648 lines
27 KiB
ReStructuredText
648 lines
27 KiB
ReStructuredText
PEP: 531
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Title: Existence checking operators
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Version: $Revision$
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Last-Modified: $Date$
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Author: Alyssa Coghlan <ncoghlan@gmail.com>
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Status: Withdrawn
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 25-Oct-2016
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Python-Version: 3.7
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Post-History: 28-Oct-2016
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Abstract
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========
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Inspired by :pep:`505` and the related discussions, this PEP proposes the addition
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of two new control flow operators to Python:
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* Existence-checking precondition ("exists-then"): ``expr1 ?then expr2``
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* Existence-checking fallback ("exists-else"): ``expr1 ?else expr2``
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as well as the following abbreviations for common existence checking
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expressions and statements:
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* Existence-checking attribute access:
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``obj?.attr`` (for ``obj ?then obj.attr``)
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* Existence-checking subscripting:
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``obj?[expr]`` (for ``obj ?then obj[expr]``)
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* Existence-checking assignment:
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``value ?= expr`` (for ``value = value ?else expr``)
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The common ``?`` symbol in these new operator definitions indicates that they
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use a new "existence checking" protocol rather than the established
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truth-checking protocol used by if statements, while loops, comprehensions,
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generator expressions, conditional expressions, logical conjunction, and
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logical disjunction.
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This new protocol would be made available as ``operator.exists``, with the
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following characteristics:
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* types can define a new ``__exists__`` magic method (Python) or
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``tp_exists`` slot (C) to override the default behaviour. This optional
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method has the same signature and possible return values as ``__bool__``.
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* ``operator.exists(None)`` returns ``False``
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* ``operator.exists(NotImplemented)`` returns ``False``
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* ``operator.exists(Ellipsis)`` returns ``False``
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* ``float``, ``complex`` and ``decimal.Decimal`` will override the existence
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check such that ``NaN`` values return ``False`` and other values (including
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zero values) return ``True``
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* for any other type, ``operator.exists(obj)`` returns True by default. Most
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importantly, values that evaluate to False in a truth checking context
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(zeroes, empty containers) will still evaluate to True in an existence
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checking context
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PEP Withdrawal
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==============
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When posting this PEP for discussion on python-ideas [4]_, I asked reviewers to
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consider 3 high level design questions before moving on to considering the
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specifics of this particular syntactic proposal:
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1. Do we collectively agree that "existence checking" is a useful
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general concept that exists in software development and is distinct
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from the concept of "truth checking"?
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2. Do we collectively agree that the Python ecosystem would benefit
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from an existence checking protocol that permits generalisation of
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algorithms (especially short circuiting ones) across different "data
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missing" indicators, including those defined in the language
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definition, the standard library, and custom user code?
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3. Do we collectively agree that it would be easier to use such a
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protocol effectively if existence-checking equivalents to the
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truth-checking "and" and "or" control flow operators were available?
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While the answers to the first question were generally positive, it quickly
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became clear that the answer to the second question is "No".
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Steven D'Aprano articulated the counter-argument well in [5]_, but the general
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idea is that when checking for "missing data" sentinels, we're almost always
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looking for a *specific* sentinel value, rather than *any* sentinel value.
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``NotImplemented`` exists, for example, due to ``None`` being a potentially
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legitimate result from overloaded arithmetic operators and exception
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handling imposing too much runtime overhead to be useful for operand coercion.
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Similarly, ``Ellipsis`` exists for multi-dimensional slicing support due to
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``None`` already have another meaning in a slicing context (indicating the use
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of the default start or stop indices, or the default step size).
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In mathematics, the value of ``NaN`` is that *programmatically* it behaves
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like a normal value of its type (e.g. exposing all the usual attributes and
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methods), while arithmetically it behaves according to the mathematical rules
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for handling ``NaN`` values.
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With that core design concept invalidated, the proposal as a whole doesn't
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make sense, and it is accordingly withdrawn.
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However, the discussion of the proposal did prompt consideration of a potential
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protocol based approach to make the existing ``and``, ``or`` and ``if-else``
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operators more flexible [6]_ without introducing any new syntax, so I'll be
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writing that up as another possible alternative to :pep:`505`.
