Menyesuaikan Akses Atribut

Pemrograman Berorientasi Objek Tingkat Menengah di Python

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
    ...

Namespace suatu objek adalah kumpulan atribut yang terkait dengan objek tersebut

__getattr__() dijalankan saat upaya mengakses ATRIBUT APA PUN di luar namespace objek

Metode ajaib, tidak dipanggil langsung
Menerima parameter name
Menjalankan logika kustom alih-alih memunculkan AttributeError

Mengatasi AttributeError

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__() adalah metode ajaib yang dijalankan saat atribut (baru atau lama) di-set atau di-update

Termasuk atribut yang diset lewat __init__()
Menerima name atribut dan value
Memanfaatkan atribut __dict__ milik objek

Mengendalikan perubahan atribut, validasi, transformasi

# 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__ menyimpan semua atribut objek; dapat dipakai untuk mengambil dan menyimpan data

Menyesuaikan penyimpanan atribut

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 dalam praktik

# 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!

Menggunakan getattr dan setattr bersama

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

Ayo berlatih!