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(spark-functions)=

# Spark-Compatible Functions

DataFusion ships Spark-compatible versions of a wide set of functions
(string, math, datetime, hash, array, aggregate) through the upstream
`datafusion-spark` crate. `datafusion-python` exposes these under
`datafusion.functions.spark` for use from the DataFrame API, and via
{py:meth}`~datafusion.SessionContext.enable_spark_functions` for use from
SQL.

## Why a Separate Namespace?

Several Spark functions share names with DataFusion built-ins but differ in
semantics. The most common divergences:

- `concat` propagates NULL. `concat('a', NULL, 'b')` returns NULL under
  Spark semantics, whereas the DataFusion default returns `'ab'`.
- `substring` is 1-indexed and supports negative positions counting from
  the end of the string.
- `round` uses HALF_UP rounding mode (`round(2.5, 0) == 3`).
- Numeric functions (`floor`, `ceil`, `mod`) follow Spark's edge-case
  handling for negative values and decimals.

Enabling Spark functions does not affect the DataFrame API: you choose which
implementation to call by which module you import from.

## DataFrame API

Import `spark` and use it like any other functions module. The Spark
functions can go anywhere you'd put a DataFusion expression — inside
`select`, `filter`, `with_column`, `aggregate`, and so on.

```python
from datafusion import SessionContext, col, lit
from datafusion.functions import spark

ctx = SessionContext()
df = ctx.from_pydict({"s": ["hello", "world"]})

# SHA-256 hash with Spark semantics
df.select(spark.sha2(col("s"), lit(256)).alias("h")).show()

# 1-indexed substring
df.select(spark.substring(col("s"), lit(1), lit(3)).alias("p")).show()
```

## SQL

To use Spark functions in SQL queries, call
{py:meth}`~datafusion.SessionContext.enable_spark_functions` on the context.
This registers every Spark UDF/UDAF/UDWF, overriding any DataFusion built-in
of the same name.

```python
from datafusion import SessionContext

ctx = SessionContext()
ctx.enable_spark_functions()

ctx.sql("SELECT sha2('hello', 256)").show()
ctx.sql("SELECT concat('a', NULL, 'b')").show()   # -> NULL, not 'ab'
```

The override applies for the lifetime of the session. To call DataFusion's
built-in versions afterwards, create a fresh `SessionContext`.

## Function Reference

The full, up-to-date list of available Spark functions — with signatures
and per-function docstrings — lives in the
{py:mod}`datafusion.functions.spark` API reference.
