pep-0532.txt

PEP: 532
Title: A circuit breaking protocol and binary operators
Version: $Revision$
Last-Modified: $Date$
Author: Nick Coghlan <ncoghlan@gmail.com>,
        Mark E. Haase <mehaase@gmail.com>
Status: Deferred
Type: Standards Track
Content-Type: text/x-rst
Created: 30-Oct-2016
Python-Version: 3.8
Post-History: 05-Nov-2016

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

Further consideration of this PEP has been deferred until Python 3.8 at the
earliest.

Abstract
========

Inspired by :pep:`335`, :pep:`505`, :pep:`531`, and the related discussions, this PEP
proposes the definition of a new circuit breaking protocol (using the
method names ``__then__`` and ``__else__``) that provides a common underlying
semantic foundation for:

* conditional expressions: ``LHS if COND else RHS``
* logical conjunction: ``LHS and RHS``
* logical disjunction: ``LHS or RHS``
* the None-aware operators proposed in :pep:`505`
* the rich comparison chaining model proposed in :pep:`535`

Taking advantage of the new protocol, it further proposes that the definition
of conditional expressions be revised to also permit the use of ``if`` and
``else`` respectively as right-associative and left-associative general
purpose short-circuiting operators:

* Right-associative short-circuiting: ``LHS if RHS``
* Left-associative short-circuiting: ``LHS else RHS``

In order to make logical inversion (``not EXPR``) consistent with the above
changes, it also proposes the introduction of a new logical inversion protocol
(using the method name ``__not__``).

To force short-circuiting of a circuit breaker without having to evaluate
the expression creating it twice, a new ``operator.short_circuit(obj)``
helper function will be added to the operator module.

Finally, a new standard ``types.CircuitBreaker`` type is proposed to decouple
an object's truth value (as used to determine control flow) from the value
it returns from short-circuited circuit breaking expressions, with the
following factory functions added to the operator module to represent
particularly common switching idioms:

* switching on ``bool(obj)``: ``operator.true(obj)``
* switching on ``not bool(obj)``: ``operator.false(obj)``
* switching on ``obj is value``: ``operator.is_sentinel(obj, value)``
* switching on ``obj is not value``: ``operator.is_not_sentinel(obj, value)``


Relationship with other PEPs
============================

This PEP builds on an extended history of work in other proposals. Some of
the key proposals are discussed below.


PEP 531: Existence checking protocol
------------------------------------

This PEP is a direct successor to :pep:`531`, replacing the existence checking
protocol and the new ``?then`` and ``?else`` syntactic operators defined there
with the new circuit breaking protocol and adjustments to conditional
expressions and the ``not`` operator.


PEP 505: None-aware operators
-----------------------------

This PEP complements the None-aware operator proposals in :pep:`505`, by offering
an underlying protocol-driven semantic framework that explains their
short-circuiting behaviour as highly optimised syntactic sugar for particular
uses of conditional expressions.

Given the changes proposed by this PEP:

* ``LHS ?? RHS`` would roughly be ``is_not_sentinel(LHS, None) else RHS``
* ``EXPR?.attr`` would roughly be ``EXPR.attr if is_not_sentinel(EXPR, None)``
* ``EXPR?[key]`` would roughly be ``EXPR[key] if is_not_sentinel(EXPR, None)``

In all three cases, the dedicated syntactic form would be optimised to avoid
actually creating the circuit breaker instance and instead implement the
underlying control flow directly. In the latter two cases, the syntactic form
would also avoid evaluating ``EXPR`` twice.

This means that while the None-aware operators would remain highly specialised
and specific to None, other sentinel values would still be usable through the
more general protocol-driven proposal in this PEP.


PEP 335: Overloadable Boolean operators
---------------------------------------

:pep:`335` proposed the ability to overload the short-circuiting ``and`` and
``or`` operators directly, with the ability to overload the semantics of
comparison chaining being one of the consequences of that change. The
proposal in an earlier version of this PEP to instead handle the element-wise
comparison use case by changing the semantic definition of comparison chaining
is drawn directly from Guido's rejection of :pep:`335` [1_].