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Relationship with other PEPs
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============================
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While this PEP was inspired by and builds on Mark Haase's excellent work in
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putting together :pep:`505`, it ultimately competes with that PEP due to
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significant differences in the specifics of the proposed syntax and semantics
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for the feature.
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It also presents a different perspective on the rationale for the change by
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focusing on the benefits to existing Python users as the typical demands of
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application and service development activities are genuinely changing. It
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isn't an accident that similar features are now appearing in multiple
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programming languages, and while it's a good idea for us to learn from how other
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language designers are handling the problem, precedents being set elsewhere
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are more relevant to *how* we would go about tackling this problem than they
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are to whether or not we think it's a problem we should address in the first
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place.
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Rationale
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=========
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Existence checking expressions
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------------------------------
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An increasingly common requirement in modern software development is the need
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to work with "semi-structured data": data where the structure of the data is
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known in advance, but pieces of it may be missing at runtime, and the software
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manipulating that data is expected to degrade gracefully (e.g. by omitting
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results that depend on the missing data) rather than failing outright.
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Some particularly common cases where this issue arises are:
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* handling optional application configuration settings and function parameters
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* handling external service failures in distributed systems
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* handling data sets that include some partial records
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It is the latter two cases that are the primary motivation for this PEP - while
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needing to deal with optional configuration settings and parameters is a design
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requirement at least as old as Python itself, the rise of public cloud
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infrastructure, the development of software systems as collaborative networks
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of distributed services, and the availability of large public and private data
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sets for analysis means that the ability to degrade operations gracefully in
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the face of partial service failures or partial data availability is becoming
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an essential feature of modern programming environments.
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At the moment, writing such software in Python can be genuinely awkward, as
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your code ends up littered with expressions like:
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* ``value1 = expr1.field.of.interest if expr1 is not None else None``
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* ``value2 = expr2["field"]["of"]["interest"] if expr2 is not None else None``
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* ``value3 = expr3 if expr3 is not None else expr4 if expr4 is not None else expr5``
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If these are only occasional, then expanding out to full statement forms may
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help improve readability, but if you have 4 or 5 of them in a row (which is a
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fairly common situation in data transformation pipelines), then replacing them
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with 16 or 20 lines of conditional logic really doesn't help matters.
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Expanding the three examples above that way hopefully helps illustrate that::
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if expr1 is not None:
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value1 = expr1.field.of.interest
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else:
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value1 = None
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if expr2 is not None:
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value2 = expr2["field"]["of"]["interest"]
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else:
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value2 = None
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if expr3 is not None:
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value3 = expr3
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else:
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if expr4 is not None:
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value3 = expr4
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else:
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value3 = expr5
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The combined impact of the proposals in this PEP is to allow the above sample
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expressions to instead be written as:
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* ``value1 = expr1?.field.of.interest``
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* ``value2 = expr2?["field"]["of"]["interest"]``
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* ``value3 = expr3 ?else expr4 ?else expr5``
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In these forms, almost all of the information presented to the reader is
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immediately relevant to the question "What does this code do?", while the
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boilerplate code to handle missing data by passing it through to the output
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or falling back to an alternative input, has shrunk to two uses of the ``?``
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symbol and two uses of the ``?else`` keyword.
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In the first two examples, the 31 character boilerplate clause
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``if exprN is not None else None`` (minimally 27 characters for a single letter
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variable name) has been replaced by a single ``?`` character, substantially
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improving the signal-to-pattern-noise ratio of the lines (especially if it
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encourages the use of more meaningful variable and field names rather than
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making them shorter purely for the sake of expression brevity).
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In the last example, two instances of the 21 character boilerplate,
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``if exprN is not None`` (minimally 17 characters) are replaced with single
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characters, again substantially improving the signal-to-pattern-noise ratio.
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Furthermore, each of our 5 "subexpressions of potential interest" is included
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exactly once, rather than 4 of them needing to be duplicated or pulled out
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to a named variable in order to first check if they exist.
