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Readability tweaks to PEP 646 (#1859)

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@ -132,8 +132,6 @@ Specification
In order to support the above use cases, we introduce ``TypeVarTuple``. This serves as a placeholder not for a single type but for an *arbitrary* number of types, and behaving like a number of ``TypeVar`` instances packed in a ``Tuple``. In order to support the above use cases, we introduce ``TypeVarTuple``. This serves as a placeholder not for a single type but for an *arbitrary* number of types, and behaving like a number of ``TypeVar`` instances packed in a ``Tuple``.
These are described in detail below.
Type Variable Tuples Type Variable Tuples
-------------------- --------------------
@ -164,7 +162,7 @@ Type variable tuples behave like a number of individual type variables packed in
The ``Shape`` type variable tuple here behaves like ``Tuple[T1, T2]``, The ``Shape`` type variable tuple here behaves like ``Tuple[T1, T2]``,
where ``T1`` and ``T2`` are type variables. To use these type variables where ``T1`` and ``T2`` are type variables. To use these type variables
as type parameters of ``Array``, we must **unpack** the type variable tuple using as type parameters of ``Array``, we must *unpack* the type variable tuple using
the star operator: ``*Shape``. The signature of ``Array`` then behaves the star operator: ``*Shape``. The signature of ``Array`` then behaves
as if we had simply written ``class Array(Generic[T1, T2]): ...``. as if we had simply written ``class Array(Generic[T1, T2]): ...``.
@ -213,8 +211,8 @@ This is, in fact, deliberately not possible: type variable tuples must
for two reasons: for two reasons:
* To avoid potential confusion about whether to use a type variable tuple * To avoid potential confusion about whether to use a type variable tuple
in a packed or unpacked form ("Hmm, should I do ``-> Shape``, in a packed or unpacked form ("Hmm, should I write '``-> Shape``',
or ``-> Tuple[Shape]``, or ``-> Tuple[*Shape]``...?") or '``-> Tuple[Shape]``', or '``-> Tuple[*Shape]``'...?")
* To improve readability: the star also functions as an explicit visual * To improve readability: the star also functions as an explicit visual
indicator that the type variable tuple is not a normal type variable. indicator that the type variable tuple is not a normal type variable.
@ -224,7 +222,7 @@ for two reasons:
Note that the use of the star operator in this context requires a grammar change, Note that the use of the star operator in this context requires a grammar change,
and is therefore available only in new versions of Python. To enable use of type and is therefore available only in new versions of Python. To enable use of type
variable tuples in older versions of Python, we introduce the ``Unpack`` type variable tuples in older versions of Python, we introduce the ``Unpack`` type
operator that can be used in place of the star operator to unpack type variable tuples: operator that can be used in place of the star operator:
:: ::
@ -244,13 +242,13 @@ To keep this PEP minimal, ``TypeVarTuple`` does not yet support specification of
* Type bounds (``TypeVar('T', bound=ParentClass)``) * Type bounds (``TypeVar('T', bound=ParentClass)``)
We leave the decision of how these arguments should behave to a future PEP, when variadic generics have been tested in the field. As of this PEP, type variable tuples are We leave the decision of how these arguments should behave to a future PEP, when variadic generics have been tested in the field. As of this PEP, type variable tuples are
**invariant**. invariant.
Behaviour when Type Parameters are not Specified Behaviour when Type Parameters are not Specified
'''''''''''''''''''''''''''''''''''''''''''''''' ''''''''''''''''''''''''''''''''''''''''''''''''
When a generic class parameterised by a type variable tuple is used without When a generic class parameterised by a type variable tuple is used without
any type parameters, it behaves as if its type parameters are ``Any, ...`` any type parameters, it behaves as if its type parameters are '``Any, ...``'
(an arbitrary number of ``Any``): (an arbitrary number of ``Any``):
:: ::
@ -263,8 +261,8 @@ any type parameters, it behaves as if its type parameters are ``Any, ...``
takes_any_array(y) # Also valid takes_any_array(y) # Also valid
This enables gradual typing: existing functions accepting, for example, This enables gradual typing: existing functions accepting, for example,
a plain ``tf.Tensor`` will still be valid even if called with a plain TensorFlow ``Tensor`` will still be valid even if ``Tensor`` is made
a parameterised ``Tensor[Height, Width]``. generic and calling code passes a ``Tensor[Height, Width]``.
