I like PEP-532. Given the opposition to non-Pythonic syntax like ?? (i.e.
PEP-505), Nick's proposal offers a Pythonic alternative that is protocol
based, more generalized, and uses built-ins and keywords to avoid
punctuation.
I agree with other posters that the terms "exists" and "missing" could lead
developers to think that it tests for NameError. Maybe "value(foo) else
bar"? I can't think of a better spelling for the inverse. Maybe the "if"
syntax described in the PEP is better: "foo.bar if value(foo)". In that
case, we wouldn't need an inverse to exists()/value()/whatever.
I also wanted to mention a couple of unmentioned benefits of this PEP:
1. It is easier to Google a name. E.g., Google "c# ??" and you'll get
nothing related to null coalescing in c#". ("C# question marks" does find
the right content, however.)
2. Dismay over the meaning of foo?.bar.baz is much clearer when expressed
as missing(foo) else foo.bar.baz -- it's very close to the ternary logic
you'd write if you didn't have a circuit breaking operator: None if foo is
None else foo.bar.baz.
On Sat, Nov 5, 2016 at 5:50 AM, Nick Coghlan <ncogh...@gmail.com> wrote:
> Hi folks,
>
> As promised, here's a follow-up to the withdrawn PEP 531 that focuses
> on coming up with a common protocol driven solution to conditional
> evaluation of subexpressions that also addresses the element-wise
> comparison chaining problem that Guido noted when rejecting PEP 335.
>
> I quite like how it came out, but see the "Risks & Concerns" section
> for a discussion of an internal inconsistency it would introduce into
> the language if accepted as currently written, and the potentially
> far-reaching consequences actually resolving that inconsistency might
> have on the way people write their Python code (if the PEP was
> subsequently approved).
>
> Regards,
> Nick.
>
> =================================
> PEP: 532
> Title: A circuit breaking operator and protocol
> Version: $Revision$
> Last-Modified: $Date$
> Author: Nick Coghlan <ncogh...@gmail.com>
> Status: Draft
> Type: Standards Track
> Content-Type: text/x-rst
> Created: 30-Oct-2016
> Python-Version: 3.7
> Post-History: 5-Nov-2016
>
> Abstract
> ========
>
> Inspired by PEP 335, PEP 505, PEP 531, and the related discussions, this
> PEP
> proposes the addition of a new protocol-driven circuit breaking operator to
> Python that allows the left operand to decide whether or not the expression
> should short circuit and return a result immediately, or else continue
> on with evaluation of the right operand::
>
> exists(foo) else bar
> missing(foo) else foo.bar()
>
> These two expressions can be read as:
>
> * "the expression result is 'foo' if it exists, otherwise it is 'bar'"
> * "the expression result is 'foo' if it is missing, otherwise it is
> 'foo.bar()'"
>
> Execution of these expressions relies on a new circuit breaking protocol
> that
> implicitly avoids repeated evaluation of the left operand while letting
> that operand fully control the result of the expression, regardless of
> whether
> it skips evaluation of the right operand or not::
>
> _lhs = LHS
> type(_lhs).__then__(_lhs) if _lhs else type(_lhs).__else__(_lhs, RHS)
>
> To properly support logical negation of circuit breakers, a new ``__not__``
> protocol method would also be introduced allowing objects to control
> the result of ``not obj`` expressions.
>
> As shown in the basic example above, the PEP further proposes the addition
> of
> builtin ``exists`` and ``missing`` circuit breakers that provide
> conditional
> branching based on whether or not an object is ``None``, but return the
> original object rather than the existence checking wrapper when the
> expression
> evaluation short circuits.
>
> In addition to being usable as simple boolean operators (e.g. as in
> ``assert all(exists, items)`` or ``if any(missing, items):``), these
> circuit
> breakers will allow existence checking fallback operations (aka
> None-coalescing
> operations) to be written as::
>
> value = exists(expr1) else exists(expr2) else expr3
>
> and existence checking precondition operations (aka None-propagating
> or None-severing operations) to be written as::
>
> value = missing(obj) else obj.field.of.interest
> value = missing(obj) else obj["field"]["of"]["interest"]
>
> A change to the definition of chained comparisons is also proposed, where
> the comparison chaining will be updated to use the circuit breaking
> operator
> rather than the logical disjunction (``and``) operator if the left hand
> comparison returns a circuit breaker as its result.
