How to initialize a Pydantic object from field values given by position instead of name?

Question

I am not able to find a simple way how to initialize a Pydantic object from field values given by position (for example in a list instead of a dictionary) so I have written class method positional_fields() to create the required dictionary from an iterable:

from typing import Optional, Iterable, Any, Dict
from pydantic import BaseModel


class StaticRoute(BaseModel):
    if_name: str
    dest_ip: str
    mask: str
    gateway_ip: str
    distance: Optional[int]
    
    @classmethod
    def positional_fields(cls, values: Iterable) -> Dict[str, Any]:
        return dict(zip(cls.__fields__, values))


input_lines = """
  route ab 10.0.0.0 255.0.0.0 10.220.196.23 1
  route gh 10.0.2.61 255.255.255.255 10.220.198.38 1
""".splitlines()

for line in input_lines:
    words = line.split()
    if words and words[0] == 'route':
        sroute = StaticRoute(**StaticRoute.positional_fields(words[1:]))
        print(sroute)

if_name='ab' dest_ip='10.0.0.0' mask='255.0.0.0' gateway_ip='10.220.196.23' distance=1
if_name='gh' dest_ip='10.0.2.61' mask='255.255.255.255' gateway_ip='10.220.198.38' distance=1

Is there a more straightforward way of achieving this?

My method expects the __fields__ dictionary to have keys in the order the fields were defined in the class. I am not sure if this is guaranteed (assuming Python 3.6+).

Answer 1

How about using dataclasses instead? Something like:

from typing import Optional

from pydantic.dataclasses import dataclass


@dataclass
class StaticRoute:
    if_name: str
    dest_ip: str
    mask: str
    gateway_ip: str
    distance: Optional[int]


words = "route if_name dest_ip mask gateway_ip 10".split()
print(StaticRoute(*words[1:])

# StaticRoute(if_name='if_name', dest_ip='dest_ip', mask='mask', gateway_ip='gateway_ip', distance=10)

Answer 2

The class method BaseModel.parse_obj() returns an object instance initialized by a dictionary. We can create a similar class method parse_iterable() which accepts an iterable instead.

from typing import Optional, Iterable, Any, Dict
from pydantic import BaseModel


class BaseModelExt(BaseModel):
    @classmethod
    def parse_iterable(cls, values: Iterable):
        return cls.parse_obj(dict(zip(cls.__fields__, values)))

    
class StaticRoute(BaseModelExt):
    if_name: str
    dest_ip: str
    mask: str
    gateway_ip: str
    distance: Optional[int]


input_lines = """
  route ab 10.0.0.0 255.0.0.0 10.220.196.23 1
  route gh 10.0.2.61 255.255.255.255 10.220.198.38 1
""".splitlines()

for line in input_lines:
    words = line.split()
    if words and words[0] == 'route':
        sroute = StaticRoute.parse_iterable(words[1:])
        print(sroute)

Note: We are still missing a confirmation if the order of BaseModel.__fields__ is guaranteed.

Answer 3

You can use a combination of a custom RootType and NamedTuple, like so:

from pydantic import BaseModel
from typing import NamedTuple, Optional


class StaticRouteTuple(NamedTuple):
    if_name: str
    dest_ip: str
    mask: str
    gateway_ip: str
    distance: Optional[int]


class StaticRoute(BaseModel):
    __root__: StaticRouteTuple

    @property
    def route(self) -> StaticRouteTuple:
        return self.__root__


input_lines = """
  route ab 10.0.0.0 255.0.0.0 10.220.196.23 1
  route gh 10.0.2.61 255.255.255.255 10.220.198.38 1
""".splitlines()

for line in input_lines:
    words = line.split()
    if words and words[0] == "route":
        sroute = StaticRoute.parse_obj(words[1:]).route
        print(sroute)

If you don't want to use a custom RootType you can use instead pyandic.parse_obj_as, example:

from pydantic import parse_obj_as
sroute = parse_obj_as(StaticRouteTuple, words[1:])