lockup¶
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 math >>> import lockup >>> lockup.reclassify_module( math ) >>> math.pi = math.e Traceback (most recent call last): ... lockup.exceptions.ImpermissibleAttributeOperation: Attempt to assign immutable attribute 'pi' on module 'math'.
>>> 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__ ) 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 ...
>>> del B.expected_functionality
Traceback (most recent call last):
...
lockup.exceptions.ImpermissibleAttributeOperation: Attempt to delete indelible attribute 'expected_functionality' on class ...
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'>
>>> 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'>]
Compatibility¶
This package has been verified to work on the following Python implementations:
It likely works on others as well, but please report if it does not.