python-peps/pep-3150.txt

PEP: 3150
Title: Statement local namespaces (aka "given" clause)
Version: $Revision$
Last-Modified: $Date$
Author: Nick Coghlan <ncoghlan@gmail.com>
Status: Withdrawn
Type: Standards Track
Content-Type: text/x-rst
Created: 2010-07-09
Python-Version: 3.3
Post-History: 2010-07-14, 2011-04-21, 2011-06-13
Resolution: TBD


Abstract
========

This PEP proposes the addition of an optional ``given`` clause to several
Python statements that do not currently have an associated code suite. This
clause will create a statement local namespace for additional names that are
accessible in the associated statement, but do not become part of the
containing namespace.

The primary motivation is to elevate ordinary assignment statements to be
on par with ``class`` and ``def`` statements where the name of the item to
be defined is presented to the reader in advance of the details of how the
value of that item is calculated.

A secondary motivation is to simplify interim calculations in module and
class level code without polluting the resulting namespaces.

There are additional emergent properties of the proposed solution which may
be of interest to some users. Most notably, it is proposed that this clause
use a new kind of scope that performs early binding of variables, potentially
replacing other techniques that achieve the same effect (such as the "default
argument hack").

The specific proposal in this PEP has been informed by various explorations
of this and related concepts over the years (e.g. [1], [2], [3], [6]), and is
inspired to some degree by the ``where`` and ``let`` clauses in Haskell. It
avoids some problems that have been identified in past proposals, but has not
yet itself been subject to the test of implementation.


PEP Deferral
============

Despite the lifting of the language moratorium (PEP 3003) for Python 3.3,
this PEP currently remains in a Deferred state. That means the PEP has to
pass at least *two* hurdles to become part of 3.3.

Firstly, I personally have to be sufficiently convinced of the PEP's value and
feasibility to return it to Draft status. While I do see merit in the concept
of statement local namespaces (otherwise I wouldn't have spent so much time
pondering the idea over the years), I also have grave doubts as to the wisdom
of actually adding it to the language (see "Key Concern" below).

Secondly, Guido van Rossum (or his delegate) will need to accept the PEP. At
the very least, that will not occur until a fully functional draft
implementation for CPython is available, and the other three major Python
implementations (PyPy, Jython, IronPython) have indicated that they consider
it feasible to implement the proposed semantics once they reach the point of
targetting 3.3 compatibility. Input from related projects with a vested
interest in Python's syntax (e.g. Cython) will also be valuable.


Proposal
========

This PEP proposes the addition of an optional ``given`` clause to the
syntax for simple statements which may contain an expression, or may
substitute for such an expression for purely syntactic purposes. The
current list of simple statements that would be affected by this
addition is as follows:

* expression statement
* assignment statement
* augmented assignment statement
* del statement
* return statement
* yield statement
* raise statement
* assert statement
* pass statement

The ``given`` clause would allow subexpressions to be referenced by
name in the header line, with the actual definitions following in
the indented clause. As a simple example::

   c = sqrt(a*a + b*b) given:
       a, b = 3, 4

The ``pass`` statement is included to provide a consistent way to skip
inclusion of a meaningful expression in the header line. While this is not
an intended use case, it isn't one that can be prevented as multiple
alternatives (such as ``...`` and ``()``) remain available even if ``pass``
itself is disallowed.


Rationale
=========

Function and class statements in Python have a unique property
relative to ordinary assignment statements: to some degree, they are
*declarative*. They present the reader of the code with some critical
information about a name that is about to be defined, before
proceeding on with the details of the actual definition in the
function or class body.