>
> While there are some practical complexities arising from the current
> handling
> of single-valued arrays in NumPy, this change should be sufficient to allow
> elementwise chained comparison operations for matrices, where the result
> is a matrix of boolean values, rather than tautologically returning
> ``True``
> or raising ``ValueError``.
>
>
> Relationship with other PEPs
> ============================
>
> 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 a single protocol driven ``else`` operator and adjustments to the
> ``not``
> operator. The existence checking use cases are taken from that PEP.
>
> It is also a direct successor to PEP 335, which proposed the ability to
> overload the ``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 this PEP to instead handle the element-wise
> comparison use case by changing the semantic definition of comparison
> chaining
> is drawn from Guido's rejection of PEP 335.
>
> This PEP competes with the dedicated null-coalescing operator in PEP 505,
> proposing that improved support for null-coalescing operations be offered
> through a more general protocol-driven short circuiting operator and
> related
> builtins, rather than through a dedicated single-purpose operator.
>
> It doesn't compete with PEP 505's proposed shorthands for existence
> checking
> attribute access and subscripting, but instead offers an alternative
> underlying
> semantic framework for defining them:
>
> * ``EXPR?.attr`` would be syntactic sugar for ``missing(EXPR) else
> EXPR.attr``
> * ``EXPR?[key]`` would be syntactic sugar for ``missing(EXPR) else
> EXPR[key]``
>
> In both cases, the dedicated syntactic form could be optimised to avoid
> actually creating the circuit breaker instance.
>
>
> Specification
> =============
>
> The circuit breaking operator (``else``)
> ----------------------------------------
>
> Circuit breaking expressions would be written using ``else`` as a new
> binary
> operator, akin to the existing ``and`` and ``or`` logical operators::
>
> LHS else RHS
>
> Ignoring the hidden variable assignment, this is semantically equivalent
> to::
>
> _lhs = LHS
> type(_lhs).__then__(_lhs) if _lhs else type(_lhs).__else__(_lhs, RHS)
>
> The key difference relative to the existing ``or`` operator is that the
> value
> determining which branch of the conditional expression gets executed *also*
> gets a chance to postprocess the results of the expressions on each of the
> branches.
>
> As part of the short-circuiting behaviour, interpreter implementations
> are expected to access only the protocol method needed for the branch
> that is actually executed, but it is still recommended that circuit
> breaker authors that always return ``True`` or always return ``False`` from
> ``__bool__`` explicitly raise ``NotImplementedError`` with a suitable
> message from branch methods that are never expected to be executed (see the
> comparison chaining use case in the Rationale section below for an example
> of that).
>
> It is proposed that the ``else`` operator use a new precedence level that
> binds
> less tightly than the ``or`` operator by adjusting the relevant line in
> Python's grammar from the current::
>
> test: or_test ['if' or_test 'else' test] | lambdef
>
> to instead be::
>
> test: else_test ['if' or_test 'else' test] | lambdef
> else_test: or_test ['else' test]
>
> The definition of ``test_nocond`` would remain unchanged, so circuit
> breaking expressions 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 epxr3`` resolves as
> ``(expr1 if cond else expr2) else epxr3``.
>
> 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.
>
>
> 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 whether or not to short
> circuit
> the expression evaluation. For example, the ``exists`` and ``missing``
> 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, ``__not__``
> implementations will be obliged to respect the following invariant::
>
> assert not bool(obj) == bool(not obj)
>
>
> Chained comparisons
> -------------------
>
> A chained comparison like ``0 < x < 10`` written as::
>
> LEFT_BOUND left_op EXPR right_op RIGHT_BOUND
>
> is currently roughly semantically equivalent to::
>
> _expr = EXPR
> _lhs_result = LEFT_BOUND left_op _expr
> _expr_result = _lhs_result and (_expr right_op RIGHT_BOUND)
>
> This PEP proposes that this be changed to explicitly check if the left
> comparison returns a circuit breaker, and if so, use ``else`` rather than
> ``and`` to implement the comparison chaining::
>
> _expr = EXPR
> _lhs_result = LEFT_BOUND left_op _expr
> if hasattr(type(_lhs_result), "__then__"):
> _expr_result = _lhs_result else (_expr right_op RIGHT_BOUND)
> else:
> _expr_result = _lhs_result and (_expr right_op RIGHT_BOUND)
>
> This allows types like NumPy arrays to control the behaviour of chained
> comparisons by returning circuit breakers from comparison operations.
