examples/schema/README.md

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# Schema tracking and validation

This examples showcases the `SchemaValidator()` adapter. At runtime, it will collect the schema of DataFrame-like objects and stored them as `pyarrow.Schema` objects. On subsequent runs, it will check if the schema of a noed output matches the stored reference schema.

## Content
## Simple example
- `dataflow.py` defines a dataflow with 3 nodes returning pandas `DataFrame` objects. The `SchemaValidator` added to the `Builder` automatically tracks their schema. After running it once, next runs will check the schema against the schemas stored on disk.
- `/schemas` includes the stored `.schema` files. They are created using the standard IPC Arrow serialization format. Unfortunately, they are not human readable. We produce a readable `schemas.json` with all the original schema and metadata. We'll be improving it's readability over time.

### Multi library example
- `multi_dataflow.py` defines a dataflow with 3 nodes returning pyarrow, ibis, and pandas dataframes and use the `SchemaValidator()` to track their schema. After running it once, you can change the `Driver` config to use `version="2"` and see the schema validation fail.
- `/multi_schemas` includes similar content as `/schemas`

You can play with the arguments `--version [1, 2, 3]` and `--no-check` to update the stored schema, trigger failing schema checks, and view the diffs.

## Output examples
### Diff examples
```python
{"col_b": "added"}
{"col_a": {"type": "bool -> double"}}
{"col_b": "removed", "col_a": {"type": "bool -> double"}}
```

### Default Pyarrow schema display
```python
{'pyarrow_table': item: string
value: decimal128(10, 2)
count: int32
-- schema metadata --
name: 'pyarrow_table'
documentation: 'Create a duckdb table in-memory and return it as a PyArro' + 7
version: 'cc8d5aba6219976c719ef0c7ac78065aef8a6c7612ea1c3ff595d0892660346' + 1, 'ibis_rename': object: string
price: decimal128(10, 2)
number: int32
-- schema metadata --
name: 'ibis_rename'
documentation: 'Rename the columns'
version: '31a5e44b3b718f994865e589642a555fee5db44ba86eaa3bcc25bc9f4242389' + 1, 'pandas_new_col': object: string
price: decimal128(4, 2)
number: int32
col_a: double
-- schema metadata --
name: 'pandas_new_col'
documentation: 'Add the column `new_col` of type float'
version: '7c679d7dbe665b55b8157f3c0a3962ea26aa0a3f6edaea44120374e33c58acb' + 1}
```

### Human-readable and JSON-serializable schema
```python
{
    "pyarrow_table": {
        "metadata": {
            "name": "pyarrow_table",
            "documentation": "Create a duckdb table in-memory and return it as a PyArrow table",
            "version": "cc8d5aba6219976c719ef0c7ac78065aef8a6c7612ea1c3ff595d08926603467",
        },
        "item": {"name": "item", "type": "string", "nullable": true, "metadata": null},
        "value": {"name": "value", "type": "decimal128(10, 2)", "nullable": true, "metadata": null},
        "count": {"name": "count", "type": "int32", "nullable": true, "metadata": null},
    },
    "ibis_rename": {
        "metadata": {
            "name": "ibis_rename",
            "documentation": "Rename the columns",
            "version": "31a5e44b3b718f994865e589642a555fee5db44ba86eaa3bcc25bc9f42423895",
        },
        "object": {"name": "object", "type": "string", "nullable": true, "metadata": null},
        "price": {"name": "price", "type": "decimal128(10, 2)", "nullable": true, "metadata": null},
        "number": {"name": "number", "type": "int32", "nullable": true, "metadata": null},
    },
    "pandas_new_col": {
        "metadata": {
            "name": "pandas_new_col",
            "documentation": "Add the column `new_col` of type float",
            "version": "7c679d7dbe665b55b8157f3c0a3962ea26aa0a3f6edaea44120374e33c58acbd",
        },
        "object": {"name": "object", "type": "string", "nullable": true, "metadata": null},
        "price": {"name": "price", "type": "decimal128(4, 2)", "nullable": true, "metadata": null},
        "number": {"name": "number", "type": "int32", "nullable": true, "metadata": null},
        "col_a": {"name": "col_a", "type": "double", "nullable": true, "metadata": null},
    },
}
```