However, initial feedback on this PEP indicated that the number of different
proposals that it covered made it difficult to read, so that part of the
proposal has been separated out as :pep:`535`.


PEP 535: Rich comparison chaining
---------------------------------

As noted above, :pep:`535` is a proposal to build on the circuit breaking protocol
defined in this PEP in order to expand the rich comparison support introduced
in :pep:`207` to also handle comparison chaining operations like
``LEFT_BOUND < VALUE < RIGHT_BOUND``.


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

The circuit breaking protocol (``if-else``)
-------------------------------------------

Conditional expressions (``LHS if COND else RHS``) are currently interpreted
as an expression level equivalent to::

    if COND:
        _expr_result = LHS
    else:
        _expr_result = RHS

This PEP proposes changing that expansion to allow the checked condition to
implement a new "circuit breaking" protocol that allows it to see, and
potentially alter, the result of either or both branches of the expression::

    _cb = COND
    _type_cb = type(cb)
    if _cb:
        _expr_result = LHS
        if hasattr(_type_cb, "__then__"):
            _expr_result = _type_cb.__then__(_cb, _expr_result)
    else:
        _expr_result = RHS
        if hasattr(_type_cb, "__else__"):
            _expr_result = _type_cb.__else__(_cb, _expr_result)

As shown, interpreter implementations would be required to access only the
protocol method needed for the branch of the conditional expression that is
actually executed. Consistent with other protocol methods, the special methods
would be looked up via the circuit breaker's type, rather than directly on the
instance.


Circuit breaking operators (binary ``if`` and binary ``else``)
--------------------------------------------------------------

The proposed name of the protocol doesn't come from the proposed changes to
the semantics of conditional expressions. Rather, it comes from the proposed
addition of ``if`` and ``else`` as general purpose protocol driven
short-circuiting operators to complement the existing ``True`` and ``False``
based short-circuiting operators (``or`` and ``and``, respectively) as well
as the ``None`` based short-circuiting operator proposed in :pep:`505` (``??``).

Together, these two operators would be known as the circuit breaking operators.

In order to support this usage, the definition of conditional expressions in
the language grammar would be updated to make both the ``if`` clause and
the ``else`` clause optional::

    test: else_test ['if' or_test ['else' test]] | lambdef
    else_test: or_test ['else' test]

Note that we would need to avoid the apparent simplification to
``else_test ('if' else_test)*`` in order to make it easier for compiler
implementations to correctly preserve the semantics of normal conditional
expressions.

The definition of the ``test_nocond`` node in the grammar (which deliberately
excludes conditional expressions) would remain unchanged, so the circuit
breaking operators would require parentheses when used in the ``if``
clause of comprehensions and generator expressions just as conditional
expressions themselves do.

This grammar definition means precedence/associativity in the otherwise
ambiguous case of ``expr1 if cond else expr2 else expr3`` resolves as
``(expr1 if cond else expr2) else epxr3``. However, a guideline will also be
added to :pep:`8` to say "don't do that", as such a construct will be inherently
confusing for readers, regardless of how the interpreter executes it.

The right-associative circuit breaking operator (``LHS if RHS``) would then
be expanded as follows::

    _cb = RHS
    _expr_result = LHS if _cb else _cb

While the left-associative circuit breaking operator (``LHS else RHS``) would
be expanded as::

    _cb = LHS
    _expr_result = _cb if _cb else RHS

The key point to note in both cases is that when the circuit breaking
expression short-circuits, the condition expression is used as the result of
the expression *unless* the condition is a circuit breaker. In the latter
case, the appropriate circuit breaker protocol method is called as usual, but
the circuit breaker itself is supplied as the method argument.

This allows circuit breakers to reliably detect short-circuiting by checking
for cases when the argument passed in as the candidate expression result is
``self``.


Overloading logical inversion (``not``)
---------------------------------------

Any circuit breaker definition will have a logical inverse that is still a
circuit breaker, but inverts the answer as to when to short circuit the
expression evaluation. For example, the ``operator.true`` and
``operator.false`` circuit breakers proposed in this PEP are each other's
logical inverse.

A new protocol method, ``__not__(self)``, will be introduced to permit circuit
breakers and other types to override ``not`` expressions to return their
logical inverse rather than a coerced boolean result.

