python-peps/pep-0227.txt

PEP: 227
Title: Statically Nested Scopes
Author: Jeremy Hylton <jeremy@alum.mit.edu>
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 01-Nov-2000
Python-Version: 2.1
Post-History:

Abstract
========

This PEP describes the addition of statically nested scoping
(lexical scoping) for Python 2.2, and as a source level option
for python 2.1.  In addition, Python 2.1 will issue warnings about
constructs whose meaning may change when this feature is enabled.

The old language definition (2.0 and before) defines exactly three
namespaces that are used to resolve names -- the local, global,
and built-in namespaces.  The addition of nested scopes allows
resolution of unbound local names in enclosing functions'
namespaces.

The most visible consequence of this change is that lambdas (and
other nested functions) can reference variables defined in the
surrounding namespace.  Currently, lambdas must often use default
arguments to explicitly creating bindings in the lambda's
namespace.

Introduction
============

This proposal changes the rules for resolving free variables in
Python functions.  The new name resolution semantics will take
effect with Python 2.2.  These semantics will also be available in
Python 2.1 by adding "from __future__ import nested_scopes" to the
top of a module.  (See :pep:`236`.)

The Python 2.0 definition specifies exactly three namespaces to
check for each name -- the local namespace, the global namespace,
and the builtin namespace.  According to this definition, if a
function A is defined within a function B, the names bound in B
are not visible in A.  The proposal changes the rules so that
names bound in B are visible in A (unless A contains a name
binding that hides the binding in B).

This specification introduces rules for lexical scoping that are
common in Algol-like languages.  The combination of lexical
scoping and existing support for first-class functions is
reminiscent of Scheme.

The changed scoping rules address two problems -- the limited
utility of lambda expressions (and nested functions in general),
and the frequent confusion of new users familiar with other
languages that support nested lexical scopes, e.g. the inability
to define recursive functions except at the module level.

The lambda expression yields an unnamed function that evaluates a
single expression.  It is often used for callback functions.  In
the example below (written using the Python 2.0 rules), any name
used in the body of the lambda must be explicitly passed as a
default argument to the lambda.

::

    from Tkinter import *
    root = Tk()
    Button(root, text="Click here",
           command=lambda root=root: root.test.configure(text="..."))

This approach is cumbersome, particularly when there are several
names used in the body of the lambda.  The long list of default
arguments obscures the purpose of the code.  The proposed
solution, in crude terms, implements the default argument approach
automatically.  The "root=root" argument can be omitted.

The new name resolution semantics will cause some programs to
behave differently than they did under Python 2.0.  In some cases,
programs will fail to compile.  In other cases, names that were
previously resolved using the global namespace will be resolved
using the local namespace of an enclosing function.  In Python
2.1, warnings will be issued for all statements that will behave
differently.

Specification
=============

Python is a statically scoped language with block structure, in
the traditional of Algol.  A code block or region, such as a
module, class definition, or function body, is the basic unit of a
program.

Names refer to objects.  Names are introduced by name binding
operations.  Each occurrence of a name in the program text refers
to the binding of that name established in the innermost function
block containing the use.

The name binding operations are argument declaration, assignment,
class and function definition, import statements, for statements,
and except clauses.  Each name binding occurs within a block
defined by a class or function definition or at the module level
(the top-level code block).

If a name is bound anywhere within a code block, all uses of the
name within the block are treated as references to the current
block.  (Note: This can lead to errors when a name is used within
a block before it is bound.)

If the global statement occurs within a block, all uses of the
name specified in the statement refer to the binding of that name
in the top-level namespace.  Names are resolved in the top-level
namespace by searching the global namespace, i.e. the namespace of
the module containing the code block, and in the builtin
namespace, i.e. the namespace of the ``__builtin__`` module.  The
global namespace is searched first.  If the name is not found
there, the builtin namespace is searched.  The global statement
must precede all uses of the name.

If a name is used within a code block, but it is not bound there
and is not declared global, the use is treated as a reference to
the nearest enclosing function region.  (Note: If a region is
contained within a class definition, the name bindings that occur
in the class block are not visible to enclosed functions.)