This also works in the opposite direction: This also works in the opposite direction:
@ -275,7 +273,7 @@ This also works in the opposite direction:
z: Array z: Array
takes_specific_array(z) takes_specific_array(z)
Thereby even if libraries are updated to use types like ``Array[Height, Width]``, This way, even if libraries are updated to use types like ``Array[Height, Width]``,
users of those libraries won't be forced to also apply type annotations to users of those libraries won't be forced to also apply type annotations to
all of their code; users still have a choice about what parts of their code all of their code; users still have a choice about what parts of their code
to type and which parts to not. to type and which parts to not.
@ -303,9 +301,10 @@ that is, ``Tuple[int, int]``, but not ``Tuple[int, ...]``.
Note that, as a result of this rule, omitting the type parameter list is the Note that, as a result of this rule, omitting the type parameter list is the
*only* way of instantiating a generic type with an arbitrary number of *only* way of instantiating a generic type with an arbitrary number of
type parameters. For example, ``Array`` may *behave* like ``Array[Any, ...]``, type parameters. (We plan to introduce a more deliberate syntax for this
but it cannot be instantiated using ``Array[Any, ...]``, because this would case in a future PEP.) For example, an unparameterised ``Array`` may
bind its type variable tuple to ``Tuple[Any, ...]``: *behave* like ``Array[Any, ...]``, but it cannot be instantiated using
``Array[Any, ...]``, because this would bind its type variable tuple to ``Tuple[Any, ...]``:
:: ::
@ -370,7 +369,9 @@ prefixed and/or suffixed:
def add_batch_axis(x: Array[*Shape]) -> Array[Batch, *Shape]: ... def add_batch_axis(x: Array[*Shape]) -> Array[Batch, *Shape]: ...
def del_batch_axis(x: Array[Batch, *Shape]) -> Array[*Shape]: ... def del_batch_axis(x: Array[Batch, *Shape]) -> Array[*Shape]: ...
def add_batch_channels(x: Array[*Shape]) -> Array[Batch, *Shape, Channels]: ... def add_batch_channels(
x: Array[*Shape]
) -> Array[Batch, *Shape, Channels]: ...
a: Array[Height, Width] a: Array[Height, Width]
b = add_batch_axis(a) # Inferred type is Array[Batch, Height, Width] b = add_batch_axis(a) # Inferred type is Array[Batch, Height, Width]
@ -396,7 +397,7 @@ Normal ``TypeVar`` instances can also be prefixed and/or suffixed:
``*args`` as a Type Variable Tuple ``*args`` as a Type Variable Tuple
---------------------------------- ----------------------------------
PEP 484 states that when a type annotation is provided for ``*args``, each argument PEP 484 states that when a type annotation is provided for ``*args``, every argument
must be of the type annotated. That is, if we specify ``*args`` to be type ``int``, must be of the type annotated. That is, if we specify ``*args`` to be type ``int``,
then *all* arguments must be of type ``int``. This limits our ability to specify then *all* arguments must be of type ``int``. This limits our ability to specify
the type signatures of functions that take heterogeneous argument types. the type signatures of functions that take heterogeneous argument types.
@ -494,15 +495,15 @@ a similar way to regular type variables:
:: ::
IntTuple = Tuple[int, *Ts] IntTuple = Tuple[int, *Ts]
NamedArray = Tuple[str, Array[*Ts]] NamedTuple = Tuple[str, Tuple[*Ts]]
IntTuple[float, bool] # Equivalent to Tuple[int, float, bool] IntTuple[float, bool] # Equivalent to Tuple[int, float, bool]
NamedArray[Height] # Equivalent to Tuple[str, Array[Height]] NamedTuple[int, int] # Equivalent to Tuple[str, Tuple[int, int]]
As this example shows, all type parameters passed to the alias are As this example shows, all type parameters passed to the alias are
bound to the type variable tuple. bound to the type variable tuple.