The *name* of the object being declared is the first thing stated
after the keyword. Other important information is also given the
honour of preceding the implementation details:

- decorators (which can greatly affect the behaviour of the created
  object, and were placed ahead of even the keyword and name as a matter
  of practicality moreso than aesthetics)
- the docstring (on the first line immediately following the header line)
- parameters, default values and annotations for function definitions
- parent classes, metaclass and optionally other details (depending on
  the metaclass) for class definitions

This PEP proposes to make a similar declarative style available for
arbitrary assignment operations, by permitting the inclusion of a
"given" suite following any simple assignment statement::

    TARGET = [TARGET2 = ... TARGETN =] EXPR given:
        SUITE

By convention, code in the body of the suite should be oriented solely
towards correctly defining the assignment operation carried out in the
header line. The header line operation should also be adequately
descriptive (e.g. through appropriate choices of variable names) to
give a reader a reasonable idea of the purpose of the operation
without reading the body of the suite.

However, while they are the initial motivating use case, limiting this
feature solely to simple assignments would be overly restrictive. Once the
feature is defined at all, it would be quite arbitrary to prevent its use
for augmented assignments, return statements, yield expressions and
arbitrary expressions that may modify the application state.

The ``given`` clause may also function as a more readable
alternative to some uses of lambda expressions and similar
constructs when passing one-off functions to operations
like ``sorted()``.

In module and class level code, the ``given`` clause will serve as a
clear and reliable replacement for usage of the ``del`` statement to keep
interim working variables from polluting the resulting namespace.

One potentially useful way to think of the proposed clause is as a middle
ground between conventional in-line code and separation of an
operation out into a dedicated function.


Keyword Choice
==============

This proposal initially used ``where`` based on the name of a similar
construct in Haskell. However, it has been pointed out that there
are existing Python libraries (such as Numpy [4]) that already use
``where`` in the SQL query condition sense, making that keyword choice
potentially confusing.

While ``given`` may also be used as a variable name (and hence would be
deprecated using the usual ``__future__`` dance for introducing
new keywords), it is associated much more strongly with the desired
"here are some extra variables this expression may use" semantics
for the new clause.

Reusing the ``with`` keyword has also been proposed. This has the
advantage of avoiding the addition of a new keyword, but also has
a high potential for confusion as the ``with`` clause and ``with``
statement would look similar but do completely different things.
That way lies C++ and Perl :)


Syntax Change
=============

Current::

   expr_stmt: testlist_star_expr (augassign (yield_expr|testlist) |
                ('=' (yield_expr|testlist_star_expr))*)
   del_stmt: 'del' exprlist
   pass_stmt: 'pass'
   return_stmt: 'return' [testlist]
   yield_stmt: yield_expr
   raise_stmt: 'raise' [test ['from' test]]
   assert_stmt: 'assert' test [',' test]


New::

   expr_stmt: testlist_star_expr (augassign (yield_expr|testlist) |
                ('=' (yield_expr|testlist_star_expr))*) [given_clause]
   del_stmt: 'del' exprlist [given_clause]
   pass_stmt: 'pass' [given_clause]
   return_stmt: 'return' [testlist] [given_clause]
   yield_stmt: yield_expr [given_clause]
   raise_stmt: 'raise' [test ['from' test]] [given_clause]
   assert_stmt: 'assert' test [',' test] [given_clause]
   given_clause: "given" ":" suite

(Note that expr_stmt in the grammar covers assignment and augmented
assignment in addition to simple expression statements)

The new clause is added as an optional element of the existing statements
rather than as a new kind of compound statement in order to avoid creating
an ambiguity in the grammar. It is applied only to the specific elements
listed so that nonsense like the following is disallowed::

   break given:
       a = b = 1

However, the precise Grammar change described above is inadequate, as it
creates problems for the definition of simple_stmt (which allows chaining of
multiple single line statements with ";" rather than "\\n").