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The existence checking precondition operator is mainly defined to provide a
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clear conceptual basis for the existence checking attribute access and
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subscripting operators:
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* ``obj?.attr`` is roughly equivalent to ``obj ?then obj.attr``
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* ``obj?[expr]`` is roughly equivalent to ``obj ?then obj[expr]``
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The main semantic difference between the shorthand forms and their expanded
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equivalents is that the common subexpression to the left of the existence
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checking operator is evaluated only once in the shorthand form (similar to
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the benefit offered by augmented assignment statements).
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Existence checking assignment
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-----------------------------
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Existence-checking assignment is proposed as a relatively straightforward
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expansion of the concepts in this PEP to also cover the common configuration
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handling idiom:
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* ``value = value if value is not None else expensive_default()``
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by allowing that to instead be abbreviated as:
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* ``value ?= expensive_default()``
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This is mainly beneficial when the target is a subscript operation or
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subattribute, as even without this specific change, the PEP would still
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permit this idiom to be updated to:
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* ``value = value ?else expensive_default()``
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The main argument *against* adding this form is that it's arguably ambiguous
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and could mean either:
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* ``value = value ?else expensive_default()``; or
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* ``value = value ?then value.subfield.of.interest``
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The second form isn't at all useful, but if this concern was deemed significant
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enough to address while still keeping the augmented assignment feature,
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the full keyword could be included in the syntax:
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* ``value ?else= expensive_default()``
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Alternatively, augmented assignment could just be dropped from the current
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proposal entirely and potentially reconsidered at a later date.
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Existence checking protocol
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---------------------------
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The existence checking protocol is including in this proposal primarily to
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allow for proxy objects (e.g. local representations of remote resources) and
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mock objects used in testing to correctly indicate non-existence of target
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resources, even though the proxy or mock object itself is not None.
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However, with that protocol defined, it then seems natural to expand it to
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provide a type independent way of checking for ``NaN`` values in numeric types
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- at the moment you need to be aware of the exact data type you're working with
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(e.g. builtin floats, builtin complex numbers, the decimal module) and use the
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appropriate operation (e.g. ``math.isnan``, ``cmath.isnan``,
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``decimal.getcontext().is_nan()``, respectively)
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Similarly, it seems reasonable to declare that the other placeholder builtin
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singletons, ``Ellipsis`` and ``NotImplemented``, also qualify as objects that
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represent the absence of data more so than they represent data.
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Proposed symbolic notation
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--------------------------
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Python has historically only had one kind of implied boolean context: truth
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checking, which can be invoked directly via the ``bool()`` builtin. As this PEP
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proposes a new kind of control flow operation based on existence checking rather
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than truth checking, it is considered valuable to have a reminder directly
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in the code when existence checking is being used rather than truth checking.
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The mathematical symbol for existence assertions is U+2203 'THERE EXISTS': ``∃``
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Accordingly, one possible approach to the syntactic additions proposed in this
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PEP would be to use that already defined mathematical notation:
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* ``expr1 ∃then expr2``
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* ``expr1 ∃else expr2``
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* ``obj∃.attr``
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* ``obj∃[expr]``
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* ``target ∃= expr``
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However, there are two major problems with that approach, one practical, and
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one pedagogical.
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The practical problem is the usual one that most keyboards don't offer any easy
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way of entering mathematical symbols other than those used in basic arithmetic
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(even the symbols appearing in this PEP were ultimately copied & pasted
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from [3]_ rather than being entered directly).
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The pedagogical problem is that the symbols for existence assertions (``∃``)
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and universal assertions (``∀``) aren't going to be familiar to most people
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the way basic arithmetic operators are, so we wouldn't actually be making the
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proposed syntax easier to understand by adopting ``∃``.
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By contrast, ``?`` is one of the few remaining unused ASCII punctuation
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characters in Python's syntax, making it available as a candidate syntactic
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marker for "this control flow operation is based on an existence check, not a
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truth check".
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Taking that path would also have the advantage of aligning Python's syntax
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with corresponding syntax in other languages that offer similar features.
