PEP 570: grammar and wording fixes (GH-923)
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Pablo Galindo
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pep-0570.rst
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pep-0570.rst
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@ -33,49 +33,49 @@ positional-only. This changed around Python 1.0, when
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all parameters suddenly became positional-or-keyword.
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This allowed users to provide arguments to a function both
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positionally or referencing the keyword used in the definition
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of it. But, this is not always desired nor even available as
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of it. But, this is not always desired nor available as
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even in current versions of Python, many CPython
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"builtin" functions still only accept positional-only arguments.
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Users might want to restrict their API to not allow for parameters
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to be referenced via keyword, as that exposes the name of the
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to be referenced via keywords, as that exposes the name of the
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parameter as part of the API. If a user of said API starts using the
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argument by keyword when calling it and then the name of the parameter
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is changed, it will be a breaking change. By using positional-only
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parameters the developer can later change the name of an argument or
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argument by keyword when calling it and then the parameter
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gets renamed, it will be a breaking change. By using positional-only
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parameters the developer can later change the name of an arguments or
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transform them to ``*args`` without breaking the API.
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Even if positional arguments only in a function can be achieved
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via using ``*args`` parameters and extracting them one by one,
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Even if making arguments positional-only in a function can be achieved
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by using ``*args`` parameters and extracting them one by one,
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the solution is far from ideal and not as expressive as the one
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proposed in this PEP, which targets to provide syntax to specify
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proposed in this PEP, which targets providing syntax to specify
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accepting a specific number of positional-only parameters. Also,
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it makes the signature of the function ambiguous as users won't
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know how many parameters the function takes by looking at help()
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know how many parameters the function takes by looking at ``help()``
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or auto-generated documentation.
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Additionally, this will bridge the gap we currently find between
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builtin functions that today allows to specify positional-only
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builtin functions that can specify positional-only
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parameters and pure Python implementations that lack the
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syntax for it. The '/' syntax is already exposed in the
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documentation for some builtins and interfaces generated by
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the argument clinic. Making positional only arguments a possibility
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in Python will bring consistency and will remove confusion from
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users that are not familiarized with the fact that positional only
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arguments are allowed in builtins and argument clinic C interfaces.
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documentation of some builtins and interfaces generated by
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the argument clinic. Making positional-only arguments a possibility
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in Python will make the language more consistent and make it clearer
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to users that positional-only arguments are allowed in builtins and argument
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clinic C interfaces.
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We find this useful in multiple situation, say for example we
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are looking at creating a function that converts from one type to
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We can find positional-only arguments useful in several situations. Example:
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we want to create a function that converts from one type to
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another::
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def as_my_type(x):
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...
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The name of the parameter provides no value whatsoever and forces
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the user to maintain it's name forever as user might rely on it
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being used as a keyword only.
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The name of the parameter provides no value whatsoever, and forces
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the developer to maintain its name forever, as users might pass ``x`` as a
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keyword.
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Another good example is APIs that want to transmit the feeling
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Another good example is an API that wants to transmit the feeling
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of ownership through positional arguments, see::
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class MyDecorator:
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@ -83,15 +83,15 @@ of ownership through positional arguments, see::
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...
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Again we get no value from using keyword arguments here and it can limit
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future evolutions of the API. Say at a later time we want the decorator
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to be able to take multiple functions, we will be forced to keep
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the original argument always or we'd potentially break users.
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By being able to define positional only arguments we can change the
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name of those at will or even change them by ``*args``.
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future evolution of the API. Say at a later time we want the decorator
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to be able to take multiple functions, we will be forced to always keep
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the original argument or we would potentially break users.
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By being able to define positional-only arguments we can change the
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name of those at will or even change them to ``*args``.
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-----------------------------------------------------
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Positional-Only Parameter Semantics In Current Python
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-----------------------------------------------------
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---------------------------------------------------
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Positional-Only Parameter Semantics In Python Today
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---------------------------------------------------
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There are many, many examples of builtins that only
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accept positional-only parameters. The resulting
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@ -109,8 +109,8 @@ arguments by name::
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File "<stdin>", line 1, in <module>
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TypeError: pow() takes no keyword arguments
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Pow clearly expresses that its arguments are only positional
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via the ``/`` marker, but this is at the moment only documentational,
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``pow()`` clearly expresses that its arguments are only positional
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via the ``/`` marker, but this at the moment is only documentational,
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Python developers cannot write such syntax.