So the above syntax change should instead be taken as a statement of intent.
Any actual proposal would need to resolve the simple_stmt parsing problem
before it could be seriously considered. This would likely require a
non-trivial restructuring of the grammar, breaking up small_stmt and
flow_stmt to separate the statements that potentially contain arbitrary
subexpressions and then allowing a single one of those statements with
a ``given`` clause at the simple_stmt level. Something along the lines of::

   stmt: simple_stmt | given_stmt | compound_stmt
   simple_stmt: small_stmt (';' (small_stmt | subexpr_stmt))* [';'] NEWLINE
   small_stmt: (pass_stmt | flow_stmt | import_stmt |
                global_stmt | nonlocal_stmt)
   flow_stmt: break_stmt | continue_stmt
   given_stmt: subexpr_stmt (given_clause |
                 (';' (small_stmt | subexpr_stmt))* [';']) NEWLINE
   subexpr_stmt: expr_stmt | del_stmt | flow_subexpr_stmt | assert_stmt
   flow_subexpr_stmt: return_stmt | raise_stmt | yield_stmt
   given_clause: "given" ":" suite

For reference, here are the current definitions at that level::

   stmt: simple_stmt | compound_stmt
   simple_stmt: small_stmt (';' small_stmt)* [';'] NEWLINE
   small_stmt: (expr_stmt | del_stmt | pass_stmt | flow_stmt |
                import_stmt | global_stmt | nonlocal_stmt | assert_stmt)
   flow_stmt: break_stmt | continue_stmt | return_stmt | raise_stmt | yield_stmt


Possible Implementation Strategy
================================

Torture Test
------------

An implementation of this PEP should support execution of the following
code at module, class and function scope::

   b = {}
   a = b[f(a)] = x given:
       x = 42
       def f(x):
           return x
   assert "x" not in locals()
   assert "f" not in locals()
   assert a == 42
   assert d[42] == 42 given:
       d = b
   assert "d" not in locals()
   y = y given:
       x = 42
       def f(): pass
       y = locals("x"), f.__name__
   assert "x" not in locals()
   assert "f" not in locals()
   assert y == (42, "f")

Most naive implementations will choke on the first complex assignment,
while less naive but still broken implementations will fail when
the torture test is executed at class scope. Renaming based strategies
struggle to support ``locals()`` correctly and also have problems with
class and function ``__name__`` attributes.

And yes, that's a perfectly well-defined assignment statement. Insane,
you might rightly say, but legal::

   >>> def f(x): return x
   ...
   >>> x = 42
   >>> b = {}
   >>> a = b[f(a)] = x
   >>> a
   42
   >>> b
   {42: 42}

Details of Proposed Semantics
-----------------------------

AKA How Class Scopes Screw You When Attempting To Implement This

The natural idea when setting out to implement this concept is to
use an ordinary nested function scope. This doesn't work for the
two reasons mentioned in the Torture Test section above:

* Non-local variables are not your friend because they ignore class scopes
  and (when writing back to the outer scope) aren't really on speaking
  terms with module scopes either.

* Return-based semantics struggle with complex assignment statements
  like the one in the torture test

The second thought is generally some kind of hidden renaming strategy. This
also creates problems, as Python exposes variables names via the ``locals()``
dictionary and class and function ``__name__`` attributes.

The most promising approach is one based on symtable analysis and
copy-in-copy-out referencing semantics to move any required name
bindings between the inner and outer scopes. The torture test above
would then translate to something like the following::

   b = {}
   def _anon1(b): # 'b' reference copied in
       x = 42
       def f(x):
           return x
       a = b[f(a)] = x
       return a # 'a' reference copied out
   a = _anon1(b)
   assert "x" not in locals()
   assert "f" not in locals()
   assert a == 42
   def _anon2(b) # 'b' reference copied in
       d = b
       assert d[42] == 42
       # Nothing to copy out (not an assignment)
   _anon2()
   assert "d" not in locals()
   def _anon3() # Nothing to copy in (no references to other variables)
       x = 42
       def f(): pass
       y = locals("x"), f.__name__
       y = y    # Assuming no optimisation of special cases
       return y # 'y' reference copied out
   y = _anon3()
   assert "x" not in locals()
   assert "f" not in locals()
   assert y == (42, "f")

However, as noted in the abstract, an actual implementation of
this idea has never been tried.