>
>
> Existence checking comparisons
> ------------------------------
>
> Two new builtins implementing the new protocol are proposed to encapsulate
> the
> notion of "existence 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").
>
> These builtins would be defined as follows::
>
> class CircuitBreaker:
> """Base circuit breaker type (available as types.CircuitBreaker)"""
> def __init__(self, value, condition, inverse_type):
> self.value = value
> self._condition = condition
> self._inverse_type = inverse_type
> def __bool__(self):
> return self._condition
> def __not__(self):
> return self._inverse_type(self.value)
> def __then__(self):
> return self.value
> def __else__(self, other):
> if other is self:
> return self.value
> return other
>
> class exists(types.CircuitBreaker):
> """Circuit breaker for 'EXPR is not None' checks"""
> def __init__(self, value):
> super().__init__(value, value is not None, missing)
>
> class missing(types.CircuitBreaker):
> """Circuit breaker for 'EXPR is None' checks"""
> def __init__(self, value):
> super().__init__(value, value is None, exists)
>
> Aside from changing the definition of ``__bool__`` to be based on
> ``is not None`` rather than normal truth checking, the key characteristic
> of
> ``exists`` is that when it is used as a circuit breaker, it is *ephemeral*:
> when it is told that short circuiting has taken place, it returns the
> original
> value, rather than the existence checking wrapper.
>
> ``missing`` is defined as the logically inverted counterpart of ``exists``:
> ``not exists(obj)`` is semantically equivalent to ``missing(obj)``.
>
> The ``__else__`` implementations for both builtin circuit breakers are
> defined
> such that the wrapper will always be removed even if you explicitly pass
> the
> circuit breaker to both sides of the ``else`` expression::
>
> breaker = exists(foo)
> assert (breaker else breaker) is foo
> breaker = missing(foo)
> assert (breaker else breaker) is foo
>
>
> Other conditional constructs
> ----------------------------
>
> No changes are proposed to if statements, while statements, conditional
> expressions, comprehensions, or generator expressions, as the boolean
> clauses
> they contain are already used for control flow purposes.
>
> However, it's worth noting that while such proposals are outside the scope
> of
> this PEP, the circuit breaking protocol defined here would be sufficient to
> support constructs like::
>
> while exists(dynamic_query()) as result:
> ... # Code using result
>
> and:
>
> if exists(re.search(pattern, text)) as match:
> ... # Code using match
>
> Leaving the door open to such a future extension is the main reason for
> recommending that circuit breaker implementations handle the ``self is
> other``
> case in ``__else__`` implementations the same way as they handle the
> short-circuiting behaviour in ``__then__``.
>
>
> Style guide recommendations
> ---------------------------
>
> The following additions to PEP 8 are proposed in relation to the new
> features
> introduced by this PEP:
>
> * In the absence of other considerations, prefer the use of the builtin
> circuit breakers ``exists`` and ``missing`` over the corresponding
> conditional expressions
>
> * Do not combine conditional expressions (``if-else``) and circuit breaking
> expressions (the ``else`` operator) in a single expression - use one or
> the
> other depending on the situation, but not both.
>
>
> Rationale
> =========
>
> Adding a new operator
> ---------------------
>
> 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
> reasons:
>
> * defining a shared protocol for both ``and`` and ``or`` was confusing, as
> ``__then__`` was the short-circuiting outcome for ``or``, while
> ``__else__``
> was the short-circuiting outcome for ``and``
> * 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
>
> Proposing a single new operator instead resolves all of those issues -
> ``__then__`` always indicates short circuiting, ``__else__`` only indicates
> "short circuiting" if the circuit breaker itself is also passed in as the
> right operand, and the 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.