To preserve the semantics of existing language optimisations (such as
eliminating double negations directly in a boolean context as redundant),
``__not__`` implementations will be required to respect the following
invariant::

    assert not bool(obj) == bool(not obj)

However, symmetric circuit breakers (those that implement all of ``__bool__``,
``__not__``, ``__then__`` and ``__else__``) would only be expected to respect
the full semantics of boolean logic when all circuit breakers involved in the
expression are using a consistent definition of "truth". This is covered
further in `Respecting De Morgan's Laws`_.


Forcing short-circuiting behaviour
----------------------------------

Invocation of a circuit breaker's short-circuiting behaviour can be forced by
using it as all three operands in a conditional expression::

    obj if obj else obj

Or, equivalently, as both operands in a circuit breaking expression::

    obj if obj
    obj else obj

Rather than requiring the using of any of these patterns, this PEP proposes
to add a dedicated function to the ``operator`` to explicitly short-circuit
a circuit breaker, while passing other objects through unmodified::

    def short_circuit(obj)
        """Replace circuit breakers with their short-circuited result

        Passes other input values through unmodified.
        """
        return obj if obj else obj


Circuit breaking identity comparisons (``is`` and ``is not``)
-------------------------------------------------------------

In the absence of any standard circuit breakers, the proposed ``if`` and
``else`` operators would largely just be unusual spellings of the existing
``and`` and ``or`` logical operators.

However, this PEP further proposes to provide a new general purpose
``types.CircuitBreaker`` type that implements the appropriate short
circuiting logic, as well as factory functions in the operator module
that correspond to the ``is`` and ``is not`` operators.

These would be defined in such a way that the following expressions produce
``VALUE`` rather than ``False`` when the conditional check fails::

    EXPR if is_sentinel(VALUE, SENTINEL)
    EXPR if is_not_sentinel(VALUE, SENTINEL)

And similarly, these would produce ``VALUE`` rather than ``True`` when the
conditional check succeeds::

    is_sentinel(VALUE, SENTINEL) else EXPR
    is_not_sentinel(VALUE, SENTINEL) else EXPR

In effect, these comparisons would be defined such that the leading
``VALUE if`` and trailing ``else VALUE`` clauses can be omitted as implied in
expressions of the following forms::

    # To handle "if" expressions, " else VALUE" is implied when omitted
    EXPR if is_sentinel(VALUE, SENTINEL) else VALUE
    EXPR if is_not_sentinel(VALUE, SENTINEL) else VALUE
    # To handle "else" expressions, "VALUE if " is implied when omitted
    VALUE if is_sentinel(VALUE, SENTINEL) else EXPR
    VALUE if is_not_sentinel(VALUE, SENTINEL) else EXPR

The proposed ``types.CircuitBreaker`` type would represent this behaviour
programmatically as follows::

    class CircuitBreaker:
        """Simple circuit breaker type"""
        def __init__(self, value, bool_value):
            self.value = value
            self.bool_value = bool(bool_value)
        def __bool__(self):
            return self.bool_value
        def __not__(self):
            return CircuitBreaker(self.value, not self.bool_value)
        def __then__(self, result):
            if result is self:
                return self.value
            return result
        def __else__(self, result):
            if result is self:
                return self.value
            return result

The key characteristic of these circuit breakers is that they are *ephemeral*:
when they are told that short circuiting has taken place (by receiving a
reference to themselves as the candidate expression result), they return the
original value, rather than the circuit breaking wrapper.