A class definition is an executable statement that may contain
uses and definitions of names.  These references follow the normal
rules for name resolution.  The namespace of the class definition
becomes the attribute dictionary of the class.

The following operations are name binding operations.  If they
occur within a block, they introduce new local names in the
current block unless there is also a global declaration.

::

    Function definition: def name ...
    Argument declaration: def f(...name...), lambda ...name...
    Class definition: class name ...
    Assignment statement: name = ...
    Import statement: import name, import module as name,
        from module import name
    Implicit assignment: names are bound by for statements and except
        clauses

There are several cases where Python statements are illegal when
used in conjunction with nested scopes that contain free
variables.

If a variable is referenced in an enclosed scope, it is an error
to delete the name.  The compiler will raise a ``SyntaxError`` for
'del name'.

If the wild card form of import (``import *``) is used in a function
and the function contains a nested block with free variables, the
compiler will raise a ``SyntaxError``.

If exec is used in a function and the function contains a nested
block with free variables, the compiler will raise a ``SyntaxError``
unless the exec explicitly specifies the local namespace for the
exec.  (In other words, "exec obj" would be illegal, but
"exec obj in ns" would be legal.)

If a name bound in a function scope is also the name of a module
global name or a standard builtin name, and the function contains
a nested function scope that references the name, the compiler
will issue a warning.  The name resolution rules will result in
different bindings under Python 2.0 than under Python 2.2.  The
warning indicates that the program may not run correctly with all
versions of Python.

Discussion
==========

The specified rules allow names defined in a function to be
referenced in any nested function defined with that function.  The
name resolution rules are typical for statically scoped languages,
with three primary exceptions:

- Names in class scope are not accessible.
- The global statement short-circuits the normal rules.
- Variables are not declared.

Names in class scope are not accessible.  Names are resolved in
the innermost enclosing function scope.  If a class definition
occurs in a chain of nested scopes, the resolution process skips
class definitions.  This rule prevents odd interactions between
class attributes and local variable access.  If a name binding
operation occurs in a class definition, it creates an attribute on
the resulting class object.  To access this variable in a method,
or in a function nested within a method, an attribute reference
must be used, either via self or via the class name.

An alternative would have been to allow name binding in class
scope to behave exactly like name binding in function scope.  This
rule would allow class attributes to be referenced either via
attribute reference or simple name.  This option was ruled out
because it would have been inconsistent with all other forms of
class and instance attribute access, which always use attribute
references.  Code that used simple names would have been obscure.

The global statement short-circuits the normal rules.  Under the
proposal, the global statement has exactly the same effect that it
does for Python 2.0.  It is also noteworthy because it allows name
binding operations performed in one block to change bindings in
another block (the module).

Variables are not declared.  If a name binding operation occurs
anywhere in a function, then that name is treated as local to the
function and all references refer to the local binding.  If a
reference occurs before the name is bound, a NameError is raised.
The only kind of declaration is the global statement, which allows
programs to be written using mutable global variables.  As a
consequence, it is not possible to rebind a name defined in an
enclosing scope.  An assignment operation can only bind a name in
the current scope or in the global scope.  The lack of
declarations and the inability to rebind names in enclosing scopes
are unusual for lexically scoped languages; there is typically a
mechanism to create name bindings (e.g. lambda and let in Scheme)
and a mechanism to change the bindings (set! in Scheme).


Examples
========

A few examples are included to illustrate the way the rules work.

