It’s a usability issue; mappings are used quite differently than sequences.
Compare to class patterns rather than sequence patterns.
On Sat, Nov 14, 2020 at 22:04 David Foster <davidf...@gmail.com> wrote:
> From PEP 636 (Structural Pattern Matching):
> > Mapping patterns: {"bandwidth": b, "latency": l} captures the
> "bandwidth" and "latency" values from a dict. Unlike sequence patterns,
> extra keys are ignored.
>
> It surprises me that ignoring extra keys would be the *default*
> behavior. This seems unsafe. Extra keys I would think would be best
> treated as suspicious by default.
>
> * Ignoring extra keys loses data silently. In the current proposal:
>
> point = {'x': 1, 'y': 2, 'z': 3)
> match point:
> case {'x': x, 'y': y}: # MATCHES, losing z O_O
> pass
> case {'x': x, 'y': y, 'z': z}: # will never match O_O
> pass
>
> * Ignoring extra keys is inconsistent with the handling of sequences: We
> don't allow extra items when using a destructuring assignment to a
> sequence:
>
> p = [1, 2]
> [x, y] = p
> [x, y, z] = p # ERROR: ValueError: not enough values to unpack
> (expected 3, got 2) :)
>
> * Ignoring extra keys in mapping patterns is inconsistent with the
> current proposal for how sequence patterns match data:
>
> point = [1, 2, 3]
> match point:
> case [x, y]: # notices extra value and does NOT match :)
> pass
> case [x, y, z]: # matches :)
> pass
>
> * Ignoring extra keys is inconsistent with TypedDict's default "total"
> matching behavior:
>
> from typing import TypedDict
>
> class Point2D(TypedDict):
> x: int
> y: int
>
> p1: Point2D = {'x': 1, 'y': 2}
> p2: Point2D = {'x': 1, 'y': 2, 'z': 3) # ERROR: Extra key 'z' for
> TypedDict "Point2D" :)
>
> * It is *possible* to force an exact key match with a pattern guard but
> it's clumsy to do so.
> It should not be clumsy to parse strictly.
>
> point = {'x': 1, 'y': 2, 'z': 3)
> match point:
> # notices extra value and does NOT match, but requires ugly
> guard :/
> case {'x': x, 'y': y, **rest} if rest == {}:
> pass
> case {'x': x, 'y': y, 'z': z, **rest} if rest == {}:
> pass
>
>
> To avoid the above problems, **I'd advocate for disallowing extra keys
> in mapping patterns by default**. For cases where extra keys want to be
> specifically allowed and ignored, I propose allowing a **_ wildcard.
>
>
> Some examples that illustrate behavior when *disallowing* extra keys in
> mapping patterns:
>
> 1. Strict parsing
>
> from typing import TypedDict, Union
>
> Point2D = TypedDict('Point2D', {'x': int, 'y': int})
> Point3D = TypedDict('Point3D', {'x': int, 'y': int, 'z': int})
>
> def parse_point(point_json: dict) -> Union[Point2D, Point3D]:
> match point_json:
> case {'x': int(x), 'y': int(y)}:
> return Point2D({'x': x, 'y': y})
> case {'x': int(x), 'y': int(y), 'z': int(z)}:
> return Point3D({'x': x, 'y': y, 'z': z})
> case _:
> raise ValueError(f'not a valid point: {point_json!r}')
>
> 2. Loose parsing, discarding unknown data.
> Common when reading JSON-like data when it's not necessary to output
> it again later.
>
> from typing import TypedDict
>
> TodoItem_ReadOnly = TypedDict('TodoItem_ReadOnly', {'title': str,
> 'completed': bool})
>
> def parse_todo_item(todo_item_json: Dict) -> TodoItem_ReadOnly:
> match todo_item_json:
> case {'title': str(title), 'completed': bool(completed), **_}:
> return TodoItem_ReadOnly({'title': title, 'completed':
> completed})
> case _:
> raise ValueError()
>
> input = {'title': 'Buy groceries', 'completed': True,
> 'assigned_to': ['me']}
> print(parse_todo_item(input)) # prints: {'title': 'Buy groceries',
> 'completed': True}
>
> 3. Loose parsing, preserving unknown data.
> Common when parsing JSON-like data when it needs to be round-tripped
> and output again later.
>
> from typing import Any, Dict, TypedDict
>
> TodoItem_ReadWrite = TypedDict('TodoItem_ReadWrite', {'title': str,
> 'completed': bool, 'extra': Dict[str, Any]})
>
> def parse_todo_item(todo_item_json: Dict) -> TodoItem_ReadWrite:
> match todo_item_json:
> case {'title': str(title), 'completed': bool(completed),
> **extra}:
> return TodoItem_ReadWrite({'title': title, 'completed':
> completed, 'extra': extra})
> case _:
> raise ValueError()
>
> def format_todo_item(item: TodoItem_ReadWrite) -> Dict:
> return {'title': item['title'], 'completed': item['completed'],
> **item['extra']}
>
> input = {'title': 'Buy groceries', 'completed': True,
> 'assigned_to': ['me']}
> output = format_todo_item(parse_todo_item(input))
> print(output) # prints: {'title': 'Buy groceries', 'completed':
> True, 'assigned_to': ['me']}
>
>
> Comments?
>
> --
> David Foster | Seattle, WA, USA
> Contributor to TypedDict support for mypy
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--
--Guido (mobile)
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