The short-circuiting detection is defined such that the wrapper will always
be removed if you explicitly pass the same circuit breaker instance to both
sides of a circuit breaking operator or use one as all three operands in a
conditional expression::

    breaker = types.CircuitBreaker(foo, foo is None)
    assert operator.short_circuit(breaker) is foo
    assert (breaker if breaker) is foo
    assert (breaker else breaker) is foo
    assert (breaker if breaker else breaker) is foo
    breaker = types.CircuitBreaker(foo, foo is not None)
    assert operator.short_circuit(breaker) is foo
    assert (breaker if breaker) is foo
    assert (breaker else breaker) is foo
    assert (breaker if breaker else breaker) is foo

The factory functions in the ``operator`` module would then make it
straightforward to create circuit breakers that correspond to identity
checks using the ``is`` and ``is not`` operators::

    def is_sentinel(value, sentinel):
        """Returns a circuit breaker switching on 'value is sentinel'"""
        return types.CircuitBreaker(value, value is sentinel)

    def is_not_sentinel(value, sentinel):
        """Returns a circuit breaker switching on 'value is not sentinel'"""
        return types.CircuitBreaker(value, value is not sentinel)


Truth checking comparisons
--------------------------

Due to their short-circuiting nature, the runtime logic underlying the ``and``
and ``or`` operators has never previously been accessible through the
``operator`` or ``types`` modules.

The introduction of circuit breaking operators and circuit breakers allows
that logic to be captured in the operator module as follows::

    def true(value):
        """Returns a circuit breaker switching on 'bool(value)'"""
        return types.CircuitBreaker(value, bool(value))

    def false(value):
        """Returns a circuit breaker switching on 'not bool(value)'"""
        return types.CircuitBreaker(value, not bool(value))

* ``LHS or RHS`` would be effectively ``true(LHS) else RHS``
* ``LHS and RHS`` would be effectively ``false(LHS) else RHS``

No actual change would take place in these operator definitions, the new
circuit breaking protocol and operators would just provide a way to make the
control flow logic programmable, rather than hardcoding the sense of the check
at development time.

Respecting the rules of boolean logic, these expressions could also be
expanded in their inverted form by using the right-associative circuit
breaking operator instead:

* ``LHS or RHS`` would be effectively ``RHS if false(LHS)``
* ``LHS and RHS`` would be effectively ``RHS if true(LHS)``


None-aware operators
--------------------

If both this PEP and :pep:`505`'s None-aware operators were accepted, then the
proposed ``is_sentinel`` and ``is_not_sentinel`` circuit breaker factories
would be used to encapsulate the notion of "None checking": seeing if a value
is ``None`` and either falling back to an alternative value (an operation known
as "None-coalescing") or passing it through as the result of the overall
expression (an operation known as "None-severing" or "None-propagating").

Given these circuit breakers, ``LHS ?? RHS`` would be roughly equivalent to
both of the following:

* ``is_not_sentinel(LHS, None) else RHS``
* ``RHS if is_sentinel(LHS, None)``

Due to the way they inject control flow into attribute lookup and subscripting
operations, None-aware attribute access and None-aware subscripting can't be
expressed directly in terms of the circuit breaking operators, but they can
still be defined in terms of the underlying circuit breaking protocol.

In those terms, ``EXPR?.ATTR[KEY].SUBATTR()`` would be semantically
equivalent to::

    _lookup_base = EXPR
    _circuit_breaker = is_not_sentinel(_lookup_base, None)
    _expr_result = _lookup_base.ATTR[KEY].SUBATTR() if _circuit_breaker

Similarly, ``EXPR?[KEY].ATTR.SUBATTR()`` would be semantically equivalent
to::

    _lookup_base = EXPR
    _circuit_breaker = is_not_sentinel(_lookup_base, None)
    _expr_result = _lookup_base[KEY].ATTR.SUBATTR() if _circuit_breaker

The actual implementations of the None-aware operators would presumably be
optimised to skip actually creating the circuit breaker instance, but the
above expansions would still provide an accurate description of the observable
behaviour of the operators at runtime.


Rich chained comparisons
------------------------

Refer to :pep:`535` for a detailed discussion of this possible use case.


Other conditional constructs
----------------------------

No changes are proposed to if statements, while statements, comprehensions,
or generator expressions, as the boolean clauses they contain are used
entirely for control flow purposes and never return a result as such.

However, it's worth noting that while such proposals are outside the scope of
this PEP, the circuit breaking protocol defined here would already be
sufficient to support constructs like::

    def is_not_none(obj):
        return is_sentinel(obj, None)

    while is_not_none(dynamic_query()) as result:
        ... # Code using result

and::

    if is_not_none(re.search(pattern, text)) as match:
        ... # Code using match

This could be done by assigning the result of
``operator.short_circuit(CONDITION)`` to the name given in the ``as`` clause,
rather than assigning ``CONDITION`` to the given name directly.