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In addition, there are some functions with particularly
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@ -128,9 +128,9 @@ interesting semantics:
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Obviously one can simulate any of these in pure Python code
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by accepting ``(*args, **kwargs)`` and parsing the arguments
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by hand. But this results in a disconnect between the
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Python function signature and what it actually accepts,
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Python function signature and what the function actually accepts,
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not to mention the work of implementing said argument parsing
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and the lack of clarity that generates in the signature.
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and the lack of clarity in the resulting signature.
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==========
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Motivation
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@ -138,28 +138,28 @@ Motivation
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The new syntax will allow developers to further control how their
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API can be consumed. It will allow restricting the usage of keyword
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Specify arguments by adding the new type of positional-only ones.
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arguments by adding the new type of positional-only ones.
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A similar PEP with a broader scope (PEP 457) was proposed
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to define the syntax. This PEP builds on top of part of it
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to define and provide an implementation for the ``/`` syntax on
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A similar PEP with a broader scope (PEP 457) was proposed earlier
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to define the syntax. This PEP builds partially on top of that,
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to define and provide an implementation for the ``/`` syntax in
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function signatures.
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Providing positional only arguments will allow for maintaining the
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Providing positional-only arguments will allow for maintaining the
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interface when creating pure Python implementation of C modules, which
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provides not only the API benefits outlined in this document but it is
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also faster, see this thread about converting keyword arguments to positional
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also faster. See this thread about converting keyword arguments to positional:
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[#thread-keyword-to-positional]_ and PEP-399 [#PEP399]_, which requires the
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same API for C accelerators than the Python implementation.
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same API for C accelerators as the Python implementation.
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There has been multiple changes in builtin functions that moved away
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There have been multiple changes in builtin functions that moved away
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from keyword arguments, like ``bool``, ``float``, ``list``, ``int``, ``tuple``
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which is a non-backward compatible. By having proper support for
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positional only arguments, this kind of APIs where it is clear that
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passing a keyword argument provides no clarity would be able to
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follow a similar approach that those builtins without breaking users.
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which is a non-backward compatible change. By having proper support for
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positional-only arguments, these kind of APIs, where it is clear that
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passing a keyword argument provides no clarity, it would be possible to
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follow a similar approach as these builtins, without breaking users.
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This is a well discussed recurring topic in the Python mailing lists:
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This is a well discussed, recurring topic on the Python mailing lists:
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* September 2018: `Anders Hovmöller: [Python-ideas] Positional-only
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parameters
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@ -193,10 +193,10 @@ and inconsistent:
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* Some functions use *both* of the above approaches. [#RANGE]_ [#ADDCH]_
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One more important idea to consider: currently in the documentation
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there's no way to tell whether a function takes positional-only
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there is no way to tell whether a function takes positional-only
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parameters. ``open()`` accepts keyword arguments, ``ord()`` does
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not, but there is no way of telling just by reading the
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documentation that this is true.
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documentation.
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====================
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Syntax And Semantics
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@ -217,10 +217,10 @@ All parameters before the ``/`` are positional-only. If ``/`` is
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not specified in a function signature, that function does not
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accept any positional-only parameters.
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The logic around optional values for positional-only argument
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Remains the same as the one for positional-or-keyword. Once
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remains the same as the one for positional-or-keyword. Once
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a positional-only argument is provided with a default,
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the following positional-only and positional-or-keyword argument
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need to have a default as well. Positional-only parameters that
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needs to have a default as well. Positional-only parameters that
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don’t have a default value are "required" positional-only parameters.
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Therefore the following are valid signatures::
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@ -249,11 +249,13 @@ A draft of the proposed grammar specification is::
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new_varargslist:
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vfpdef ['=' test] (',' vfpdef ['=' test])* ',' '/' [',' [varargslist]] | varargslist
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It will be added to the actual typedargslist and varargslist but for easier discussion is
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presented as new_typedargslist and new_varargslist. Also, notice that using a construction
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using two new rules (new_varargslist and new_varargslist) is not possible with the current
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parser as the rule is not LL(1). This is the reason the rule needs to be include in the
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existing typedargslist and varargslist (in the same way keyword-only arguments were introduced).