>
> As a result of that design simplification, the new protocol and operator
> would
> even allow us to expose ``operator.true`` and ``operator.false``
> as circuit breaker definitions if we chose to do so::
>
> class true(types.CircuitBreaker):
> """Circuit breaker for 'bool(EXPR)' checks"""
> def __init__(self, value):
> super().__init__(value, bool(value), when_false)
>
> class false(types.CircuitBreaker):
> """Circuit breaker for 'not bool(EXPR)' checks"""
> def __init__(self, value):
> super().__init__(value, not bool(value), when_true)
>
> Given those circuit breakers:
>
> * ``LHS or RHS`` would be roughly ``operator.true(LHS) else RHS``
> * ``LHS and RHS`` would be roughly ``operator.false(LHS) else RHS``
>
>
> 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 an existing keyword
> -------------------------
>
> Using an existing keyword has the benefit of allowing the new expression to
> be introduced without a ``__future__`` statement.
>
> ``else`` is semantically appropriate for the proposed new protocol, and the
> only syntactic ambiguity introduced arises when the new operator is
> combined
> with the explicit ``if-else`` conditional expression syntax.
>
>
> Element-wise chained comparisons
> --------------------------------
>
> In ultimately rejecting PEP 335, Guido van Rossum noted [1_]:
>
> The NumPy folks brought up a somewhat separate issue: for them,
> the most common use case is chained comparisons (e.g. A < B < C).
>
> To understand this observation, we first need to look at how comparisons
> work
> with NumPy arrays::
>
> >>> import numpy as np
> >>> increasing = np.arange(5)
> >>> increasing
> array([0, 1, 2, 3, 4])
> >>> decreasing = np.arange(4, -1, -1)
> >>> decreasing
> array([4, 3, 2, 1, 0])
> >>> increasing < decreasing
> array([ True, True, False, False, False], dtype=bool)
>
> Here we see that NumPy array comparisons are element-wise by default,
> comparing
> each element in the lefthand array to the corresponding element in the
> righthand
> array, and producing a matrix of boolean results.
>
> If either side of the comparison is a scalar value, then it is broadcast
> across
> the array and compared to each individual element::
>
> >>> 0 < increasing
> array([False, True, True, True, True], dtype=bool)
> >>> increasing < 4
> array([ True, True, True, True, False], dtype=bool)
>
> However, this broadcasting idiom breaks down if we attempt to use chained
> comparisons::
>
> >>> 0 < increasing < 4
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> ValueError: The truth value of an array with more than one element
> is ambiguous. Use a.any() or a.all()
>
> The problem is that internally, Python implicitly expands this chained
> comparison into the form::
>
> >>> 0 < increasing and increasing < 4
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> ValueError: The truth value of an array with more than one element
> is ambiguous. Use a.any() or a.all()
>
> And NumPy only permits implicit coercion to a boolean value for
> single-element
> arrays where ``a.any()`` and ``a.all()`` can be assured of having the same
> result::
>
> >>> np.array([False]) and np.array([False])
> array([False], dtype=bool)
> >>> np.array([False]) and np.array([True])
> array([False], dtype=bool)
> >>> np.array([True]) and np.array([False])
> array([False], dtype=bool)
> >>> np.array([True]) and np.array([True])
> array([ True], dtype=bool)
>
> The proposal in this PEP would allow this situation to be changed by
> updating
> the definition of element-wise comparison operations in NumPy to return a
> dedicated subclass that implements the new circuit breaking protocol and
> also
> changes the result array's interpretation in a boolean context to always
> return ``False`` and hence never trigger the short-circuiting behaviour::
>
> class ComparisonResultArray(np.ndarray):
> def __bool__(self):
> return False
> def _raise_NotImplementedError(self):
> msg = ("Comparison array truth values are ambiguous outside "
> "chained comparisons. Use a.any() or a.all()")
> raise NotImplementedError(msg)
> def __not__(self):
> self._raise_NotImplementedError()
> def __then__(self):
> self._raise_NotImplementedError()
> def __else__(self, other):
> return np.logical_and(self, other.view(ComparisonResultArray))
>
> With this change, the chained comparison example above would be able to
> return::
>
> >>> 0 < increasing < 4
> ComparisonResultArray([ False, True, True, True, False], dtype=bool)
>
>
> Existence checking expressions
> ------------------------------
>
> An increasingly common requirement in modern software development is the
> need
> to work with "semi-structured data": data where the structure of the data
> is
> known in advance, but pieces of it may be missing at runtime, and the
> software
> manipulating that data is expected to degrade gracefully (e.g. by omitting
> results that depend on the missing data) rather than failing outright.