Style guide recommendations
---------------------------

The following additions to :pep:`8` are proposed in relation to the new features
introduced by this PEP:

* Avoid combining conditional expressions (``if-else``) and the standalone
  circuit breaking operators (``if`` and ``else``) in a single expression -
  use one or the other depending on the situation, but not both.

* Avoid using conditional expressions (``if-else``) and the standalone
  circuit breaking operators (``if`` and ``else``) as part of ``if``
  conditions in ``if`` statements and the filter clauses of comprehensions
  and generator expressions.


Rationale
=========

Adding new operators
--------------------

Similar to :pep:`335`, early drafts of this PEP focused on making the existing
``and`` and ``or`` operators less rigid in their interpretation, rather than
proposing new operators. However, this proved to be problematic for a few key
reasons:

* the ``and`` and ``or`` operators have a long established and stable meaning,
  so readers would inevitably be surprised if their meaning now became
  dependent on the type of the left operand. Even new users would be confused
  by this change due to 25+ years of teaching material that assumes the
  current well-known semantics for these operators
* Python interpreter implementations, including CPython, have taken advantage
  of the existing semantics of ``and`` and ``or`` when defining runtime and
  compile time optimisations, which would all need to be reviewed and
  potentially discarded if the semantics of those operations changed
* it isn't clear what names would be appropriate for the new methods needed
  to define the protocol

Proposing short-circuiting binary variants of the existing ``if-else`` ternary
operator instead resolves all of those issues:

* the runtime semantics of ``and`` and ``or`` remain entirely unchanged
* while the semantics of the unary ``not`` operator do change, the invariant
  required of ``__not__`` implementations means that existing expression
  optimisations in boolean contexts will remain valid.
* ``__else__`` is the short-circuiting outcome for ``if`` expressions due to
  the absence of a trailing ``else`` clause
* ``__then__`` is the short-circuiting outcome for ``else`` expressions due to
  the absence of a leading ``if`` clause (this connection would be even clearer
  if the method name was ``__if__``, but that would be ambiguous given the
  other uses of the ``if`` keyword that won't invoke the circuit breaking
  protocol)


Naming the operator and protocol
--------------------------------

The names "circuit breaking operator", "circuit breaking protocol" and
"circuit breaker" are all inspired by the phrase "short circuiting operator":
the general language design term for operators that only conditionally
evaluate their right operand.

The electrical analogy is that circuit breakers in Python detect and handle
short circuits in expressions before they trigger any exceptions similar to the
way that circuit breakers detect and handle short circuits in electrical
systems before they damage any equipment or harm any humans.

The Python level analogy is that just as a ``break`` statement lets you
terminate a loop before it reaches its natural conclusion, a circuit breaking
expression lets you terminate evaluation of the expression and produce a result
immediately.


Using existing keywords
-----------------------

Using existing keywords has the benefit of allowing the new operators to
be introduced without a ``__future__`` statement.

``if`` and ``else`` are semantically appropriate for the proposed new protocol,
and the only additional syntactic ambiguity introduced arises when the new
operators are combined with the explicit ``if-else`` conditional expression
syntax.

The PEP handles that ambiguity by explicitly specifying how it should be
handled by interpreter implementers, but proposing to point out in :pep:`8`
that even though interpreters will understand it, human readers probably
won't, and hence it won't be a good idea to use both conditional expressions
and the circuit breaking operators in a single expression.


Naming the protocol methods
---------------------------

Naming the ``__else__`` method was straightforward, as reusing the operator
keyword name results in a special method name that is both obvious and
unambiguous.

Naming the ``__then__`` method was less straightforward, as there was another
possible option in using the keyword-based name ``__if__``.

The problem with ``__if__`` is that there would continue to be many cases
where the ``if`` keyword appeared, with an expression to its immediate right,
but the ``__if__`` special method would not be invoked. Instead, the
``bool()`` builtin and its underlying special methods (``__bool__``,
``__len__``) would be invoked, while ``__if__`` had no effect.