Importantly for our ``Array`` example (see `Summary Examples`_), this Importantly for our original ``Array`` example (see `Summary Examples`_), this
allows us to define convenience aliases for arrays of a fixed shape allows us to define convenience aliases for arrays of a fixed shape
or datatype: or datatype:
@ -524,7 +525,7 @@ tuple in the alias is set empty:
:: ::
IntTuple[()] # Equivalent to Tuple[int] IntTuple[()] # Equivalent to Tuple[int]
NamedArray[()] # Equivalent to Tuple[str, Array[()]] NamedTuple[()] # Equivalent to Tuple[str, Tuple[()]]
If the type parameter list is omitted entirely, the alias is If the type parameter list is omitted entirely, the alias is
compatible with arbitrary type parameters: compatible with arbitrary type parameters:
@ -535,7 +536,9 @@ compatible with arbitrary type parameters:
x: Float32Array[Height, Width] = Array() x: Float32Array[Height, Width] = Array()
takes_float_array_of_any_shape(x) # Valid takes_float_array_of_any_shape(x) # Valid
def takes_float_array_with_specific_shape(y: Float32Array[Height, Width]): ... def takes_float_array_with_specific_shape(
y: Float32Array[Height, Width]
): ...
y: Float32Array = Array() y: Float32Array = Array()
takes_float_array_with_specific_shape(y) # Valid takes_float_array_with_specific_shape(y) # Valid
@ -556,7 +559,8 @@ Normal ``TypeVar`` instances can also be used in such aliases:
Overloads for Accessing Individual Types Overloads for Accessing Individual Types
---------------------------------------- ----------------------------------------
For situations where we require access to each individual type, overloads can be used with individual ``TypeVar`` instances in place of the type variable tuple: For situations where we require access to each individual type in the type variable tuple,
overloads can be used with individual ``TypeVar`` instances in place of the type variable tuple:
:: ::
@ -580,16 +584,16 @@ For situations where we require access to each individual type, overloads can be
(For array shape operations in particular, having to specify (For array shape operations in particular, having to specify
overloads for each possible rank is, of course, a rather cumbersome overloads for each possible rank is, of course, a rather cumbersome
solution. However, it's the best we can do without additional type solution. However, it's the best we can do without additional type
manipulation mechanisms, which are beyond the scope of this PEP.) manipulation mechanisms. We plan to introduce these in a future PEP.)
Rationale and Rejected Ideas Rationale and Rejected Ideas
============================ ============================
Supporting Variadicity Through aliases Supporting Variadicity Through Aliases
-------------------------------------- --------------------------------------
As noted in the introduction, it **is** possible to avoid variadic generics As noted in the introduction, it *is* possible to avoid variadic generics
by simply defining aliases for each possible number of type parameters: by simply defining aliases for each possible number of type parameters:
:: ::
@ -636,7 +640,7 @@ of the following examples should type-check correctly:
y: Array y: Array
takes_specific_array(y) takes_specific_array(y)
Note that this is in contrast to the behaviour of the only existing Note that this is in contrast to the behaviour of the only currently-existing
variadic type in Python, ``Tuple``: variadic type in Python, ``Tuple``:
:: ::
@ -715,28 +719,6 @@ Footnotes
shape begins with 'time × batch', then ``videos_batch[1][0]`` would select the shape begins with 'time × batch', then ``videos_batch[1][0]`` would select the
same frame. same frame.