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Drawing from the existing summary in :pep:`505` and the Wikipedia articles on
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the "safe navigation operator [1]_ and the "null coalescing operator" [2]_,
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we see:
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* The ``?.`` existence checking attribute access syntax precisely aligns with:
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* the "safe navigation" attribute access operator in C# (``?.``)
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* the "optional chaining" operator in Swift (``?.``)
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* the "safe navigation" attribute access operator in Groovy (``?.``)
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* the "conditional member access" operator in Dart (``?.``)
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* The ``?[]`` existence checking attribute access syntax precisely aligns with:
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* the "safe navigation" subscript operator in C# (``?[]``)
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* the "optional subscript" operator in Swift (``?[].``)
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* The ``?else`` existence checking fallback syntax semantically aligns with:
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* the "null-coalescing" operator in C# (``??``)
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* the "null-coalescing" operator in PHP (``??``)
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* the "nil-coalescing" operator in Swift (``??``)
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To be clear, these aren't the only spelling of these operators used in other
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languages, but they're the most common ones, and the ``?`` symbol is the most
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common syntactic marker by far (presumably prompted by the use of ``?`` to
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introduce the "then" clause in C-style conditional expressions, which many
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of these languages also offer).
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Proposed keywords
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-----------------
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Given the symbolic marker ``?``, it would be syntactically unambiguous to spell
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the existence checking precondition and fallback operations using the same
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keywords as their truth checking counterparts:
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* ``expr1 ?and expr2`` (instead of ``expr1 ?then expr2``)
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* ``expr1 ?or expr2`` (instead of ``expr1 ?else expr2``)
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However, while syntactically unambiguous when written, this approach makes
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the code incredibly hard to *pronounce* (What's the pronunciation of "?"?) and
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also hard to *describe* (given reused keywords, there's no obvious shorthand
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terms for "existence checking precondition (?and)" and "existence checking
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fallback (?or)" that would distinguish them from "logical conjunction (and)"
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and "logical disjunction (or)").
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We could try to encourage folks to pronounce the ``?`` symbol as "exists",
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making the shorthand names the "exists-and expression" and the
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"exists-or expression", but there'd be no way of guessing those names purely
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from seeing them written in a piece of code.
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Instead, this PEP takes advantage of the proposed symbolic syntax to introduce
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a new keyword (``?then``) and borrow an existing one (``?else``) in a way
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that allows people to refer to "then expressions" and "else expressions"
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without ambiguity.
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These keywords also align well with the conditional expressions that are
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semantically equivalent to the proposed expressions.
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For ``?else`` expressions, ``expr1 ?else expr2`` is equivalent to::
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_lhs_result = expr1
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_lhs_result if operator.exists(_lhs_result) else expr2
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Here the parallel is clear, since the ``else expr2`` appears at the end of
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both the abbreviated and expanded forms.
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For ``?then`` expressions, ``expr1 ?then expr2`` is equivalent to::
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_lhs_result = expr1
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expr2 if operator.exists(_lhs_result) else _lhs_result
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Here the parallel isn't as immediately obvious due to Python's traditionally
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anonymous "then" clauses (introduced by ``:`` in ``if`` statements and suffixed
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by ``if`` in conditional expressions), but it's still reasonably clear as long
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as you're already familiar with the "if-then-else" explanation of conditional
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control flow.
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Risks and concerns
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==================
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Readability
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-----------
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Learning to read and write the new syntax effectively mainly requires
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internalising two concepts:
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* expressions containing ``?`` include an existence check and may short circuit
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* if ``None`` or another "non-existent" value is an expected input, and the
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correct handling is to propagate that to the result, then the existence
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checking operators are likely what you want
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Currently, these concepts aren't explicitly represented at the language level,
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so it's a matter of learning to recognise and use the various idiomatic
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patterns based on conditional expressions and statements.
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Magic syntax
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------------
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There's nothing about ``?`` as a syntactic element that inherently suggests
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``is not None`` or ``operator.exists``. The main current use of ``?`` as a
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symbol in Python code is as a trailing suffix in IPython environments to
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request help information for the result of the preceding expression.
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However, the notion of existence checking really does benefit from a pervasive
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visual marker that distinguishes it from truth checking, and that calls for
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a single-character symbolic syntax if we're going to do it at all.
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Conceptual complexity
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---------------------
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This proposal takes the currently ad hoc and informal concept of "existence
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checking" and elevates it to the status of being a syntactic language feature
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with a clearly defined operator protocol.