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It will be added to the actual typedargslist and varargslist but for easier
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discussion it is presented as new_typedargslist and new_varargslist. Also,
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notice that using a construction with two new rules (new_varargslist and
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new_varargslist) is not possible with the current parser as the rule is not
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LL(1). This is the reason the rule needs to be included in the existing
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typedargslist and varargslist (in the same way keyword-only arguments were
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introduced).
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==============
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@ -285,21 +287,21 @@ After marker proposal
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---------------------
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A complaint against the proposal is the fact that the modifier of
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the signature impacts the "already passed" tokens.
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the signature impacts the tokens already passed.
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This might make confusing to "human parsers" to read functions
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This might make it confusing to users to read functions
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with many arguments. Example::
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def really_bad_example_of_a_python_function(fist_long_argument, second_long_argument,
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third_long_argument, /):
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It is not until you reach the end of the signature that the reader
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realized the ``/`` and therefore the fact that the arguments are
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It is not until reaching the end of the signature that the reader
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realizes the ``/``, and therefore the fact that the arguments are
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position-only. This deviates from how the keyword-only marker works.
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That said we could not find an implementation that would modify the
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arguments after the marker, as that will force the one before the
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marker to be position only as well. Example::
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marker to be position-only as well. Example::
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def (x, y, /, z):
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@ -307,48 +309,50 @@ If we define that ``/`` makes only z position-only it won't be possible
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to call x and y via keyword argument. Finding a way to work around it
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will add confusion given that at the moment keyword arguments cannot be
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followed by positional arguments. ``/`` will therefore make both the
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preceding and following position-only.
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preceding and following parameters position-only.
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-------------------
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Per-argument marker
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-------------------
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Using a per argument marker might be an option as well. The approach
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Using a per-argument marker might be an option as well. The approach
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basically adds a token to each of the arguments that are position only
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and requires those to be placed together. Example::
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def (.arg1, .arg2, arg3):
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Note the dot on arg1 and arg2. Even if this approach might look easier
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to read it has been discarded as ``/`` goes further inline with the
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keyword-only approach and is less error prone.
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to read, it has been discarded as ``/`` goes further in line with the
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keyword-only approach and is less error-prone.
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There are some libraries that use leading underscore[#leading-underscore]_
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to mark those arguments as positional only.
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to mark those arguments as positional-only.
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----------------
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Using decorators
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----------------
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It has been suggested on python-ideas [#python-ideas-decorator-based]_ to provide
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a decorator written in Python as an implementation for this feature. This approach
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has the advantage that keeps parameter declaration more easy to read but also
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introduces an asymmetry on how parameter behaviour is declared. Also, as the ``/``
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syntax is already introduced for C functions, this inconsistency will make more
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difficult to implement all tools and modules that deal with this syntax including
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but not limited to, the argument clinic, the inspect module and the ast module.
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Another disadvantage of this approach is that calling the decorated functions
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will be slower than the functions generated if the feature was implemented directly
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in C.
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It has been suggested on python-ideas [#python-ideas-decorator-based]_ to
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provide a decorator written in Python as an implementation for this feature.
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This approach has the advantage that keeps parameter declaration more easy to
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read but also introduces an asymmetry on how parameter behaviour is declared.
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Also, as the ``/`` syntax is already introduced for C functions, this
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inconsistency will make it more difficult to implement all tools and modules
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that deal with this syntax including but not limited to, the argument clinic,
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the inspect module and the ast module. Another disadvantage of this approach
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is that calling the decorated functions will be slower than the functions
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generated if the feature was implemented directly in C.
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======
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Thanks
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======
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Credit for most of the content of this PEP is contained in Larry Hastings’s PEP 457.
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Credit for most of the content of this PEP is contained in Larry Hastings’s
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PEP 457.
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Credit for the use of '/' as the separator between positional-only and positional-or-keyword
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parameters go to Guido van Rossum, in a proposal from 2012. [#GUIDO]_
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Credit for the use of '/' as the separator between positional-only and
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positional-or-keyword parameters go to Guido van Rossum, in a proposal from
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2012. [#GUIDO]_
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Credit for discussion about the simplification of the grammar goes to
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Braulio Valdivieso.
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