References
==========
.. [#typing193] Python typing issue #193: https://github.com/python/typing/issues/193
.. [#pep-612] PEP 612, "Parameter Specification Variables":
https://www.python.org/dev/peps/pep-0612
.. [#numeric-stack] Static typing of Python numeric stack:
https://paper.dropbox.com/doc/Static-typing-of-Python-numeric-stack-summary-6ZQzTkgN6e0oXko8fEWwN
.. [#typing-ideas] Ideas for array shape typing in Python: https://docs.google.com/document/d/1vpMse4c6DrWH5rq2tQSx3qwP_m_0lyn-Ij4WHqQqRHY/edit
.. [#syntax-proposal] Shape annotation syntax proposal:
https://docs.google.com/document/d/1But-hjet8-djv519HEKvBN6Ik2lW3yu0ojZo6pG9osY/edit
.. [#arbitrary_len] Discussion on Python typing-sig mailing list: https://mail.python.org/archives/list/typing-sig@python.org/thread/SQVTQYWIOI4TIO7NNBTFFWFMSMS2TA4J/
.. _cpython/23527: https://github.com/python/cpython/pull/24527
.. _mrahtz/cpython/pep637+646: https://github.com/mrahtz/cpython/tree/pep637%2B646
Acknowledgements Acknowledgements
================ ================
@ -751,24 +733,45 @@ Thank you especially to **Lucio**, for suggesting the star syntax, which has mad
Resources Resources
========= =========
Discussions on variadic generics in Python started in 2016 with `Issue 193`__ Discussions on variadic generics in Python started in 2016 with Issue 193
on the python/typing GitHub repository. on the python/typing GitHub repository [#typing193]_.
__ https://github.com/python/typing/issues/193
Inspired by this discussion, **Ivan Levkivskyi** made a concrete proposal Inspired by this discussion, **Ivan Levkivskyi** made a concrete proposal
at PyCon 2019, summarised in `Type system improvements`__ at PyCon 2019, summarised in notes on 'Type system improvements' [#type-improvements]_
and `Static typing of Python numeric stack`__. and 'Static typing of Python numeric stack' [#numeric-stack]_.
__ https://paper.dropbox.com/doc/Type-system-improvements-HHOkniMG9WcCgS0LzXZAe
__ https://paper.dropbox.com/doc/Static-typing-of-Python-numeric-stack-summary-6ZQzTkgN6e0oXko8fEWwN
Expanding on these ideas, **Mark Mendoza** and **Vincent Siles** gave a presentation on Expanding on these ideas, **Mark Mendoza** and **Vincent Siles** gave a presentation on
`Variadic Type Variables for Decorators and Tensors`__ at the 2019 Python 'Variadic Type Variables for Decorators and Tensors' [#variadic-type-variables]_ at the 2019 Python
Typing Summit. Typing Summit.
__ https://github.com/facebook/pyre-check/blob/ae85c0c6e99e3bbfc92ec55104bfdc5b9b3097b2/docs/Variadic_Type_Variables_for_Decorators_and_Tensors.pdf
References
==========
.. [#typing193] Python typing issue #193:
https://github.com/python/typing/issues/193
.. [#type-improvements] Ivan Levkivskyi, 'Type system improvements', PyCon 2019:
https://paper.dropbox.com/doc/Type-system-improvements-HHOkniMG9WcCgS0LzXZAe
.. [#numeric-stack] Ivan Levkivskyi, 'Static typing of Python numeric stack', PyCon 2019:
https://paper.dropbox.com/doc/Static-typing-of-Python-numeric-stack-summary-6ZQzTkgN6e0oXko8fEWwN
.. [#typing-ideas] Stephan Hoyer, 'Ideas for array shape typing in Python':
https://docs.google.com/document/d/1vpMse4c6DrWH5rq2tQSx3qwP_m_0lyn-Ij4WHqQqRHY/edit
.. [#variadic-type-variables] Mark Mendoza, 'Variadic Type Variables for Decorators and Tensors', Python Typing Summit 2019:
https://github.com/facebook/pyre-check/blob/ae85c0c6e99e3bbfc92ec55104bfdc5b9b3097b2/docs/Variadic_Type_Variables_for_Decorators_and_Tensors.pdf
.. [#syntax-proposal] Matthew Rahtz et al., 'Shape annotation syntax proposal':
https://docs.google.com/document/d/1But-hjet8-djv519HEKvBN6Ik2lW3yu0ojZo6pG9osY/edit
.. [#arbitrary_len] Discussion on Python typing-sig mailing list:
https://mail.python.org/archives/list/typing-sig@python.org/thread/SQVTQYWIOI4TIO7NNBTFFWFMSMS2TA4J/
.. _cpython/23527: https://github.com/python/cpython/pull/24527
.. _mrahtz/cpython/pep637+646: https://github.com/mrahtz/cpython/tree/pep637%2B646
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