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In many ways, this should actually *reduce* the overall conceptual complexity
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of the language, as many more expectations will map correctly between truth
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checking with ``bool(expr)`` and existence checking with
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``operator.exists(expr)`` than currently map between truth checking and
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existence checking with ``expr is not None`` (or ``expr is not NotImplemented``
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in the context of operand coercion, or the various NaN-checking operations
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in mathematical libraries).
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As a simple example of the new parallels introduced by this PEP, compare::
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all_are_true = all(map(bool, iterable))
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at_least_one_is_true = any(map(bool, iterable))
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all_exist = all(map(operator.exists, iterable))
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at_least_one_exists = any(map(operator.exists, iterable))
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Design Discussion
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=================
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Subtleties in chaining existence checking expressions
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-----------------------------------------------------
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Similar subtleties arise in chaining existence checking expressions as already
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exist in chaining logical operators: the behaviour can be surprising if the
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right hand side of one of the expressions in the chain itself returns a
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value that doesn't exist.
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As a result, ``value = arg1 ?then f(arg1) ?else default()`` would be dubious for
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essentially the same reason that ``value = cond and expr1 or expr2`` is dubious:
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the former will evaluate ``default()`` if ``f(arg1)`` returns ``None``, just
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as the latter will evaluate ``expr2`` if ``expr1`` evaluates to ``False`` in
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a boolean context.
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Ambiguous interaction with conditional expressions
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--------------------------------------------------
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In the proposal as currently written, the following is a syntax error:
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* ``value = f(arg) if arg ?else default``
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While the following is a valid operation that checks a second condition if the
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first doesn't exist rather than merely being false:
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* ``value = expr1 if cond1 ?else cond2 else expr2``
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The expression chaining problem described above means that the argument can be
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made that the first operation should instead be equivalent to:
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* ``value = f(arg) if operator.exists(arg) else default``
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requiring the second to be written in the arguably clearer form:
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* ``value = expr1 if (cond1 ?else cond2) else expr2``
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Alternatively, the first form could remain a syntax error, and the existence
|
|
checking symbol could instead be attached to the ``if`` keyword:
|
|
|
|
* ``value = expr1 if? cond else expr2``
|
|
|
|
|
|
Existence checking in other truth-checking contexts
|
|
---------------------------------------------------
|
|
|
|
The truth-checking protocol is currently used in the following syntactic
|
|
constructs:
|
|
|
|
* logical conjunction (and-expressions)
|
|
* logical disjunction (or-expressions)
|
|
* conditional expressions (if-else expressions)
|
|
* if statements
|
|
* while loops
|
|
* filter clauses in comprehensions and generator expressions
|
|
|
|
In the current PEP, switching from truth-checking with ``and`` and ``or`` to
|
|
existence-checking is a matter of substituting in the new keywords, ``?then``
|
|
and ``?else`` in the appropriate places.
|
|
|
|
For other truth-checking contexts, it proposes either importing and
|
|
using the ``operator.exists`` API, or else continuing with the current idiom
|
|
of checking specifically for ``expr is not None`` (or the context appropriate
|
|
equivalent).
|
|
|
|
The simplest possible enhancement in that regard would be to elevate the
|
|
proposed ``exists()`` API from an operator module function to a new builtin
|
|
function.
|
|
|
|
Alternatively, the ``?`` existence checking symbol could be supported as a
|
|
modifier on the ``if`` and ``while`` keywords to indicate the use of an
|
|
existence check rather than a truth check.
|
|
|
|
However, it isn't at all clear that the potential consistency benefits gained
|
|
for either suggestion would justify the additional disruption, so they've
|
|
currently been omitted from the proposal.
|
|
|
|
|
|
Defining expected invariant relations between ``__bool__`` and ``__exists__``
|
|
-----------------------------------------------------------------------------
|
|
|
|
The PEP currently leaves the definition of ``__bool__`` on all existing types
|
|
unmodified, which ensures the entire proposal remains backwards compatible,
|
|
but results in the following cases where ``bool(obj)`` returns ``True``, but
|
|
the proposed ``operator.exists(obj)`` would return ``False``:
|
|
|
|
* ``NaN`` values for ``float``, ``complex``, and ``decimal.Decimal``
|
|
* ``Ellipsis``
|
|
* ``NotImplemented``
|
|
|
|
The main argument for potentially changing these is that it becomes easier to
|
|
reason about potential code behaviour if we have a recommended invariant in
|
|
place saying that values which indicate they don't exist in an existence
|
|
checking context should also report themselves as being ``False`` in a truth
|
|
checking context.