>
> Some particularly common cases where this issue arises are:
>
> * handling optional application configuration settings and function
> parameters
> * handling external service failures in distributed systems
> * handling data sets that include some partial records
>
> At the moment, writing such software in Python can be genuinely awkward, as
> your code ends up littered with expressions like:
>
> * ``value1 = expr1.field.of.interest if expr1 is not None else None``
> * ``value2 = expr2["field"]["of"]["interest"] if expr2 is not None else
> None``
> * ``value3 = expr3 if expr3 is not None else expr4 if expr4 is not
> None else expr5``
>
> PEP 531 goes into more detail on some of the challenges of working with
> this
> kind of data, particularly in data transformation pipelines where dealing
> with
> potentially missing content is the norm rather than the exception.
>
> The combined impact of the proposals in this PEP is to allow the above
> sample
> expressions to instead be written as:
>
> * ``value1 = missing(expr1) else expr1.field.of.interest``
> * ``value2 = missing(expr2) else expr2.["field"]["of"]["interest"]``
> * ``value3 = exists(expr3) else exists(expr4) else expr5``
>
> In these forms, significantly more of the text presented to the reader is
> immediately relevant to the question "What does this code do?", while the
> boilerplate code to handle missing data by passing it through to the output
> or falling back to an alternative input, has shrunk to two uses of the new
> ``missing`` builtin, and two uses of the new ``exists`` builtin.
>
> In the first two examples, the 31 character boilerplate suffix
> ``if exprN is not None else None`` (minimally 27 characters for a single
> letter
> variable name) has been replaced by a 19 character ``missing(expr1) else``
> prefix (minimally 15 characters with a single letter variable name),
> markedly
> improving the signal-to-pattern-noise ratio of the lines (especially if it
> encourages the use of more meaningful variable and field names rather than
> making them shorter purely for the sake of expression brevity). The
> additional
> syntactic sugar proposals in PEP 505 would further reduce this boilerplate
> to
> a single ``?`` character that also eliminated the repetition of the
> expession
> being checked for existence.
>
> In the last example, not only are two instances of the 21 character
> boilerplate,
> `` if exprN is not None`` (minimally 17 characters) replaced with the
> 8 character function call ``exists()``, but that function call is placed
> directly around the original expression, eliminating the need to duplicate
> it
> in the conditional existence check.
>
>
> 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 a new operator and adjusts the definition of chained
> comparison
> rather than impacting the existing ``and`` and ``or`` operators
> * the changes to the ``not`` unary operator are defined in such a way that
> control flow optimisations based on the existing semantics remain valid
> * rather than the cryptic ``??``, it uses ``else`` as the operator keyword
> in
> exactly the same sense as it is already used in conditional expressions
> * it defines a general purpose short-circuiting binary operator that can
> even
> be used to express the existing semantics of ``and`` and ``or`` rather
> than
> focusing solely and inflexibly on existence checking
> * it names the proposed builtins in such a way that they provide a strong
> mnemonic hint as to when the expression containing them will
> short-circuit
> and skip evaluating the right operand
>
>
> Possible confusion with conditional expressions
> -----------------------------------------------
>
> The proposal in this PEP is essentially for an "implied ``if``" where if
> you
> omit the ``if`` clause from a conditional expression, you invoke the
> circuit
> breaking protocol instead. That is::
>
> exists(foo) else calculate_default()
>
> invokes the new protocol, but::
>
> foo.field.of.interest if exists(foo) else calculate_default()
>
> bypasses it entirely, *including* the non-short-circuiting ``__else__``
> method.
>
> This mostly wouldn't be a problem for the proposed ``types.CircuitBreaker``
> implementation (and hence the ``exists`` and ``missing`` builtins), as the
> only purpose the extended protocol serves in that case is to remove the
> wrapper in the short-circuiting case - the ``__else__`` method passes the
> right operand through unchanged.
