属性アクセスのカスタマイズ

Python 中級オブジェクト指向プログラミング

Jake Roach

Data Engineer

AttributeError

class Student:
  def __init__(self, student_name, major):
    self.student_name = student_name
    self.major = major
...

karina = Student("Karina", "Literature")
student.residence_hall  # Attempt to access an attribute that does not exist

...
AttributeError: 'Student' object has no attribute 'residence_hall'

getattr()

# Rest of the class definition above

  def __getattr__(self, name):
    # Implement logic here
    ...

オブジェクトの名前空間は、そのオブジェクトに結び付く属性の集合です

__getattr__() は、オブジェクトの名前空間外の属性にアクセスしようとしたときに実行される

マジックメソッド。直接は呼ばれない
name 引数を取る
AttributeError を投げる代わりに独自の処理を実装できる

AttributeError の解消

class Student:
  def __init__(self, student_name, major):
    self.student_name = student_name
    self.major = major

  def __getattr__(self, name):
      print(f"""{name} does not exist in this object's namespace, try setting 
            a value for {name} first""")

karina.residence_hall  # Now, try to retrieve the residence_hall attribute again

residence_hall does not exist in this object's namespace, try setting a value for 
  residence_hall first

setattr()

__setattr__() は、新規/既存の属性が設定・更新されると実行されるマジックメソッド

__init__() で設定される属性も含む
属性の name と value を受け取る
オブジェクトの __dict__ を利用する

属性変更の制御、検証、変換に利用

# Rest of the class definition above

  def __setattr__(self, name, value):
    # Implement logic here
    ...

    # Use __dict__ to create/update 
    # the attribute
    self.__dict__[name] = value

__dict__ はオブジェクトの全属性を保持し、取得・保存に使える

属性の保存をカスタマイズ

class Student:
  def __init__(self, student_name, major):
    self.student_name = student_name
    self.major = major

  def __setattr__(self, name, value):
    # If value is a string, set the attribute using the __dict__ attribute
    if isinstance(value, str):
      print(f"Setting {name} = {value}")
      self.__dict__[name] = value

    else: # Otherwise, raise an exception noting an incorrect data type
        raise Exception("Unexpected data type!")

setattr の動作

# Set an attribute using a value of type 'str'
karina.residence_hall = "Honors College South"
print(karina.residence_hall)

Setting residence_hall = Honors College South
Honors College South

# Set an attribute using a value of type 'int'
karina.student_id = 19301872

...
    raise Exception("Unexpected data type!")
Exception: Unexpected data type!

getattr と setattr の併用

class Student:
    ...   

    def __getattr__(self, name):
        # Set the attribute with a placeholder
        self.__setattr__(name, None)
        return None

    def __setattr__(self, name, value):
        if value is None:  # Print a message denoting a placeholder
            print(f"Setting placeholder for {name}")

        self.__dict__[name] = value  # Set the attribute

Let's practice!