query_first_or_default_async

query_first_or_default_async can execute a query and serialize the first result, or return a default value if the result set contains no records.

Parameters

All command methods also accept keyword-only options=; see Command options.

name	type	description	optional	default
sql	str	the sql query str to execute
default	Any	any object to return if the result set is empty
params	ParamType	params to substitute in the query		None
model	Any	the callable to serialize the model; callable must be able to accept column names as kwargs.		dict
mapper	Callable[[RawRow], Any]	callable that receives a RawRow and returns a projected value. Mutually exclusive with model.		None

param= remains accepted as a 1.x compatibility alias for params=. Pass only one of the two names.

Parameter Shapes

params accepts one parameter record: a mapping, mapping subclass, mutable mapping, or object/dataclass with attributes matching the placeholder names. Top-level list params are only for execute and execute_async; read and scalar methods raise InvalidParameterShapeException for top-level lists before opening a cursor.

params=None, param=None, or omitting both names means there is no parameter object. If the SQL contains pydapper placeholders such as ?id?, every referenced placeholder must be supplied or pydapper raises MissingParameterException before calling the DBAPI. An empty mapping is a real parameter record with no keys. A list inside one parameter record, such as {"ids": []} or {"ids": [1, 2, 3]}, is one value and is reserved for future IN list expansion support.

Rows

When rows are returned as dictionaries, they are insertion-ordered dict[str, Any] values. Key order follows the column order reported by the DB-API cursor.

Column names must be unique exactly as the driver reports them. If a result includes duplicate names, pydapper raises DuplicateColumnException with columns, duplicate_columns, and duplicate_indexes attributes. Alias joined columns instead of using ambiguous select * joins.

Missing keys on returned dict rows raise normal Python KeyError. When custom model construction is requested with model= or models=, pydapper uses the same column-name keyword argument mapping path for results with unique column names.

Use mapper= when column-name mapping is too restrictive. The mapper receives a RawRow with columns, values, and as_dict(). RawRow preserves duplicate column names and positional values in cursor order, so mapper functions can intentionally project joined rows, nested objects, aliases, or duplicate names. Positional indexing and slicing read from values; name indexing and RawRow.as_dict() require unique column names and raise DuplicateColumnException when a dict would be ambiguous.

First, Single and Default

Be careful to use the right method. first and single methods are very different. For default methods, callable defaults that accept no arguments are called only when no row is returned.

method	no item	one item	many items
first	NoResultException	item	first item
single	NoResultException	item	MoreThanOneResultException
first_or_default	default	item	first item
single_or_default	default	item	MoreThanOneResultException

Example

Execute a query and map the first result to a dataclass.

import asyncio
import datetime
from dataclasses import dataclass

from pydapper import connect_async


@dataclass
class Task:
    id: int
    description: str
    due_date: datetime.date
    owner_id: int


sentinel = object()


async def main():
    async with connect_async() as commands:
        task = await commands.query_first_or_default_async(
            "select * from task where id = -1", model=Task, default=sentinel
        )

    if task is sentinel:
        print("No results found!")
    # No results found!


asyncio.run(main())

(This script is complete, it should run "as is")