lockup 1.0.3

Immutable and concealed attributes for classes, modules, and namespaces.

API Documentation (stable) | API Documentation (current) | Code of Conduct | Contribution Guide

Overview

Enables the creation of classes, modules, and namespaces on which the following properties are true:

• All attributes are immutable. Immutability increases code safety by discouraging monkey-patching and preventing accidental or deliberate changes to state.

  >>> import sys
  >>> import lockup
  >>> lockup.reclassify_module( sys )  # doctest: +SKIP
  >>> sys.stderr = sys.stdout          # doctest: +SKIP
  Traceback (most recent call last):
  ...
  lockup.exceptions.ImpermissibleAttributeOperation: Attempt to assign immutable attribute 'stderr' on module 'sys'.

  >>> import lockup
  >>> ns = lockup.create_namespace( some_constant = 6 )
  >>> ns.some_constant = 13
  Traceback (most recent call last):
  ...
  lockup.exceptions.ImpermissibleAttributeOperation: Attempt to assign immutable attribute 'some_constant' on class 'lockup.Namespace'.

• Non-public attributes are concealed. Concealment means that functions, such as dir, can report a subset of attributes that are intended for programmers to use (without directly exposing internals).

  >>> import lockup
  >>> dir( lockup )
  ['Class', 'Module', 'NamespaceClass', 'base', 'create_namespace', 'exceptions', 'reclassify_module']
  >>> len( dir( lockup ) )
  7
  >>> len( lockup.__dict__ )  # doctest: +SKIP
  18

Quick Tour

Module

Let us consider the mutable os module from the Python standard library and how we can alter “constants” that may be used in many places:

>>> import os
>>> os.EX_OK
0
>>> del os.EX_OK
>>> os.EX_OK
Traceback (most recent call last):
...
AttributeError: module 'os' has no attribute 'EX_OK'
>>> os.EX_OK = 0
>>> type( os )
<class 'module'>

Now, let us see what protection it gains from becoming immutable:

>>> import os
>>> import lockup
>>> lockup.reclassify_module( os )
>>> del os.EX_OK
Traceback (most recent call last):
...
lockup.exceptions.ImpermissibleAttributeOperation: Attempt to delete indelible attribute 'EX_OK' on module 'os'.
>>> os.EX_OK = 255
Traceback (most recent call last):
...
lockup.exceptions.ImpermissibleAttributeOperation: Attempt to assign immutable attribute 'EX_OK' on module 'os'.
>>> type( os )
<class 'lockup.Module'>

Class Factory

Let us monkey-patch a mutable class:

>>> class A:
...     def expected_functionality( self ): return 42
...
>>> a = A( )
>>> a.expected_functionality( )
42
>>> def monkey_patch( self ):
...     return 'I selfishly change behavior upon which other consumers depend.'
...
>>> A.expected_functionality = monkey_patch
>>> a = A( )
>>> a.expected_functionality( )
'I selfishly change behavior upon which other consumers depend.'

Now, let us try to monkey-patch an immutable class:

>>> import lockup
>>> class B( metaclass = lockup.Class ):
...     def expected_functionality( self ): return 42
...
>>> b = B( )
>>> b.expected_functionality( )
42
>>> def monkey_patch( self ):
...     return 'I selfishly change behavior upon which other consumers depend.'
...
>>> B.expected_functionality = monkey_patch
Traceback (most recent call last):
...
lockup.exceptions.ImpermissibleAttributeOperation: Attempt to assign immutable attribute 'expected_functionality' on class ...
>>> type( B )
<class 'lockup.Class'>
>>> del type( B ).__setattr__
Traceback (most recent call last):
...
lockup.exceptions.ImpermissibleAttributeOperation: Attempt to delete indelible attribute '__setattr__' on class 'lockup.Class'.
>>> issubclass( type( B ), type )
True

Note

Only class attributes are immutable. Instances of immutable classes will have mutable attributes without additional intervention beyond the scope of this package.

Namespace Factory

An alternative to types.SimpleNamespace is provided. First, let us observe the behaviors on a standard namespace:

>>> import types
>>> sn = types.SimpleNamespace( run = lambda: 42 )
>>> sn
namespace(run=<function <lambda> at ...>)
>>> sn.run( )
42
>>> type( sn )
<class 'types.SimpleNamespace'>
>>> dir( sn )
['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'run']
>>> sn.__dict__
{'run': <function <lambda> at ...>}
>>> type( sn.run )
<class 'function'>
>>> sn.run = lambda: 666
>>> sn.run( )
666
>>> sn( )
Traceback (most recent call last):
...
TypeError: 'types.SimpleNamespace' object is not callable

Now, let us compare those behaviors to an immutable namespace:

>>> import lockup
>>> ns = lockup.create_namespace( run = lambda: 42 )
>>> ns
NamespaceClass( 'Namespace', ('object',), { ... } )
>>> ns.run( )
42
>>> type( ns )
<class 'lockup.NamespaceClass'>
>>> ns.__dict__
mappingproxy({...})
>>> type( ns.run )
<class 'function'>
>>> ns.run = lambda: 666
Traceback (most recent call last):
...
lockup.exceptions.ImpermissibleAttributeOperation: Attempt to assign immutable attribute 'run' on class 'lockup.Namespace'.
>>> ns.__dict__[ 'run' ] = lambda: 666
Traceback (most recent call last):
...
TypeError: 'mappingproxy' object does not support item assignment
>>> ns( )
Traceback (most recent call last):
...
lockup.exceptions.ImpermissibleOperation: Impermissible instantiation of class 'lockup.Namespace'.

Also of note is that we can define namespace classes directly, allowing us to capture imports for internal use in a module without publicly exposing them as part of the module API, for example:

    >>> import lockup
>>> class __( metaclass = lockup.NamespaceClass ):
...     from os import O_RDONLY, O_RDWR
...
>>> __.O_RDONLY
0

The above technique is used internally within this package itself.

Exceptions

Exceptions can be intercepted with appropriate builtin exception classes or with package exception classes:

>>> import os
>>> import lockup
>>> from lockup.exceptions import InvalidOperation
>>> os.O_RDONLY
0
>>> lockup.reclassify_module( os )
>>> try: os.O_RDONLY = 15
... except AttributeError as exc:
...     type( exc ).mro( )
...
[<class 'lockup.exceptions.ImpermissibleAttributeOperation'>, <class 'lockup.exceptions.ImpermissibleOperation'>, <class 'lockup.exceptions.InvalidOperation'>, <class 'lockup.exceptions.Exception0'>, <class 'TypeError'>, <class 'AttributeError'>, <class 'Exception'>, <class 'BaseException'>, <class 'object'>]
>>> try: os.does_not_exist
... except InvalidOperation as exc:
...     type( exc ).mro( )
...
[<class 'lockup.exceptions.InaccessibleAttribute'>, <class 'lockup.exceptions.InaccessibleEntity'>, <class 'lockup.exceptions.InvalidOperation'>, <class 'lockup.exceptions.Exception0'>, <class 'AttributeError'>, <class 'Exception'>, <class 'BaseException'>, <class 'object'>]

Changelog

v1.0.3

Documentation

• Improve presentation of overview in README.

• Properly fence Python code blocks in README for correct rendering on PyPI.

v1.0.2

Documentation

• Provide direct link in README to stable API documentation.

v1.0.1

Documentation

• Improve wording of introduction in README.

• Add badges in README for supported Python versions, current release, code coverage, and license.

• Provide direct links in README to current API documentation, code of conduct, and contribution guide.