Covert to defaultdict instead of plain dict

[4l1fe]

• cattrs version: 23.2.3
• Python version: 3.11.2
• Operating System: Ubuntu 22.04

Description

I expect the method SelectorConfig.load() to convert the toml text to the defaultdict type in SelectorConfig.properties. Instead, it is converted as dict

How to configure structurizing to make it recognizing the exact type which is DefaultDict[str, LineStringProperties] ?

What I Did

config_converter = TomlkitConverter()

@attrs.define
class LineStringProperties:
    pinned: bool = False
    comment: str = ''
    theme_group: str = LIGHT_THEME

@attrs.define
class SelectorConfig:
    properties: DefaultDict[str, LineStringProperties] = attrs.field(factory=lambda: defaultdict(LineStringProperties))

   @staticmethod
    def load(config_path: Path) -> 'SelectorConfig':
        if not config_path.exists():
            return SelectorConfig()

        text = config_path.read_text()
        config = config_converter.loads(text, SelectorConfig)
        return config

[Tinche]

Hi,

that's an interesting question. I think nobody's asked for it over the years (or I've forgotten about it); we should probably support it since it's a standard library thing.

First you need a predicate function to see if a type is a defaultdict. Note that on 3.9+ (you're on 3.11) you don't need to use typing.DefaultDict, you can just use collections.defaultdict in type hints.

Something like this:

from collections import defaultdict
from typing import DefaultDict, get_origin

def is_defaultdict(type: Any) -> bool:
    """Is this type a defaultdict?"""
    return is_subclass(get_origin(type), (defaultdict, DefaultDict))

Now you need a structure hook factory. We can wrap an existing hook factory, cattrs.gen.make_mapping_structure_fn. You'll need the c Converter to be in scope.

from functools import partial
from typing import get_args

from cattrs.gen import make_mapping_structure_fn
from cattrs.dispatch import StructureHook

def structure_defaultdict_factory(type: type[defaultdict]) -> StructureHook:
    value_type = get_args(type)[1]
    return make_mapping_structure_fn(type, c, partial(defaultdict, value_type))

And then you tie to all together:

c = TomlkitConverter()
c.register_structure_hook_factory(is_defaultdict, structure_defaultdict_factory)

Now you can structure defaultdicts. Since defaultdicts take an arbitrary lambda when created it won't work for all defaultdicts, but should for most.

I'll look into adding this to the next version.

[4l1fe]

Thank you

Hi,

that's an interesting question. I think nobody's asked for it over the years (or I've forgotten about it); we should probably support it since it's a standard library thing.

First you need a predicate function to see if a type is a defaultdict. Note that on 3.9+ (you're on 3.11) you don't need to use typing.DefaultDict, you can just use collections.defaultdict in type hints.

Hey, thx, i didn't know it.

The problem

Something like this:

from collections import defaultdict
from typing import DefaultDict, get_origin

def is_defaultdict(type: Any) -> bool:
    """Is this type a defaultdict?"""
    return is_subclass(get_origin(type), (defaultdict, DefaultDict))

Now you need a structure hook factory. We can wrap an existing hook factory, cattrs.gen.make_mapping_structure_fn. You'll need the c Converter to be in scope.

from functools import partial
from typing import get_args

from cattrs.gen import make_mapping_structure_fn
from cattrs.dispatch import StructureHook

def structure_defaultdict_factory(type: type[defaultdict]) -> StructureHook:
    value_type = get_args(type)[1]
    return make_mapping_structure_fn(type, c, partial(defaultdict, value_type))

And then you tie to all together:

c = TomlkitConverter()
c.register_structure_hook_factory(is_defaultdict, structure_defaultdict_factory)

Now you can structure defaultdicts. Since defaultdicts take an arbitrary lambda when created it won't work for all defaultdicts, but should for most.

In short, it works, thx for the working solution, but here what i'm thinking about it:

• it is very dynamic solution that covers all the defaultdict cases recursively yet is very implicit
• to bring the implemention into code i have to import many additional dependecies
• many of the dependencies take time to comprehend their responsibility
• after that, the factory magic is not intuitive and takes even more time to comprehend 🙂
• the question i ask myself is how the attrs field's attributes are reused? Are they ignored or partially applicaple or whatever?

• i ended up with an explicit solution without the dynamic factory, but it doesnt work if i write type hintting. It means that i have to drop cattrs or rely on the dynamic factory ONLY removing the simple, explicit code. Here is code that doesn't work

  def to_defaultdict(default_factory, data):
    obj = defaultdict(default_factory)
  
    for name, properties in data.items():
        obj[name] = default_factory(**properties)
  
    return obj
  
  @attrs.define
  class LineStringProperties:
    pinned: bool = False
    comment: str = ''
    theme_group: str = LIGHT_THEME
  
  @attrs.define
  class SelectorConfig:
    properties: defaultdict[str, LineStringProperties] = attrs.field(converter=partial(to_defaultdict, LineStringProperties),
                                                                     default=to_defaultdict(LineStringProperties, {}))
  
    @staticmethod
    def load(config_path: Path) -> 'SelectorConfig':
        if not config_path.exists():
            return toml_converter.structure({}, SelectorConfig)
  
        text = config_path.read_text()
        config = toml_converter.loads(text, SelectorConfig)
  
        return config

  The idea here is simply to add an attrs converter.

  The toml sample for better understanding:

  [properties]
  [properties."alabaster_dark.toml"]
  pinned = true
  comment = ""
  theme_group = "light"
  
  [properties."alabaster.toml"]
  pinned = true
  comment = ""
  theme_group = "light"

  it's getting broken on creating obj[name] = default_factory(**properties) inside def to_defaultdict(default_factory, data) raising the error while calling config = toml_converter.loads(text, SelectorConfig):

    |     obj[name] = default_factory(**properties)
  |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  | TypeError: __main__.LineStringProperties() argument after ** must be a mapping, not LineStringProperties
  +------------------------------------

  Imcompatibility is having type hinting and an attrs converter at the same time while structuring with toml_converter.structure().

The quesiton

I'll look into adding this to the next version.

You mean the defaultdict case will be covered without the hook factory magic?

[Tinche]

You mean the defaultdict case will be covered without the hook factory magic?

Yeah, simple cases we can handle automatically. I'm working on a PR already.

I can look at your other comments later, really busy with work ATM.

[4l1fe]

You mean the defaultdict case will be covered without the hook factory magic?

Yeah, simple cases we can handle automatically. I'm working on a PR already.

I can look at your other comments later, really busy with work ATM.

No worries