::

    >>> def make_adder(base):
    ...     def adder(x):
    ...         return base + x
    ...     return adder
    >>> add5 = make_adder(5)
    >>> add5(6)
    11

    >>> def make_fact():
    ...     def fact(n):
    ...         if n == 1:
    ...             return 1L
    ...         else:
    ...             return n * fact(n - 1)
    ...     return fact
    >>> fact = make_fact()
    >>> fact(7)
    5040L

    >>> def make_wrapper(obj):
    ...     class Wrapper:
    ...         def __getattr__(self, attr):
    ...             if attr[0] != '_':
    ...                 return getattr(obj, attr)
    ...             else:
    ...                 raise AttributeError, attr
    ...     return Wrapper()
    >>> class Test:
    ...     public = 2
    ...     _private = 3
    >>> w = make_wrapper(Test())
    >>> w.public
    2
    >>> w._private
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
    AttributeError: _private

An example from Tim Peters demonstrates the potential pitfalls of
nested scopes in the absence of declarations::

    i = 6
    def f(x):
        def g():
            print i
        # ...
        # skip to the next page
        # ...
        for i in x:  # ah, i *is* local to f, so this is what g sees
            pass
        g()

The call to ``g()`` will refer to the variable i bound in ``f()`` by the for
loop.  If ``g()`` is called before the loop is executed, a NameError will
be raised.

Backwards compatibility
=======================

There are two kinds of compatibility problems caused by nested
scopes.  In one case, code that behaved one way in earlier
versions behaves differently because of nested scopes.  In the
other cases, certain constructs interact badly with nested scopes
and will trigger SyntaxErrors at compile time.

The following example from Skip Montanaro illustrates the first
kind of problem::

    x = 1
    def f1():
        x = 2
        def inner():
            print x
        inner()

Under the Python 2.0 rules, the print statement inside ``inner()``
refers to the global variable x and will print 1 if ``f1()`` is
called.  Under the new rules, it refers to the ``f1()``'s namespace,
the nearest enclosing scope with a binding.

The problem occurs only when a global variable and a local
variable share the same name and a nested function uses that name
to refer to the global variable.  This is poor programming
practice, because readers will easily confuse the two different
variables.  One example of this problem was found in the Python
standard library during the implementation of nested scopes.

To address this problem, which is unlikely to occur often, the
Python 2.1 compiler (when nested scopes are not enabled) issues a
warning.

The other compatibility problem is caused by the use of ``import *``
and 'exec' in a function body, when that function contains a
nested scope and the contained scope has free variables.  For
example::

    y = 1
    def f():
        exec "y = 'gotcha'" # or from module import *
        def g():
            return y
        ...

At compile-time, the compiler cannot tell whether an exec that
operates on the local namespace or an ``import *`` will introduce
name bindings that shadow the global y.  Thus, it is not possible
to tell whether the reference to y in ``g()`` should refer to the
global or to a local name in ``f()``.

In discussion of the python-list, people argued for both possible
interpretations.  On the one hand, some thought that the reference
in ``g()`` should be bound to a local y if one exists.  One problem
with this interpretation is that it is impossible for a human
reader of the code to determine the binding of y by local
inspection.  It seems likely to introduce subtle bugs.  The other
interpretation is to treat exec and import * as dynamic features
that do not effect static scoping.  Under this interpretation, the
exec and import * would introduce local names, but those names
would never be visible to nested scopes.  In the specific example
above, the code would behave exactly as it did in earlier versions
of Python.

Since each interpretation is problematic and the exact meaning
ambiguous, the compiler raises an exception.  The Python 2.1
compiler issues a warning when nested scopes are not enabled.

A brief review of three Python projects (the standard library,
Zope, and a beta version of PyXPCOM) found four backwards
compatibility issues in approximately 200,000 lines of code.
There was one example of case #1 (subtle behavior change) and two
examples of ``import *`` problems in the standard library.

(The interpretation of the ``import *`` and exec restriction that was
implemented in Python 2.1a2 was much more restrictive, based on
language that in the reference manual that had never been
enforced.  These restrictions were relaxed following the release.)

Compatibility of C API
======================

The implementation causes several Python C API functions to
change, including ``PyCode_New()``.  As a result, C extensions may
need to be updated to work correctly with Python 2.1.

locals() / vars()
=================

These functions return a dictionary containing the current scope's
local variables.  Modifications to the dictionary do not affect
the values of variables.  Under the current rules, the use of
``locals()`` and ``globals()`` allows the program to gain access to all
the namespaces in which names are resolved.