|
|
|
|
Failing to define such an invariant would lead to arguably odd outcomes like
|
|
``float("NaN") ?else 0.0`` returning ``0.0`` while ``float("NaN") or 0.0``
|
|
returns ``NaN``.
|
|
|
|
|
|
Limitations
|
|
===========
|
|
|
|
Arbitrary sentinel objects
|
|
--------------------------
|
|
|
|
This proposal doesn't attempt to provide syntactic support for the "sentinel
|
|
object" idiom, where ``None`` is a permitted explicit value, so a
|
|
separate sentinel object is defined to indicate missing values::
|
|
|
|
_SENTINEL = object()
|
|
def f(obj=_SENTINEL):
|
|
return obj if obj is not _SENTINEL else default_value()
|
|
|
|
This could potentially be supported at the expense of making the existence
|
|
protocol definition significantly more complex, both to define and to use:
|
|
|
|
* at the Python layer, ``operator.exists`` and ``__exists__`` implementations
|
|
would return the empty tuple to indicate non-existence, and otherwise return
|
|
a singleton tuple containing a reference to the object to be used as the
|
|
result of the existence check
|
|
* at the C layer, ``tp_exists`` implementations would return NULL to indicate
|
|
non-existence, and otherwise return a ``PyObject *`` pointer as the
|
|
result of the existence check
|
|
|
|
Given that change, the sentinel object idiom could be rewritten as::
|
|
|
|
class Maybe:
|
|
SENTINEL = object()
|
|
def __init__(self, value):
|
|
self._result = (value,) is value is not self.SENTINEL else ()
|
|
def __exists__(self):
|
|
return self._result
|
|
|
|
def f(obj=Maybe.SENTINEL):
|
|
return Maybe(obj) ?else default_value()
|
|
|
|
However, I don't think cases where the 3 proposed standard sentinel values (i.e.
|
|
``None``, ``Ellipsis`` and ``NotImplemented``) can't be used are going to be
|
|
anywhere near common enough for the additional protocol complexity and the loss
|
|
of symmetry between ``__bool__`` and ``__exists__`` to be worth it.
|
|
|
|
|
|
Specification
|
|
=============
|
|
|
|
The Abstract already gives the gist of the proposal and the Rationale gives
|
|
some specific examples. If there's enough interest in the basic idea, then a
|
|
full specification will need to provide a precise correspondence between the
|
|
proposed syntactic sugar and the underlying conditional expressions that is
|
|
sufficient to guide the creation of a reference implementation.
|
|
|
|
...TBD...
|
|
|
|
|
|
Implementation
|
|
==============
|
|
|
|
As with :pep:`505`, actual implementation has been deferred pending in-principle
|
|
interest in the idea of adding these operators - the implementation isn't
|
|
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...
|
|
|
|
|
|
References
|
|
==========
|
|
|
|
.. [1] Wikipedia: Safe navigation operator
|
|
(https://en.wikipedia.org/wiki/Safe_navigation_operator)
|
|
|
|
.. [2] Wikipedia: Null coalescing operator
|
|
(https://en.wikipedia.org/wiki/Null_coalescing_operator)
|
|
|
|
.. [3] FileFormat.info: Unicode Character 'THERE EXISTS' (U+2203)
|
|
(http://www.fileformat.info/info/unicode/char/2203/index.htm)
|
|
|
|
.. [4] python-ideas discussion thread
|
|
(https://mail.python.org/pipermail/python-ideas/2016-October/043415.html)
|
|
|
|
.. [5] Steven D'Aprano's critique of the proposal
|
|
(https://mail.python.org/pipermail/python-ideas/2016-October/043453.html)
|
|
|
|
.. [6] Considering a link to the idea of overloadable Boolean operators
|
|
(https://mail.python.org/pipermail/python-ideas/2016-October/043447.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/
|