PEP 531 updates after more time to reflect
- proposed precondition operator is now "?then" - discusses the possible use of mathematical existence notation - discusses some of the implications of different design decisions - assorted other fixes and clarifications
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pep-0531.txt
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pep-0531.txt
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@ -8,25 +8,35 @@ 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 logical operators to Python:
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of two new control flow operators to Python:
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* Existence-checking fallback: ``expr1 ?else expr2``
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* Existence-checking precondition: ``expr1 ?and expr2``
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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: ``obj?.attr`` (for ``obj ?and obj.attr``)
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* Existence-checking subscripting: ``obj?[expr]`` (for ``obj ?and obj[expr]``)
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* Existence-checking assignment: ``target ?= expr``
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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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These expressions will be defined in terms of a new "existence" protocol,
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accessible as ``operator.exists``, with the following characteristics:
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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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@ -34,12 +44,13 @@ accessible as ``operator.exists``, with the following characteristics:
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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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* Python's builtin and standard library numeric types will override the
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existence check such that ``NaN`` values return ``False`` and other
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values return ``True``
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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 boolean context (zeroes,
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empty containers) evaluate to True in an existence checking context
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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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Relationship with other PEPs
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@ -47,15 +58,15 @@ Relationship with other PEPs
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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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differences in the specifics of the proposed syntax and semantics for the
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feature.
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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 it's a good idea for us to learn from how other
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language designers are handling the problem, but precedents set elsewhere
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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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@ -91,39 +102,71 @@ 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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* ``value = expr.field.of.interest if expr is not None else None``
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* ``value = expr["field"]["of"]["interest"] if expr is not None else None``
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* ``value = expr1 if expr1 is not None else expr2 if expr2 is not None else expr3``
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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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_expr1 = expr1
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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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_expr2 = expr2
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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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_expr3 = expr3
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if _expr3 is not None:
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value3 = _expr3
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else:
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_expr4 = expr4
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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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* ``value = expr?.field.of.interest``
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* ``value = expr?["field"]["of"]["interest"]``
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* ``value = expr1 ?else expr2 ?else expr3``
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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 the first two examples, the 30 character boilerplate clause
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`` if expr is not None else None`` (minimally 27 characters for a single letter
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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 expr1 is not None`` (minimally 17 characters) are replaced with single
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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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common conceptual basis for the existence checking attribute access and
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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 ?and obj.attr``
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* ``obj?[expr]``is roughly equivalent to ``obj ?and obj[expr]``
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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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@ -140,18 +183,31 @@ handling idiom:
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* ``value = value if value is not None else expensive_default()``
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allowing that to instead be abbreviated as:
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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
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Even without this specific change, the PEP would still
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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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@ -172,16 +228,50 @@ 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 moreso than they represent data.
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Proposed syntax
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---------------
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Without a mathematical precedent to draw on (as Python historically has for
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other operations), the proposed use of ``?`` as the key syntactic marker for
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this feature is primarily derived from the corresponding syntax in other
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languages that offer similar features.
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Proposed symbolic notation
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--------------------------
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Drawing from the excellent 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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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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@ -203,17 +293,60 @@ we see:
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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 ``?`` is far and away
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the most common syntactic marker (presumably prompted by the use of ``?`` in
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C-style conditional expressions, which many of these languages also offer).
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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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``?else`` is proposed over ``?or`` for the existence checking fallback syntax
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simply because it reads more clearly as "choose the first subexpression that
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exists" when multiple instances of the expression are chained together.
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``?and`` is proposed as the spelling for the existence checking precondition
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syntax as it semantically relates to ``?else`` in the same way that ``and``
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relates to ``or``.
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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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@ -225,7 +358,7 @@ Readability
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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 ``?`` will return None if their input is None
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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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@ -234,6 +367,7 @@ 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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@ -242,11 +376,11 @@ There's nothing about ``?`` as a syntactic element that inherently suggests
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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 make the most sense
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as a modifier on existing operators (aside from the proposed spelling of
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the fallback operator as ``?else`` rather than ``?or``), and that calls for
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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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@ -259,7 +393,8 @@ 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).
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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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@ -269,14 +404,117 @@ As a simple example of the new parallels introduced by this PEP, compare::
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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
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checking symbol could instead be attached to the ``if`` keyword:
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* ``value = expr1 if? cond else expr2``
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Existence checking in other truth-checking contexts
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---------------------------------------------------
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The truth-checking protocol is currently used in the following syntactic
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constructs:
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* logical conjunction (and-expressions)
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* logical disjunction (or-expressions)
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* conditional expressions (if-else expressions)
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* if statements
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* while loops
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* filter clauses in comprehensions and generator expressions
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|
||||
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 currently attempt to provide syntactic support for the
|
||||
"sentinel object" idiom, where ``None`` is a permitted explicit value, so a
|
||||
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()
|
||||
|
|
@ -306,8 +544,8 @@ Given that change, the sentinel object idiom could be rewritten as::
|
|||
def f(obj=Maybe.SENTINEL):
|
||||
return Maybe(obj) ?else default_value()
|
||||
|
||||
However, I don't think cases where none of the 3 standard sentinel values (i.e.
|
||||
``None``, ``Ellipsis`` and ``NotImplemented``) can be used are going to be
|
||||
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.
|
||||
|
||||
|
|
@ -318,7 +556,8 @@ 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.
|
||||
proposed syntactic sugar and the underlying conditional expressions that is
|
||||
sufficient to guide the creation of a reference implementation.
|
||||
|
||||
...TBD...
|
||||
|
||||
|
|
@ -346,13 +585,15 @@ References
|
|||
.. [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)
|
||||
|
||||
|
||||
Copyright
|
||||
=========
|
||||
|
||||
This document has been placed in the public domain.
|
||||
|
||||
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/
|
||||
|
||||
|
||||
..
|
||||
|
|
|
|||
Loading…
Reference in New Issue