Maxlen property always set to None, when deque item type is provided.

[maciekglowka]

Initial Checks

• I have searched GitHub for a duplicate issue and I'm sure this is something new
• I have searched Google & StackOverflow for a solution and couldn't find anything
• I have read and followed the docs and still think this is a bug
• I am confident that the issue is with pydantic (not my code, or another library in the ecosystem like FastAPI or mypy)

Description

The maxlen property of the deque field seems to be set to None when the queue item type is provided.

In the example below I first create an untyped deque and upon instancing everything goes well - the maxlen is set to 15.
However when the model definitions is more specific: deque[int] or Deque[int] then the maxlen value cannot be set.

Example Code

>>> from pydantic import BaseModel
>>> from collections import deque
>>> class D(BaseModel):
...     q: deque = deque(maxlen=10)
... 
>>> d = D(q=deque(maxlen=15))
>>> d
D(q=deque([], maxlen=15))
>>> 
>>> 
>>> class DI(BaseModel):
...     q: deque[int] = deque(maxlen=10)
... 
>>> d = DI(q=deque(maxlen=15))
>>> d
DI(q=deque([]))
>>> 
>>> from typing import Deque
>>> 
>>> class DT(BaseModel):
...     q: Deque[int] = deque(maxlen=10)
... 
>>> d = DT(q=deque(maxlen=15))
>>> d
DT(q=deque([]))

Python, Pydantic & OS Version

pydantic version: 1.10.5
pydantic compiled: True
install path: /opt/venv/lib/python3.11/site-packages/pydantic
python version: 3.11.4 (main, Jun 13 2023, 15:08:32) [GCC 10.2.1 20210110]
platform: Linux-5.15.90.1-microsoft-standard-WSL2-x86_64-with-glibc2.31
optional deps. installed: ['typing-extensions']

Affected Components

• Compatibility between releases
• Data validation/parsing
• Data serialization - .dict() and .json()
• JSON Schema
• Dataclasses
• Model Config
• Field Types - adding or changing a particular data type
• Function validation decorator
• Generic Models
• Other Model behaviour - construct(), pickling, private attributes, ORM mode
• Plugins and integration with other tools - mypy, FastAPI, python-devtools, Hypothesis, VS Code, PyCharm, etc.

Selected Assignee: @davidhewitt

[maciekglowka]

Some more cases:

>>> from collections import deque
>>> from pydantic import BaseModel
>>> class D(BaseModel):
...     q: deque[int] = deque(maxlen=10)
... 
>>> d = D()
>>> d
D(q=deque([], maxlen=10))

>>> d.q = deque(maxlen=25)
>>> d
D(q=deque([], maxlen=25))

So it seems that it is validation issue on model creation only.

I do not know the codebase here but potentially:

https://github.com/pydantic/pydantic/blob/f35c780cc3cf073c400ee038b8b6dd6d7b5703d9/pydantic/v1/fields.py#L954C25-L954C25

[hramezani]

Thanks @maciekglowka for this issue 🙏

As you may know, Pydantic V2 is out and we are not actively working on V1.

I've checked your first example code on V2, all of them returning D(q=deque([], maxlen=15))

Same for you second example. In V2 you can have validation on assignment to a deque field like:

from collections import deque
from typing import Deque

from typing_extensions import Annotated
from pydantic import BaseModel, ConfigDict, Field

class D(BaseModel):
    q: Annotated[Deque[int], Field(deque(maxlen=10), max_length=10)]

    model_config = ConfigDict(validate_assignment=True)

d = D()
d.q = deque([1] * 25)

Note: We still accept bug fixes on V1 but it should be an easy fix.

[maciekglowka]

Thanks for checking.
Yeah I've just forked the repo. If I'll be able to fix this I'm gonna make a PR.

[maciekglowka]

@hramezani I've done a very simple fix (PR passes tests) - I am just rewriting the single attribute. Perhaps the rewrite should be less manual? But I am not really sure what happens under the hood in Pydantic.

[hramezani]

Fixed in 2aaddf6