# Build

Struct.build(data) validates data and constructs a dataclass instance. Signature.build(data) returns constructed keyword arguments; the caller invokes or awaits the function.

from dataclasses import dataclass
from pytypehint import signature_of, struct_of

@dataclass
class Config:
    count: int = 1

def run(config: Config):
    return config.count

config = struct_of(Config).build({"count": 2})
kwargs = signature_of(run).build({"config": {"count": 2}})
assert run(**kwargs) == 2

Input must be an exact dict, with exact Python values. Nested dataclasses take dictionaries, not instances; lists and tuples keep their respective types. Missing fields use defaults; unknown or required missing keys fail. For portable data, use schema.build(schema.decode(data)).

# Union selection

Different runtime types select themselves. Shared types need a discriminator; contents and option order never select an input variant.

Dataclass discriminators use the class name. Other wrappers use Shape.option_id() and must contain exactly $type and $value. Discriminators are accepted only where needed, including inside sequence items. A lone dataclass rejects $type as an unexpected key. When dataclass and wrapped options coexist, $value distinguishes the wrapper from a dataclass dictionary. See option identities.

resolve preserves validated discriminators; build removes them while constructing the selected values. Defaults are Python values and need no wrapper.

# Errors and construction

Validation fails at the first error. SchemaTypeError and SchemaValueError subclass TypeError and ValueError and expose path and leaf:

cart: items: [0]: size: default: too large: 145, maximum 100
terms: $value: [1]: expected str, got int

Supplied input is validated before construction. Nested dictionaries become instances, and lists and tuples are rebuilt with their constructed contents. Missing defaults are served at their own depth. Constructor and __post_init__ exceptions propagate unchanged; use __post_init__ for cross-field checks. Do not mutate input during construction, including from a constructor: supplied values are validated once and are not checked again after mutation.