Pythonのgetattr()とは

目標

Pythonのgetattr()関数について理解する。

getattr(Object, name [, default])

Return the value of the named attribute of object. name must be a string. If the string is the name of one of the object’s attributes, the result is the value of that attribute. For example, getattr(x, 'foobar') is equivalent to x.foobar. If the named attribute does not exist, default is returned if provided, otherwise AttributeError is raised.

Python document > library > functions

• この関数は、オブジェクトの指定した名前(name)の属性の値を返します。
  • このnameは文字列でなければなりません。
• 例えば、getattr(x, ‘foobar’)はx.foobarと同じ値を返します。
• 指定された属性が存在しない場合は、defaultが用意されていればその値を、それ以外の場合はAttributeErrorが返されます。

実行例

サンプルコードです。Dogクラスは、name, ageというインスタンス変数を持ち、sit(), bark()というインスタンスメソッドを持ちます。

class Dog():
    def __init__(self, name, age):
        self.name = name
        self.age = age
    def sit(self):
        self.tired = False
        print("sit down")
    def bark(self, count):
        self.tired = True
        for i in range(count):
            print("bow wow")

dog1 = Dog("Poppy", 3)
val1 = getattr(dog1, "name")
val2 = getattr(dog1, "age")
val3 = getattr(dog1, "tired", "unknown")
val4 = getattr(dog1, "sit")()
val5 = getattr(dog1, "bark")(2)
val6 = getattr(dog1, "sit")
val7 = getattr(dog1, "bark")
val8 = getattr(dog1, "tired", "unknown")

属性の取得

インスタンス名と属性名を引数に与えることで インスタンス変数の値を取得できます。

# val1 = getattr(dog1, "name")
print(val1)
>> Poppy

# val2 = getattr(dog1, "age")
print(val2)
>> 3

Get default value

存在しないメソッドや変数を取得し用とした場合は、第三引数(default)に指定した値を返す。

# val3 = getattr(dog1, "tired", "unknown")
print(val3)
>> unknown

インスタンスメソッドの実行

インスタンスメソッドの実行、

# val4 = getattr(dog1, "sit")()
print(val4)
>> None

# val5 = getattr(dog1, "bark")(2)
print(val5)
>> None

インスタンスメソッドの取得

インスタンスメソッドを取得し、()を付けることで、メソッドを実行することができます。

# val6 = getattr(dog1, "sit")
print(val6)
>> <bound method Dog.sit of <__main__.Dog object at 0x7f192a93f1d0>>
val6()
>> sit down

取得したメソッドに引数を渡すこともできます。

# val7 = getattr(dog1, "bark")
print(val7)
>> <bound method Dog.bark of <__main__.Dog object at 0x7fb2169ae1d0>>
val7(3)
>> bow wow
>> bow wow
>> bow wow

メソッドを実行した後の変化

# val8 = getattr(dog1, "tired", "unknown")
print(val8)
>> True

先ほどはunknownだったインスタンス変数tiredの値が、getattr(dog1, bark)(2) によってTrueに変化しました。

GOAL

To understand getattr() function in python.

getattr(Object, name [, default])

Return the value of the named attribute of object. name must be a string. If the string is the name of one of the object’s attributes, the result is the value of that attribute. For example, getattr(x, 'foobar') is equivalent to x.foobar. If the named attribute does not exist, default is returned if provided, otherwise AttributeError is raised.

Python document > library > functions

What will happen

class Dog():
    def __init__(self, name, age):
        self.name = name
        self.age = age
    def sit(self):
        self.tired = False
        print("sit down")
    def bark(self, count):
        self.tired = True
        for i in range(count):
            print("bow wow")

dog1 = Dog("Poppy", 3)
val1 = getattr(dog1, "name")
val2 = getattr(dog1, "age")
val3 = getattr(dog1, "tired", "unknown")
val4 = getattr(dog1, "sit")()
val5 = getattr(dog1, "bark")(2)
val6 = getattr(dog1, "sit")
val7 = getattr(dog1, "bark")
val8 = getattr(dog1, "tired", "unknown")

Get attribute

# val1 = getattr(dog1, "name")
print(val1)
>> Poppy

# val2 = getattr(dog1, "age")
print(val2)
>> 3

Get default value

# val3 = getattr(dog1, "tired", "unknown")
print(val3)
>> unknown

Execute function

# val4 = getattr(dog1, "sit")()
print(val4)
>> None

# val5 = getattr(dog1, "bark")(2)
print(val5)
>> None

Get function

# val6 = getattr(dog1, "sit")
print(val6)
>> <bound method Dog.sit of <__main__.Dog object at 0x7f192a93f1d0>>
val6()
>> sit down

# val7 = getattr(dog1, "bark")
print(val7)
>> <bound method Dog.bark of <__main__.Dog object at 0x7fb2169ae1d0>>
val7(3)
>> bow wow
>> bow wow
>> bow wow

After Executing function

# val8 = getattr(dog1, "tired", "unknown")
print(val8)
>> True

getattr(dog1, sit)() and getattr(dog1, bark)(2) changed internal variable “tired”.