Python: Access Control
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The document discusses access control in Python object-oriented programming. It explains that in most languages, attributes and methods can be public, protected, or private, but in Python there is no built-in access control. By convention, prefixing with a single underscore _ indicates internal use only, while double underscore __ makes Python attempt to make it private. The document provides an example SecretString class to demonstrate how attributes prefixed with __ cannot be directly accessed but can still be retrieved by external objects in more complex ways.
Python: Access Control
- 2. Access Control • In most other object-orientated programming languages, methods and attributes of an object can be classified as: – PRIVATE: Meaning only the object can access it. – PROTECTED: Meaning only that class and sub-classes can access it. – PUBLIC: Meaning any other object can access it.
- 3. Access Control • In most other object-orientated programming languages, methods and attributes of an object can be classified as: – PRIVATE: Meaning only the object can access it. – PROTECTED: Meaning only that class and sub-classes can access it. – PUBLIC: Meaning any other object can access it.
- 4. Access Control • Python doesn’t work like that! • If want to make a method or attribute private, the easiest way of doing it is to add a comment in the docstrings at the start of the class indicating which methods or attributes are for internal use only.
- 5. Access Control • Nonetheless, by convention, if we prefix a method or attribute with a single underscore ( _ ) we are telling everyone that this method or attribute should not be used by other objects except in the most dire circumstances. • But there is nothing in the interpreter to stop external objects from access a method or attribute with an underscore ( _ ).
- 6. Access Control • Nonetheless, by convention, if we prefix a method or attribute with a single underscore ( _ ) we are telling everyone that this method or attribute should not be used by other objects except in the most dire circumstances. • But there is nothing in the interpreter to stop external objects from access a method or attribute with an underscore ( _ ).
- 7. Access Control • If we want to more strongly suggest that the methods or attributes are for internal use only we can use a double- underscore ( _ _ ) which means Python will try a little harder to make it private, but still… • Let’s look at an example:
- 8. Access Control class SecretString: ‘‘‘A not-at-all secure way to store a secret string.’’’ def __init__(self, plain_string, pass_phrase): self.__plain_string = plain_string self.__pass_phrase = pass_phrase # End __init__ Continued
- 9. Access Control def decrypt(self, pass_phrase): '''Only show the string if the pass_phrase is correct.''' if pass_phrase == self.__pass_phrase: # THEN return self.__plain_string else: return '' # Endif; # End decrypt. # End Class. Continued
- 10. Access Control >>> secret_string = SecretString("ACME: Top Secret", “secretpass") >>> print(secret_string.decrypt("secretpass")) ACME: Top Secret
- 11. Access Control >>> print(secret_string.__plain_string) Traceback (most recent call last): File "<pyshell#9>", line 1, in <module> print(secret_string.__plain_string) AttributeError: 'SecretString' object has no attribute '__plain_string‘ >>> print(secret_string.plain_string) Traceback (most recent call last): File "<pyshell#10>", line 1, in <module> print(secret_string.plain_string) AttributeError: 'SecretString' object has no attribute 'plain_string'
- 12. Access Control >>> print(secret_string._SecretString_ _plain_string) ACME: Top Secret BUT:
- 13. Access Control • OK, so the double underscore won’t 100% guarantee that a method or attribute can’t be accessed by another class, but it does make it more awkward: secret_string._SecretString_ _plain_string • And should be sufficient to discourage other programmers for using objects you wish to keep internal.
- 14. etc.