Comments (2)
Howdy!
You're right, the structure hook for lists (and sequences, mutablesequences, homogenous tuples) is very simple; you can see it here. In the case of no detailed validation, it boils down to a single list comprehension. This is for a comple of reasons:
- performance
- if we were to check, it's a little unclear what to check for. A certain JSON library might be guaranteed to return only lists, but we support a wide variety of formats and libraries
Also the docs don't list Iterable as something that gets structured to a list
We don't actually check if the input type is an iterable. We just iterate over it. But I get your point.
is there a way to make it more strict
This is a very good question. In my mind the core job of cattrs is for this to be easy to do.
Here's how you would do this today (let's say you decide to perform an additional validation step with an isinstance
against a Sequence
):
from collections.abc import Sequence
from cattrs import Converter
from cattrs._compat import is_sequence
c = Converter()
def seq_hook(val, type):
if not isinstance(val, Sequence):
raise ValueError(f"Not a {type}")
return c._structure_list(val, type)
c.register_structure_hook_func(is_sequence, seq_hook)
c.structure({}, list[int])
This will be composable: it will affect all lists. That said it kinda sucks.
- The predicate in an internal module (
cattrs._compat
) for historical reasons. It'd be better if it was in a public place. - The structure hook is, again, a private method. It's pretty old and hasn't gotten modernized since it was "good enough". It'd be better if it was a hook factory and documented.
- There's no documentation for it. It'd be better if there was a note about this in the docs.
These are all pretty easy fixes and will make the library materially better, so I'm going to assign this issue to the next release to ensure these get done.
from cattrs.
Thank you for a detailed response! It may also be worth noting in the docs that all Iterable
s and not just Sequence
s are structured into lists by default.
from cattrs.
Related Issues (20)
- Constructor selection for structuring HOT 7
- Tagged Union: How to do structure/unstructure hook(s) for particular member type? HOT 4
- Question: Correct use of converters HOT 3
- Documentation does not explain unstruct_collection_overrides' input HOT 4
- Detailed validation exception groups with hook errors HOT 2
- cattrs effectively disables attrs validation HOT 3
- Covert to defaultdict instead of plain dict HOT 4
- How to recursively unstructure with hooks? HOT 5
- Dataclass structuring fails with misleading error message.
- Default field conversion conflicts with user-defined field converter function HOT 3
- [macOS] Error when trying to run tests: `ImportError: cannot import name 'CodecOptions' from 'bson'` HOT 2
- How to hook into structuring of a simple dict? HOT 6
- Incorrect dispatch in Python 3.12 HOT 4
- Why does structuring of datetime in attrs class need a structure hook? HOT 1
- `prefer_attrib_converters` doesn't work when the field is aliased HOT 2
- Register structure hook only for optional types HOT 2
- TYPE_CHECKING and init=False HOT 5
- Problem with tagged union example HOT 3
- Subclass disambiguation for nested structures. HOT 3
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from cattrs.