>
> However, this discrepancy could potentially be eliminated entirely by also
> updating conditional expressions to use the circuit breaking protocol if
> the condition defines those methods. In that case, ``__then__`` would need
> to be updated to accept the left operand as a parameter, with
> short-circuiting
> indicated by passing in the circuit breaker itself::
>
> class CircuitBreaker:
> """Base circuit breaker type (available as types.CircuitBreaker)"""
> def __init__(self, value, condition, inverse_type):
> self.value = value
> self._condition = condition
> self._inverse_type = inverse_type
> def __bool__(self):
> return self._condition
> def __not__(self):
> return self._inverse_type(self.value)
> def __then__(self, other):
> if other is not self:
> return other
> return self.value # Short-circuit, remove the wrapper
> def __else__(self, other):
> if other is not self:
> return other
> return self.value # Short-circuit, remove the wrapper
>
> With this symmetric protocol, the definition of conditional expressions
> could be updated to also make the ``else`` clause optional::
>
> test: else_test ['if' or_test ['else' test]] | lambdef
> else_test: or_test ['else' test]
>
> (We would avoid the apparent simplification to ``else_test ('if'
> else_test)*``
> in order to make it easier to correctly preserve the semantics of normal
> conditional expressions)
>
> Given that expanded definition, the following statements would be
> functionally equivalent::
>
> foo = calculate_default() if missing(foo)
> foo = calculate_default() if foo is None else foo
>
> Just as the base proposal already makes the following equivalent::
>
> foo = exists(foo) else calculate_default()
> foo = foo if foo is not None else calculate_default()
>
> The ``if`` based circuit breaker form has the virtue of reading
> significantly
> better when used for conditional imperative commands like debug messages::
>
> print(some_expensive_query()) if verbosity > 2
>
> If we went down this path, then ``operator.true`` would need to be declared
> as the nominal implicit circuit breaker when the condition didn't define
> the
> circuit breaker protocol itself (so the above example would produce
> ``None``
> if the debugging message was printed, and ``False`` otherwise)
>
> The main objection to this expansion of the proposal is that it makes it a
> more intrusive change that may potentially affect the behaviour of existing
> code, while the main point in its favour is that allowing both ``if`` and
> ``else`` as circuit breaking operators and also supporting the circuit
> breaking
> protocol for normal conditional expressions would be significantly more
> self-consistent than special-casing a bare ``else`` as a stand-alone
> operator.
>
>
> Design Discussion
> =================
>
> Arbitrary sentinel objects
> --------------------------
>
> Unlike PEPs 505 and 531, this proposal readily handles custom sentinel
> objects::
>
> class defined(types.CircuitBreaker):
> MISSING = object()
> def __init__(self, value):
> super().__init__(self, value is not self.MISSING, undefined)
>
> class undefined(types.CircuitBreaker):
> def __init__(self, value):
> super().__init__(self, value is defined.MISSING, defined)
>
> # Using the sentinel to check whether or not an argument was supplied
> def my_func(arg=defined.MISSING):
> arg = defined(arg) else calculate_default()
>
>
> Implementation
> ==============
>
> As with PEP 505, actual implementation has been deferred pending
> in-principle
> interest in the idea of making these changes - aside from the possible
> syntactic ambiguity concerns covered by the grammer proposals above, the
> implementation isn't really the hard part of these proposals, the hard part
> is deciding whether or not this is a change where the long term benefits
> for
> new and existing Python users outweigh the short term costs involved in the
> wider ecosystem (including developers of other implementations, language
> curriculum developers, and authors of other Python related educational
> material) adjusting to the change.
>
> ...TBD...
>
>
> Acknowledgements
> ================
>
> Thanks go to Mark E. Haase for feedback on and contributions to earlier
> drafts
> of this proposal. However, his clear and exhaustive explanation of the
> original
> protocol design that modified the semantics of ``if-else`` conditional
> expressions to use an underlying ``__then__``/``__else__`` protocol helped
> convince me it was too complicated to keep, so this iteration contains
> neither
> that version of the protocol, nor Mark's explanation of it.
>
>
> References
> ==========
>
> .. [1] PEP 335 rejection notification
> (http://mail.python.org/pipermail/python-dev/2012-March/117510.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/
>
>
> --
> Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia
> _______________________________________________
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>
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