PEP 406 – Improved Encapsulation of Import State

PEP: 406
Title: Improved Encapsulation of Import State
Author: Nick Coghlan <ncoghlan at gmail.com>, Greg Slodkowicz <jergosh at gmail.com>
Status: Withdrawn
Type: Standards Track
Created: 04-Jul-2011
Python-Version: 3.4
Post-History: 31-Jul-2011, 13-Nov-2011, 4-Dec-2011

Abstract
PEP Withdrawal
Rationale
Proposal
Specification
Open Issues
Reference Implementation
References
Copyright

Abstract

This PEP proposes the introduction of a new ‘ImportEngine’ class as part of importlib which would encapsulate all state related to importing modules into a single object. Creating new instances of this object would then provide an alternative to completely replacing the built-in implementation of the import statement, by overriding the __import__() function. To work with the builtin import functionality and importing via import engine objects, this PEP proposes a context management based approach to temporarily replacing the global import state.

The PEP also proposes inclusion of a GlobalImportEngine subclass and a globally accessible instance of that class, which “writes through” to the process global state. This provides a backwards compatible bridge between the proposed encapsulated API and the legacy process global state, and allows straightforward support for related state updates (e.g. selectively invalidating path cache entries when sys.path is modified).

PEP Withdrawal

The import system has seen substantial changes since this PEP was originally written, as part of PEP 420 in Python 3.3 and PEP 451 in Python 3.4.

While providing an encapsulation of the import state is still highly desirable, it is better tackled in a new PEP using PEP 451 as a foundation, and permitting only the use of PEP 451 compatible finders and loaders (as those avoid many of the issues of direct manipulation of global state associated with the previous loader API).

Rationale

Currently, most state related to the import system is stored as module level attributes in the sys module. The one exception is the import lock, which is not accessible directly, but only via the related functions in the imp module. The current process global import state comprises:

sys.modules
sys.path
sys.path_hooks
sys.meta_path
sys.path_importer_cache
the import lock (imp.lock_held()/acquire_lock()/release_lock())

Isolating this state would allow multiple import states to be conveniently stored within a process. Placing the import functionality in a self-contained object would also allow subclassing to add additional features (e.g. module import notifications or fine-grained control over which modules can be imported). The engine would also be subclassed to make it possible to use the import engine API to interact with the existing process-global state.

The namespace PEPs (especially PEP 402) raise a potential need for additional process global state, in order to correctly update package paths as sys.path is modified.

Finally, providing a coherent object for all this state makes it feasible to also provide context management features that allow the import state to be temporarily substituted.

Proposal

We propose introducing an ImportEngine class to encapsulate import functionality. This includes an __import__() method which can be used as an alternative to the built-in __import__() when desired and also an import_module() method, equivalent to importlib.import_module() 3.

Since there are global import state invariants that are assumed and should be maintained, we introduce a GlobalImportState class with an interface identical to ImportEngine but directly accessing the current global import state. This can be easily implemented using class properties.

Specification

ImportEngine API

The proposed extension consists of the following objects:

importlib.engine.ImportEngine

from_engine(self, other)

Create a new import object from another ImportEngine instance. The new object is initialised with a copy of the state in other. When called on importlib engine.sysengine, from_engine() can be used to create an ImportEngine object with a copy of the global import state.

__import__(self, name, globals={}, locals={}, fromlist=[], level=0)

Reimplementation of the builtin __import__() function. The import of a module will proceed using the state stored in the ImportEngine instance rather than the global import state. For full documentation of __import__ functionality, see 2 . __import__() from ImportEngine and its subclasses can be used to customise the behaviour of the import statement by replacing __builtin__.__import__ with ImportEngine().__import__.

import_module(name, package=None)

A reimplementation of importlib.import_module() which uses the import state stored in the ImportEngine instance. See 3 for a full reference.

modules, path, path_hooks, meta_path, path_importer_cache

Instance-specific versions of their process global sys equivalents

importlib.engine.GlobalImportEngine(ImportEngine)

Convenience class to provide engine-like access to the global state. Provides __import__(), import_module() and from_engine() methods like ImportEngine but writes through to the global state in sys.

To support various namespace package mechanisms, when sys.path is altered, tools like pkgutil.extend_path should be used to also modify other parts of the import state (in this case, package __path__ attributes). The path importer cache should also be invalidated when a variety of changes are made.

The ImportEngine API will provide convenience methods that automatically make related import state updates as part of a single operation.

Global variables

importlib.engine.sysengine

A precreated instance of GlobalImportEngine. Intended for use by importers and loaders that have been updated to accept optional engine parameters and with ImportEngine.from_engine(sysengine) to start with a copy of the process global import state.

No changes to finder/loader interfaces

Rather than attempting to update the PEP 302 APIs to accept additional state, this PEP proposes that ImportEngine support the content management protocol (similar to the context substitution mechanisms in the decimal module).

The context management mechanism for ImportEngine would:

On entry: * Acquire the import lock * Substitute the global import state with the import engine’s own state
On exit: * Restore the previous global import state * Release the import lock

The precise API for this is TBD (but will probably use a distinct context management object, along the lines of that created by decimal.localcontext).

Open Issues

API design for falling back to global import state

The current proposal relies on the from_engine() API to fall back to the global import state. It may be desirable to offer a variant that instead falls back to the global import state dynamically.

However, one big advantage of starting with an “as isolated as possible” design is that it becomes possible to experiment with subclasses that blur the boundaries between the engine instance state and the process global state in various ways.

Builtin and extension modules must be process global

Due to platform limitations, only one copy of each builtin and extension module can readily exist in each process. Accordingly, it is impossible for each ImportEngine instance to load such modules independently.

The simplest solution is for ImportEngine to refuse to load such modules, raising ImportError. GlobalImportEngine would be able to load them normally.

ImportEngine will still return such modules from a prepopulated module cache - it’s only loading them directly which causes problems.

Scope of substitution

Related to the previous open issue is the question of what state to substitute when using the context management API. It is currently the case that replacing sys.modules can be unreliable due to cached references and there’s the underlying fact that having independent copies of some modules is simply impossible due to platform limitations.

As part of this PEP, it will be necessary to document explicitly:

Which parts of the global import state can be substituted (and declare code which caches references to that state without dealing with the substitution case buggy)
Which parts must be modified in-place (and hence are not substituted by the ImportEngine context management API, or otherwise scoped to ImportEngine instances)

Reference Implementation

A reference implementation 4 for an earlier draft of this PEP, based on Brett Cannon’s importlib has been developed by Greg Slodkowicz as part of the 2011 Google Summer of Code. Note that the current implementation avoids modifying existing code, and hence duplicates a lot of things unnecessarily. An actual implementation would just modify any such affected code in place.

That earlier draft of the PEP proposed change the PEP 302 APIs to support passing in an optional engine instance. This had the (serious) downside of not correctly affecting further imports from the imported module, hence the change to the context management based proposal for substituting the global state.