With the boolean protocol already playing a part in conditional expressions and
the new circuit breaking protocol, the less ambiguous name ``__then__`` was
chosen based on the terminology commonly used in computer science and
programming language design to describe the first clause of an ``if``
statement.


Making binary ``if`` right-associative
--------------------------------------

The precedent set by conditional expressions means that a binary
short-circuiting ``if`` expression must necessarily have the condition on the
right as a matter of consistency.

With the right operand always being evaluated first, and the left operand not
being evaluated at all if the right operand is true in a boolean context,
the natural outcome is a right-associative operator.


Naming the standard circuit breakers
------------------------------------

When used solely with the left-associative circuit breaking operator,
explicit circuit breaker names for unary checks read well if they start with
the preposition ``if_``::

    operator.if_true(LHS) else RHS
    operator.if_false(LHS) else RHS

However, incorporating the ``if_`` doesn't read as well when performing
logical inversion::

    not operator.if_true(LHS) else RHS
    not operator.if_false(LHS) else RHS

Or when using the right-associative circuit breaking operator::

    LHS if operator.if_true(RHS)
    LHS if operator.if_false(RHS)

Or when naming a binary comparison operation::

    operator.if_is_sentinel(VALUE, SENTINEL) else EXPR
    operator.if_is_not_sentinel(VALUE, SENTINEL) else EXPR

By contrast, omitting the preposition from the circuit breaker name gives a
result that reads reasonably well in all forms for unary checks::

    operator.true(LHS) else RHS       # Preceding "LHS if " implied
    operator.false(LHS) else RHS      # Preceding "LHS if " implied
    not operator.true(LHS) else RHS   # Preceding "LHS if " implied
    not operator.false(LHS) else RHS  # Preceding "LHS if " implied
    LHS if operator.true(RHS)         # Trailing " else RHS" implied
    LHS if operator.false(RHS)        # Trailing " else RHS" implied
    LHS if not operator.true(RHS)     # Trailing " else RHS" implied
    LHS if not operator.false(RHS)    # Trailing " else RHS" implied

And also reads well for binary checks::

    operator.is_sentinel(VALUE, SENTINEL) else EXPR
    operator.is_not_sentinel(VALUE, SENTINEL) else EXPR
    EXPR if operator.is_sentinel(VALUE, SENTINEL)
    EXPR if operator.is_not_sentinel(VALUE, SENTINEL)


Risks and concerns
==================

This PEP has been designed specifically to address the risks and concerns
raised when discussing PEPs 335, 505 and 531.

* it defines new operators and adjusts the definition of chained comparison
  (in a separate PEP) rather than impacting the existing ``and`` and ``or``
  operators
* the proposed new operators are general purpose short-circuiting binary
  operators that can even be used to express the existing semantics of ``and``
  and ``or`` rather than focusing solely and inflexibly on identity checking
  against ``None``
* the changes to the ``not`` unary operator and the ``is`` and ``is not``
  binary comparison operators are defined in such a way that control flow
  optimisations based on the existing semantics remain valid

One consequence of this approach is that this PEP *on its own* doesn't produce
much in the way of direct benefits to end users aside from making it possible
to omit some common ``None if`` prefixes and ``else None`` suffixes from
particular forms of conditional expression.

Instead, what it mainly provides is a common foundation that would allow the
None-aware operator proposals in :pep:`505` and the rich comparison chaining
proposal in :pep:`535` to be pursued atop a common underlying semantic framework
that would also be shared with conditional expressions and the existing ``and``
and ``or`` operators.


Design Discussion
=================

Protocol walk-through
---------------------

The following diagram illustrates the core concepts behind the circuit
breaking protocol (although it glosses over the technical detail of looking
up the special methods via the type rather than the instance):

.. image:: pep-0532/circuit-breaking-protocol.svg
   :class: invert-in-dark-mode
   :alt: diagram of circuit breaking protocol applied to ternary expression

We will work through the following expression::

    >>> def is_not_none(obj):
    ...     return operator.is_not_sentinel(obj, None)
    >>> x if is_not_none(data.get("key")) else y

``is_not_none`` is a helper function that invokes the proposed
``operator.is_not_sentinel`` ``types.CircuitBreaker`` factory with ``None`` as
the sentinel value. ``data`` is a container (such as a builtin ``dict``
instance) that returns ``None`` when the ``get()`` method is called with an
unknown key.

