
    
    def __str__(self):
        return str(self.elements)
    
    def __add__(self, anotherList):
      return len(self.elements) + len(anotherList.elements)

x = TypedList('Hello', ['some', 'original', 'stuff'])
print(x)  # ['some', 'original', 'stuff']

y = TypedList('', [''] * 5)
print(y)  # ['', '', '', '', '']

y[3] = "15"
y[4] = "hello"
print(y)  # ['', '', '', '15', 'hello']

print(x + y)  # 8

class TypedList(list):
    def __init__(self, example_element, initial_list):
        self.type = type(example_element)
        if not isinstance(initial_list, list):
            raise TypeError("Second argument of TypedLisst must be a list.")
        for element in initial_list:
            self.__check(element)
        super().__init__(initial_list)

    def __check(self, element):
        if type(element) != self.type:
            raise TypeError(
                "Attempted to add an element of incorrect type to a TypedList.")

    def __setitem__(self, i, element):
        self.__check(element)
        super().__setitem__(i, element)

    def __getitem__(self, i):
        return super().__getitem__(i)

x = TypedList('Hello', ['Some', 'original', 'stuff'])
print(x)  # ['Some', 'original', 'stuff']
y = TypedList('', [''] * 5)
print(y)  # ['', '', '', '', '']
print(x + y)  # ['Some', 'original', 'stuff', '', '', '', '', '']

Decimal, Binary, and Hexadecimal

Numeral System 記數系統

• hex(number) - converts the specified integer number into a hexadecimal value. The return string always starts with the prefix 0x.
• bin(number) - returns the binary version of a specified integer number. The result will always start with the prefix 0b.

print(hex(255))  # 0xff
print(bin(255))  # 0b11111111

Encoding and UTF-8