Detailed Semantics #1: Early Binding of Variable References
-----------------------------------------------------------

The copy-in-copy-out semantics mean that all variable references from a
``given`` clause will exhibit early binding behaviour, in contrast to the
late binding typically seen with references to closure variables and globals
in ordinary functions. This behaviour will allow the ``given`` clause to
be used as a substitute for the default argument hack when early binding
behaviour is desired::

  # Current Python (late binding)
  seq = []
  for i in range(10):
    def f():
      return i
    seq.append(f)
  assert [f() for f in seq] == [9]*10

  # Current Python (early binding via default argument hack)
  seq = []
  for i in range(10):
    def f(_i=i):
      return i
    seq.append(f)
  assert [f() for f in seq] == list(range(10))

  # Early binding via given clause
  seq = []
  for i in range(10):
    seq.append(f) given:
      def f():
        return i
  assert [f() for f in seq] == list(range(10))

Note that the current intention is for the copy-in/copy-out semantics to
apply only to names defined in the local scope containing the ``given``
clause. Name in outer scopes will be referenced as normal.

This intention is subject to revision based on feedback and practicalities
of implementation.

Detailed Semantics #2: Handling of ``nonlocal`` and ``global``
--------------------------------------------------------------

``nonlocal`` and ``global`` will largely operate as if the anonymous
functions were defined as in the expansion above. However, they will also
override the default early-binding semantics from names from the containing
scope.

This intention is subject to revision based on feedback and practicalities
of implementation.

Detailed Semantics #3: Handling of ``break`` and ``continue``
-------------------------------------------------------------

``break`` and ``continue`` will operate as if the anonymous functions were
defined as in the expansion above. They will be syntax errors if they occur
in the ``given`` clause suite but will work normally if they appear within
a ``for`` or ``while`` loop as part of that suite.

Detailed Semantics #4: Handling of ``return`` and ``yield``
-------------------------------------------------------------

``return`` and ``yield`` are explicitly disallowed in the ``given`` clause
suite and will be syntax errors if they occur. They will work normally if
they appear within a ``def`` statement within that suite.


Examples
========

Defining "one-off" classes which typically only have a single instance::

  # Current Python (instantiation after definition)
  class public_name():
    ... # However many lines
  public_name = public_name(*params)

  # Becomes:
  public_name = MeaningfulClassName(*params) given:
    class MeaningfulClassName():
      ... # Should trawl the stdlib for an example of doing this

Calculating attributes without polluting the local namespace (from os.py)::

  # Current Python (manual namespace cleanup)
  def _createenviron():
    ... # 27 line function

  environ = _createenviron()
  del _createenviron

  # Becomes:
  environ = _createenviron() given:
      def _createenviron():
        ... # 27 line function

Replacing default argument hack (from functools.lru_cache)::

  # Current Python (default argument hack)
  def decorating_function(user_function,
                 tuple=tuple, sorted=sorted, len=len, KeyError=KeyError):
    ... # 60 line function
  return decorating_function

  # Becomes:
  return decorating_function given:
    # Cell variables rather than locals, but should give similar speedup
    tuple, sorted, len, KeyError = tuple, sorted, len, KeyError
    def decorating_function(user_function):
      ... # 60 line function

  # This example also nicely makes it clear that there is nothing in the
  # function after the nested function definition. Due to additional
  # nested functions, that isn't entirely clear in the current code.


Anticipated Objections
======================

* Two Ways To Do It: a lot of code may now be written with values
  defined either before the expression where they are used or
  afterwards in a ``given`` clause, creating two ways to do it,
  without an obvious way of choosing between them.

* Out of Order Execution: the ``given`` clause makes execution
  jump around a little strangely, as the body of the ``given``
  clause is executed before the simple statement in the clause
  header. The closest any other part of Python comes to this
  is the out of order evaluation in list comprehensions,
  generator expressions and conditional expressions.

