## testing.assert_contract()


Assert that a node's contract is proved, and that it holds on `data`.


Usage

``` python
testing.assert_contract(node: Node, data: None = None, expected: None = None, engine: str = 'duckdb') -> None
 
testing.assert_contract(node: Node, data: Mapping[str, Rows], expected: Rows | None = None, engine: str = 'duckdb') -> pa.Table
```


Every contracted transform the node is built from must have its postconditions proved and every precondition on another node's result proved from that node's guarantees. A transform with `on_unproved="error"` (the default) already raises when the node is built; this catches `"warn"` and `"check"`, which let an unproved contract through.

With `data`, the node runs on those rows: they must meet the preconditions (a violation is a mistake in the test data, and is reported with the offending rows), and the output must meet the postconditions on `engine`. With `expected`, the output must also be those rows, compared as bags (doubles to 12 significant digits), with the missing and unexpected rows listed when it is not.


## Parameters


`node: Node`  
A node: a source or node piped into contracted transforms (`source(...).pipe(paid)`).

`data: Mapping[str, Rows] | None = None`  
The rows of each table the node reads, by name: its sources, and the stored nodes it reads ([Node.write](Node.write.md#sverify.Node.write)). Default: none, which only checks the proof.

`expected: Rows | None = None`  
The rows the node should give on `data`: dicts by column name, tuples in column order, or an Arrow table.

`engine: str = ``"duckdb"`  
Where to run the node: `"duckdb"` (the default), `"polars"`, `"datafusion"` or `"spark"`.


## Returns


`pa.Table | None`  
The node's output on `data`, or None without `data`.


## Raises


`AssertionError`  
The contract is not proved, the data violates a precondition, or the output violates a postcondition or differs from `expected`.

`ValueError`  
`data` misses a table the node reads, or has one it does not read.

`TypeError`  
[node](nodes.NodeStatus.md#sverify.nodes.NodeStatus.node) is not a node, or `expected` is given without `data`.


## Examples

``` python
import sverify as sv
from ibis import _
from sverify.testing import assert_contract, source

@sv.transform(
    requires={"orders": [_.amount >= 0]},
    ensures=[_.doubled >= 0],
)
def doubled(orders):
    return orders.select(orders.id, doubled=orders.amount * 2)

orders = source([], {"id": "int64", "amount": "int64"}, name="orders")
assert_contract(orders.pipe(doubled))
assert_contract(
    orders.pipe(doubled),
    data={"orders": [(1, 30), (2, 0)]},
    expected=[(1, 60), (2, 0)],
)
```