An analogous function will not be provided for nested scopes.
Under this proposal, it will not be possible to gain
dictionary-style access to all visible scopes.

Warnings and Errors
===================

The compiler will issue warnings in Python 2.1 to help identify
programs that may not compile or run correctly under future
versions of Python.  Under Python 2.2 or Python 2.1 if the
``nested_scopes`` future statement is used, which are collectively
referred to as "future semantics" in this section, the compiler
will issue SyntaxErrors in some cases.

The warnings typically apply when a function that contains a
nested function that has free variables.  For example, if function
F contains a function G and G uses the builtin ``len()``, then F is a
function that contains a nested function (G) with a free variable
(len).  The label "free-in-nested" will be used to describe these
functions.

import * used in function scope
-------------------------------

The language reference specifies that ``import *`` may only occur
in a module scope.  (Sec. 6.11)  The implementation of C
Python has supported ``import *`` at the function scope.

If ``import *`` is used in the body of a free-in-nested function,
the compiler will issue a warning.  Under future semantics,
the compiler will raise a ``SyntaxError``.

bare exec in function scope
---------------------------

The exec statement allows two optional expressions following
the keyword "in" that specify the namespaces used for locals
and globals.  An exec statement that omits both of these
namespaces is a bare exec.

If a bare exec is used in the body of a free-in-nested
function, the compiler will issue a warning.  Under future
semantics, the compiler will raise a ``SyntaxError``.

local shadows global
--------------------

If a free-in-nested function has a binding for a local
variable that (1) is used in a nested function and (2) is the
same as a global variable, the compiler will issue a warning.

Rebinding names in enclosing scopes
-----------------------------------

There are technical issues that make it difficult to support
rebinding of names in enclosing scopes, but the primary reason
that it is not allowed in the current proposal is that Guido is
opposed to it.  His motivation: it is difficult to support,
because it would require a new mechanism that would allow the
programmer to specify that an assignment in a block is supposed to
rebind the name in an enclosing block; presumably a keyword or
special syntax (x := 3) would make this possible.  Given that this
would encourage the use of local variables to hold state that is
better stored in a class instance, it's not worth adding new
syntax to make this possible (in Guido's opinion).

The proposed rules allow programmers to achieve the effect of
rebinding, albeit awkwardly.  The name that will be effectively
rebound by enclosed functions is bound to a container object.  In
place of assignment, the program uses modification of the
container to achieve the desired effect::

    def bank_account(initial_balance):
        balance = [initial_balance]
        def deposit(amount):
            balance[0] = balance[0] + amount
            return balance
        def withdraw(amount):
            balance[0] = balance[0] - amount
            return balance
        return deposit, withdraw

Support for rebinding in nested scopes would make this code
clearer.  A class that defines ``deposit()`` and ``withdraw()`` methods
and the balance as an instance variable would be clearer still.
Since classes seem to achieve the same effect in a more
straightforward manner, they are preferred.

Implementation
==============

The implementation for C Python uses flat closures [1]_.  Each def
or lambda expression that is executed will create a closure if the
body of the function or any contained function has free
variables.  Using flat closures, the creation of closures is
somewhat expensive but lookup is cheap.

The implementation adds several new opcodes and two new kinds of
names in code objects.  A variable can be either a cell variable
or a free variable for a particular code object.  A cell variable
is referenced by containing scopes; as a result, the function
where it is defined must allocate separate storage for it on each
invocation.  A free variable is referenced via a function's
closure.

The choice of free closures was made based on three factors.
First, nested functions are presumed to be used infrequently,
deeply nested (several levels of nesting) still less frequently.
Second, lookup of names in a nested scope should be fast.
Third, the use of nested scopes, particularly where a function
that access an enclosing scope is returned, should not prevent
unreferenced objects from being reclaimed by the garbage
collector.

References
==========

.. [1] Luca Cardelli.  Compiling a functional language.  In Proc. of
       the 1984 ACM Conference on Lisp and Functional Programming,
       pp. 208-217, Aug. 1984
       http://citeseer.ist.psu.edu/cardelli84compiling.html

Copyright
=========