* Harmful to Introspection: poking around in module and class internals
  is an invaluable tool for white-box testing and interactive debugging.
  The ``given`` clause will be quite effective at preventing access to
  temporary state used during calculations (although no more so than
  current usage of ``del`` statements in that regard)

These objections should not be dismissed lightly - the proposal
in this PEP needs to be subjected to the test of application to
a large code base (such as the standard library) in a search
for examples where the readability of real world code is genuinely
enhanced.

New PEP 8 guidelines would also need to be developed to provide
appropriate direction on when to use the ``given`` clause over
ordinary variable assignments. Some thoughts on possible guidelines are
provided at [7]


Possible Additions
==================

* The current proposal allows the addition of a ``given`` clause only
  for simple statements. Extending the idea to allow the use of
  compound statements would be quite possible, but doing so raises
  serious readability concerns (as values defined in the ``given``
  clause may be used well before they are defined, exactly the kind
  of readability trap that other features like decorators and ``with``
  statements are designed to eliminate)

* Currently only the outermost clause of comprehensions and generator
  expressions can reference the surrounding namespace when executed
  at class level. If this proposal is implemented successfully, the
  associated namespace semantics could allow that restriction to be
  lifted. There would be backwards compatibility implications in doing
  so as existing code may be relying on the behaviour of ignoring
  class level variables, but the idea is worth considering.


Reference Implementation
========================

None as yet. If you want a crash course in Python namespace
semantics and code compilation, feel free to try ;)


Key Concern
===========

If a decision on the acceptance or rejection of this PEP had to be made
immediately, rejection would be far more likely. Unlike the previous
major syntax addition to Python (PEP 343's ``with`` statement), this
PEP as yet has no "killer application" of common code that is clearly and
obviously improved through the use of the new syntax. The ``with`` statement
(in conjunction with the generator enhancements in PEP 342) allowed
exception handling to be factored out into context managers in a way
that had never before been possible. Code using the new statement was
not only easier to read, but much easier to write correctly in the
first place.

In the case of this PEP. however, the "Two Ways to Do It" objection is a
strong one. While the ability to break out subexpresions of a statement
without having to worry about name clashes with the rest of a
function or script and without distracting from the operation that is
the ultimate aim of the statement is potentially nice to have as a
language feature, it doesn't really provide significant expressive power
over and above what is already possible by assigning subexpressions to
ordinary local variables before the statement of interest. In particular,
explaining to new Python programmers when it is best to use a ``given``
clause and when to use normal local variables is likely to be challenging
and an unnecessary distraction.

"It might be kinda, sorta, nice to have, sometimes" really isn't a strong
argument for a new syntactic construct (particularly one this complicated).
"Status quo wins a stalemate" [5] is a very useful design principle, and I'm
not yet convinced that this PEP clears that hurdle.

The case for it has definitely strengthened over time though, which is why
this PEP remains Deferred rather than Rejected.


TO-DO
=====

* Mention two-suite in-order variants (and explain why they're even more
  pointless than the specific idea in the PEP)
* Mention PEP 359 and possible uses for locals() in the ``given`` clause


References
==========

.. [1] http://mail.python.org/pipermail/python-ideas/2010-June/007476.html

.. [2] http://mail.python.org/pipermail/python-ideas/2010-July/007584.html

.. [3] http://mail.python.org/pipermail/python-ideas/2009-July/005132.html

.. [4] http://mail.python.org/pipermail/python-ideas/2010-July/007596.html

.. [5] http://www.boredomandlaziness.org/2011/02/status-quo-wins-stalemate.html

.. [6] http://mail.python.org/pipermail/python-ideas/2011-April/009863.html

.. [7] http://mail.python.org/pipermail/python-ideas/2011-April/009869.html

Copyright
=========

This document has been placed in the public domain.



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