We can rewrite the example to give a name to the circuit breaker instance::

    >>> maybe_value = is_not_none(data.get("key"))
    >>> x if maybe_value else y

Here the ``maybe_value`` circuit breaker instance corresponds to ``breaker``
in the diagram.

The ternary condition is evaluated by calling ``bool(maybe_value)``, which is
the same as Python's existing behavior. The change in behavior is that instead
of directly returning one of the operands ``x`` or ``y``, the circuit breaking
protocol passes the relevant operand to the circuit breaker used in the
condition.

If ``bool(maybe_value)`` evaluates to ``True`` (i.e. the requested
key exists and its value is not ``None``) then the interpreter calls
``type(maybe_value).__then__(maybe_value, x)``. Otherwise, it calls
``type(maybe_value).__else__(maybe_value, y)``.

The protocol also applies to the new ``if`` and ``else`` binary operators,
but in these cases, the interpreter needs a way to indicate the missing third
operand. It does this by re-using the circuit breaker itself in that role.

Consider these two expressions::

    >>> x if data.get("key") is None
    >>> x if operator.is_sentinel(data.get("key"), None)

The first form of this expression returns ``x`` if ``data.get("key") is None``,
but otherwise returns ``False``, which almost certainly isn't what we want.

By contrast, the second form of this expression still returns ``x`` if
``data.get("key") is None``, but otherwise returns ``data.get("key")``, which
is significantly more useful behaviour.

We can understand this behavior by rewriting it as a ternary expression with
an explicitly named circuit breaker instance::

    >>> maybe_value = operator.is_sentinel(data.get("key"), None)
    >>> x if maybe_value else maybe_value

If ``bool(maybe_value)`` is ``True`` (i.e. ``data.get("key")`` is ``None``),
then the interpreter calls ``type(maybe_value).__then__(maybe_value, x)``. The
implementation of ``types.CircuitBreaker.__then__`` doesn't see anything that
indicates short-circuiting has taken place, and hence returns ``x``.

By contrast, if ``bool(maybe_value)`` is ``False`` (i.e. ``data.get("key")``
is *not* ``None``),  the interpreter calls
``type(maybe_value).__else__(maybe_value, maybe_value)``. The implementation of
``types.CircuitBreaker.__else__`` detects that the instance method has received
itself as its argument and returns the wrapped value (i.e. ``data.get("key")``)
rather than the circuit breaker.

The same logic applies to ``else``, only reversed::

    >>> is_not_none(data.get("key")) else y

This expression returns ``data.get("key")`` if it is not ``None``, otherwise it
evaluates and returns ``y``. To understand the mechanics, we rewrite the
expression as follows::

    >>> maybe_value = is_not_none(data.get("key"))
    >>> maybe_value if maybe_value else y

If ``bool(maybe_value)`` is ``True``, then the expression short-circuits and
the interpreter calls ``type(maybe_value).__else__(maybe_value, maybe_value)``.
The implementation of ``types.CircuitBreaker.__then__`` detects that the
instance method has received itself as its argument and returns the wrapped
value (i.e. ``data.get("key")``) rather than the circuit breaker.

If ``bool(maybe_value)`` is ``True``, the interpreter calls
``type(maybe_value).__else__(maybe_value, y)``. The implementation of
``types.CircuitBreaker.__else__`` doesn't see anything that indicates
short-circuiting has taken place, and hence returns ``y``.


Respecting De Morgan's Laws
---------------------------

Similar to ``and`` and ``or``, the binary short-circuiting operators will
permit multiple ways of writing essentially the same expression. This
seeming redundancy is unfortunately an implied consequence of defining the
protocol as a full boolean algebra, as boolean algebras respect a pair of
properties known as "De Morgan's Laws": the ability to express the results
of ``and`` and ``or`` operations in terms of each other and a suitable
combination of ``not`` operations.

