polars-genson 0.1.2

A Polars plugin for JSON schema inference using genson-rs.

Polars Genson

A Polars plugin for JSON schema inference from string columns using genson-rs. Infer both JSON schemas and Polars schemas directly from JSON data.

Installation

pip install polars-genson[polars]

On older CPUs run:

pip install polars-genson[polars-lts-cpu]

Features

• JSON Schema Inference: Generate JSON schemas from JSON strings in Polars columns
• Polars Schema Inference: Directly infer Polars data types and schemas from JSON data
• Multiple JSON Objects: Handle columns with varying JSON schemas across rows
• Complex Types: Support for nested objects, arrays, and mixed types
• Flexible Input: Support for both single JSON objects and arrays of objects
• Polars Integration: Native Polars plugin with familiar API

Usage

The plugin adds a genson namespace to Polars DataFrames for schema inference.

Quick Start

import polars as pl
import polars_genson
import json

# Create a DataFrame with JSON strings
df = pl.DataFrame({
    "json_data": [
        '{"name": "Alice", "age": 30, "scores": [95, 87]}',
        '{"name": "Bob", "age": 25, "city": "NYC", "active": true}',
        '{"name": "Charlie", "age": 35, "metadata": {"role": "admin"}}'
    ]
})

print("Input DataFrame:")
print(df)

shape: (3, 1)
┌─────────────────────────────────┐
│ json_data                       │
│ ---                             │
│ str                             │
╞═════════════════════════════════╡
│ {"name": "Alice", "age": 30, "… │
│ {"name": "Bob", "age": 25, "ci… │
│ {"name": "Charlie", "age": 35,… │
└─────────────────────────────────┘

JSON Schema Inference

# Infer JSON schema from the JSON column
schema = df.genson.infer_json_schema("json_data")

print("Inferred JSON schema:")
print(json.dumps(schema, indent=2))

{
  "$schema": "http://json-schema.org/schema#",
  "properties": {
    "name": {
      "type": "string"
    },
    "age": {
      "type": "integer"
    },
    "scores": {
      "items": {
        "type": "integer"
      },
      "type": "array"
    }
    "city": {
      "type": "string"
    },
    "active": {
      "type": "boolean"
    },
    "metadata": {
      "properties": {
        "role": {
          "type": "string"
        }
      },
      "required": [
        "role"
      ],
      "type": "object"
    },
  },
  "required": [
    "age",
    "name"
  ],
  "type": "object"
}

Polars Schema Inference

Directly infer Polars data types and schemas:

# Infer Polars schema from the JSON column
polars_schema = df.genson.infer_polars_schema("json_data")

print("Inferred Polars schema:")
print(polars_schema)

Schema({
    'name': String,
    'age': Int64,
    'scores': List(Int64),
    'city': String,
    'active': Boolean,
    'metadata': Struct({'role': String}),
})

The Polars schema inference automatically handles:

• ✅ Complex nested structures with proper Struct types
• ✅ Typed arrays like List(Int64), List(String)
• ✅ Mixed data types (integers, floats, booleans, strings)
• ✅ Optional fields present in some but not all objects
• ✅ Deep nesting with multiple levels of structure

Advanced Usage

JSON Schema Options

# Use the expression directly for more control
result = df.select(
    polars_genson.infer_json_schema(
        pl.col("json_data"),
        merge_schemas=False,  # Get individual schemas instead of merged
    ).alias("individual_schemas")
)

# Or use with different options
schema = df.genson.infer_json_schema(
    "json_data",
    ignore_outer_array=False,  # Treat top-level arrays as arrays
    ndjson=True,              # Handle newline-delimited JSON
    schema_uri="AUTO",        # Specify a schema URI
    merge_schemas=True        # Merge all schemas (default)
)

Polars Schema Options

# Infer Polars schema with options
polars_schema = df.genson.infer_polars_schema(
    "json_data",
    ignore_outer_array=True,  # Treat top-level arrays as streams of objects
    ndjson=False,            # Not newline-delimited JSON
    debug=False              # Disable debug output
)

# Note: merge_schemas=False not yet supported for Polars schemas

Method Reference

The genson namespace provides two main methods:

infer_json_schema(column, **kwargs) -> dict

Returns a JSON schema (as a Python dict) following the JSON Schema specification.

Parameters:

• column: Name of the column containing JSON strings
• ignore_outer_array: Whether to treat top-level arrays as streams of objects (default: True)
• ndjson: Whether to treat input as newline-delimited JSON (default: False)
• merge_schemas: Whether to merge schemas from all rows (default: True)
• debug: Whether to print debug information (default: False)

infer_polars_schema(column, **kwargs) -> pl.Schema

Returns a Polars schema with native data types for direct use with Polars operations.

Parameters:

• column: Name of the column containing JSON strings
• ignore_outer_array: Whether to treat top-level arrays as streams of objects (default: True)
• ndjson: Whether to treat input as newline-delimited JSON (default: False)
• debug: Whether to print debug information (default: False)

Note: merge_schemas=False is not yet supported for Polars schema inference.

Examples

Working with Complex JSON

# Complex nested JSON with arrays of objects
df = pl.DataFrame({
    "complex_json": [
        '{"user": {"profile": {"name": "Alice", "preferences": {"theme": "dark"}}}, "posts": [{"title": "Hello", "likes": 5}]}',
        '{"user": {"profile": {"name": "Bob", "preferences": {"theme": "light"}}}, "posts": [{"title": "World", "likes": 3}, {"title": "Test", "likes": 1}]}'
    ]
})

schema = df.genson.infer_polars_schema("complex_json")
print(schema)

Schema({
    'user': Struct({
        'profile': Struct({
            'name': String, 
            'preferences': Struct({'theme': String})
        })
    }),
    'posts': List(Struct({'likes': Int64, 'title': String})),
})

Using Inferred Schema

# You can use the inferred schema for validation or DataFrame operations
inferred_schema = df.genson.infer_polars_schema("json_data")

# Use with other Polars operations
print(f"Schema has {len(inferred_schema)} fields:")
for name, dtype in inferred_schema.items():
    print(f"  {name}: {dtype}")

Standalone CLI Tool

The project also includes a standalone command-line tool for JSON schema inference:

cd genson-cli
cargo run -- input.json

Or from stdin:

echo '{"name": "test", "value": 42}' | cargo run

Contributing

This crate is part of the polars-genson project. See the main repository for the contribution and development docs.

License

MIT License

• Contains vendored and slightly adapted copy of the Apache 2.0 licensed fork of genson-rs crate