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PEP 267 -- Optimized Access to Module Namespaces

PEP: 267
Title: Optimized Access to Module Namespaces
Author: Jeremy Hylton <jeremy at>
Status: Deferred
Type: Standards Track
Created: 23-May-2001
Python-Version: 2.2


    While this PEP is a nice idea, no-one has yet emerged to do the work of
    hashing out the differences between this PEP, PEP 266 and PEP 280.
    Hence, it is being deferred.


    This PEP proposes a new implementation of global module namespaces
    and the builtin namespace that speeds name resolution.  The
    implementation would use an array of object pointers for most
    operations in these namespaces.  The compiler would assign indices
    for global variables and module attributes at compile time.

    The current implementation represents these namespaces as
    dictionaries.  A global name incurs a dictionary lookup each time
    it is used; a builtin name incurs two dictionary lookups, a failed
    lookup in the global namespace and a second lookup in the builtin

    This implementation should speed Python code that uses
    module-level functions and variables.  It should also eliminate
    awkward coding styles that have evolved to speed access to these

    The implementation is complicated because the global and builtin
    namespaces can be modified dynamically in ways that are impossible
    for the compiler to detect.  (Example: A module's namespace is
    modified by a script after the module is imported.)  As a result,
    the implementation must maintain several auxiliary data structures
    to preserve these dynamic features.


    This PEP proposes a new implementation of attribute access for
    module objects that optimizes access to module variables known at
    compile time.  The module will store these variables in an array
    and provide an interface to lookup attributes using array offsets.
    For globals, builtins, and attributes of imported modules, the
    compiler will generate code that uses the array offsets for fast

    [describe the key parts of the design: dlict, compiler support,
    stupid name trick workarounds, optimization of other module's

    The implementation will preserve existing semantics for module
    namespaces, including the ability to modify module namespaces at
    runtime in ways that affect the visibility of builtin names.

DLict design

    The namespaces are implemented using a data structure that has
    sometimes gone under the name dlict.  It is a dictionary that has
    numbered slots for some dictionary entries.  The type must be
    implemented in C to achieve acceptable performance.  The new
    type-class unification work should make this fairly easy.  The
    DLict will presumably be a subclass of dictionary with an
    alternate storage module for some keys.

    A Python implementation is included here to illustrate the basic

        """A dictionary-list hybrid"""

        import types

        class DLict:
            def __init__(self, names):
                assert isinstance(names, types.DictType)
                self.names = {}
                self.list = [None] * size
                self.empty = [1] * size
                self.dict = {}
                self.size = 0

            def __getitem__(self, name):
                i = self.names.get(name)
                if i is None:
                    return self.dict[name]
                if self.empty[i] is not None:
                    raise KeyError, name
                return self.list[i]

            def __setitem__(self, name, val):
                i = self.names.get(name)
                if i is None:
                    self.dict[name] = val
                    self.empty[i] = None
                    self.list[i] = val
                    self.size += 1

            def __delitem__(self, name):
                i = self.names.get(name)
                if i is None:
                    del self.dict[name]
                    if self.empty[i] is not None:
                        raise KeyError, name
                    self.empty[i] = 1
                    self.list[i] = None
                    self.size -= 1

            def keys(self):
                if self.dict:
                    return self.names.keys() + self.dict.keys()
                    return self.names.keys()

            def values(self):
                if self.dict:
                    return self.names.values() + self.dict.values()
                    return self.names.values()

            def items(self):
                if self.dict:
                    return self.names.items()
                    return self.names.items() + self.dict.items()

            def __len__(self):
                return self.size + len(self.dict)

            def __cmp__(self, dlict):
                c = cmp(self.names, dlict.names)
                if c != 0:
                    return c
                c = cmp(self.size, dlict.size)
                if c != 0:
                    return c
                for i in range(len(self.names)):
                    c = cmp(self.empty[i], dlict.empty[i])
                    if c != 0:
                        return c
                    if self.empty[i] is None:
                        c = cmp(self.list[i], dlict.empty[i])
                        if c != 0:
                            return c
                return cmp(self.dict, dlict.dict)

            def clear(self):
                for i in range(len(self.names)):
                    if self.empty[i] is None:
                        self.empty[i] = 1
                        self.list[i] = None

            def update(self):

            def load(self, index):
                """dlict-special method to support indexed access"""
                if self.empty[index] is None:
                    return self.list[index]
                    raise KeyError, index # XXX might want reverse mapping

            def store(self, index, val):
                """dlict-special method to support indexed access"""
                self.empty[index] = None
                self.list[index] = val

            def delete(self, index):
                """dlict-special method to support indexed access"""
                self.empty[index] = 1
                self.list[index] = None

Compiler issues

    The compiler currently collects the names of all global variables
    in a module.  These are names bound at the module level or bound
    in a class or function body that declares them to be global.

    The compiler would assign indices for each global name and add the
    names and indices of the globals to the module's code object.
    Each code object would then be bound irrevocably to the module it
    was defined in.  (Not sure if there are some subtle problems with

    For attributes of imported modules, the module will store an
    indirection record.  Internally, the module will store a pointer
    to the defining module and the offset of the attribute in the
    defining module's global variable array.  The offset would be
    initialized the first time the name is looked up.

Runtime model

    The PythonVM will be extended with new opcodes to access globals
    and module attributes via a module-level array.

    A function object would need to point to the module that defined
    it in order to provide access to the module-level global array.

    For module attributes stored in the dlict (call them static
    attributes), the get/delattr implementation would need to track
    access to these attributes using the old by-name interface.  If a
    static attribute is updated dynamically, e.g.

        mod.__dict__["foo"] = 2

    The implementation would need to update the array slot instead of
    the backup dict.

Backwards compatibility

    The dlict will need to maintain meta-information about whether a
    slot is currently used or not.  It will also need to maintain a
    pointer to the builtin namespace.  When a name is not currently
    used in the global namespace, the lookup will have to fail over to
    the builtin namespace.

    In the reverse case, each module may need a special accessor
    function for the builtin namespace that checks to see if a global
    shadowing the builtin has been added dynamically.  This check
    would only occur if there was a dynamic change to the module's
    dlict, i.e. when a name is bound that wasn't discovered at

    These mechanisms would have little if any cost for the common case
    whether a module's global namespace is not modified in strange
    ways at runtime.  They would add overhead for modules that did
    unusual things with global names, but this is an uncommon practice
    and probably one worth discouraging.

    It may be desirable to disable dynamic additions to the global
    namespace in some future version of Python.  If so, the new
    implementation could provide warnings.

Related PEPs

    PEP 266, Optimizing Global Variable/Attribute Access, proposes a
    different mechanism for optimizing access to global variables as
    well as attributes of objects.  The mechanism uses two new opcodes
    TRACK_OBJECT and UNTRACK_OBJECT to create a slot in the local
    variables array that aliases the global or object attribute.  If
    the object being aliases is rebound, the rebind operation is
    responsible for updating the aliases.

    The objecting tracking approach applies to a wider range of
    objects than just module.  It may also have a higher runtime cost,
    because each function that uses a global or object attribute must
    execute extra opcodes to register its interest in an object and
    unregister on exit; the cost of registration is unclear, but
    presumably involves a dynamically resizable data structure to hold
    a list of callbacks.

    The implementation proposed here avoids the need for registration,
    because it does not create aliases.  Instead it allows functions
    that reference a global variable or module attribute to retain a
    pointer to the location where the original binding is stored.  A
    second advantage is that the initial lookup is performed once per
    module rather than once per function call.


    This document has been placed in the public domain.