For ``and`` and ``or`` in Python, these invariants can be described as follows::

    assert bool(A and B) == bool(not (not A or not B))
    assert bool(A or B) == bool(not (not A and not B))

That is, if you take one of the operators, invert both operands, switch to the
other operator, and then invert the overall result, you'll get the same
answer (in a boolean sense) as you did from the original operator. (This may
seem redundant, but in many situations it actually lets you eliminate double
negatives and find tautologically true or false subexpressions, thus reducing
the overall expression size).

For circuit breakers, defining a suitable invariant is complicated by the
fact that they're often going to be designed to eliminate themselves from the
expression result when they're short-circuited, which is an inherently
asymmetric behaviour. Accordingly, that inherent asymmetry needs to be
accounted for when mapping De Morgan's Laws to the expected behaviour of
symmetric circuit breakers.

One way this complication can be addressed is to wrap the operand that would
otherwise short-circuit in ``operator.true``, ensuring that when ``bool`` is
applied to the overall result, it uses the same definition of truth that was
used to decide which branch to evaluate, rather than applying ``bool`` directly
to the circuit breaker's input value.

Specifically, for the new short-circuiting operators, the following properties
would be reasonably expected to hold for any well-behaved symmetric circuit
breaker that implements both ``__bool__`` and ``__not__``::

    assert bool(B if true(A)) == bool(not (true(not A) else not B))
    assert bool(true(A) else B) == bool(not (not B if true(not A)))

Note the order of operations on the right hand side (applying ``true``
*after* inverting the input circuit breaker) - this ensures that an
assertion is actually being made about ``type(A).__not__``, rather than
merely being about the behaviour of ``type(true(A)).__not__``.

At the very least, ``types.CircuitBreaker`` instances would respect this
logic, allowing existing boolean expression optimisations (like double
negative elimination) to continue to be applied.


Arbitrary sentinel objects
--------------------------

Unlike PEPs 505 and 531, the proposal in this PEP readily handles custom
sentinel objects::

    _MISSING = object()

    # Using the sentinel to check whether or not an argument was supplied
    def my_func(arg=_MISSING):
        arg = make_default() if is_sentinel(arg, _MISSING) # "else arg" implied


Implicitly defined circuit breakers in circuit breaking expressions
-------------------------------------------------------------------

A never-posted draft of this PEP explored the idea of special casing the
``is`` and ``is not`` binary operators such that they were automatically
treated as circuit breakers when used in the context of a circuit breaking
expression. Unfortunately, it turned out that this approach necessarily
resulted in one of two highly undesirable outcomes:

A. the return type of these expressions changed universally from ``bool`` to
   ``types.CircuitBreaker``, potentially creating a backwards compatibility
   problem (especially when working with extension module APIs that
   specifically look for a builtin boolean value with ``PyBool_Check`` rather
   than passing the supplied value through ``PyObject_IsTrue`` or using
   the ``p`` (predicate) format in one of the argument parsing functions)
B. the return type of these expressions became *context dependent*, meaning
   that other routine refactorings (like pulling a comparison operation out
   into a local variable) could have a significant impact on the runtime
   semantics of a piece of code

Neither of those possible outcomes seems warranted by the proposal in this PEP,
so it reverted to the current design where circuit breaker instances must be
created explicitly via API calls, and are never produced implicitly.


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

As with :pep:`505`, actual implementation has been deferred pending in-principle
interest in the idea of making these changes.

...TBD...


Acknowledgements
================

Thanks go to Steven D'Aprano for his detailed critique [2_] of the initial
draft of this PEP that inspired many of the changes in the second draft, as
well as to all of the other participants in that discussion thread [3_]


References
==========

.. [1] PEP 335 rejection notification
   (https://mail.python.org/pipermail/python-dev/2012-March/117510.html)

.. [2] Steven D'Aprano's critique of the initial draft
   (https://mail.python.org/pipermail/python-ideas/2016-November/043615.html)

.. [3] python-ideas thread discussing initial draft
   (https://mail.python.org/pipermail/python-ideas/2016-November/043563.html)

Copyright
=========

This document has been placed in the public domain under the terms of the
CC0 1.0 license: https://creativecommons.org/publicdomain/zero/1.0/


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