Add missing contains string function

Expose the upstream DataFusion `contains(string, search_str)` function
which returns true if search_str is found within string (case-sensitive).

Note: the other functions from #1450 (instr, position, substring_index)
already exist — instr and position are aliases for strpos, and
substring_index is exposed as substr_index.

Closes #1450

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: timsaucer

crates/core/src/functions.rs

@@ -494,6 +494,11 @@ expr_fn!(length, string);
 expr_fn!(char_length, string);
 expr_fn!(chr, arg, "Returns the character with the given code.");
 expr_fn_vec!(coalesce);
+expr_fn!(
+    contains,
+    string search_str,
+    "Return true if search_str is found within string (case-sensitive)."
+);
 expr_fn!(cos, num);
 expr_fn!(cosh, num);
 expr_fn!(cot, num);
@@ -960,6 +965,7 @@ pub(crate) fn init_module(m: &Bound<'_, PyModule>) -> PyResult<()> {
     m.add_wrapped(wrap_pyfunction!(col))?;
     m.add_wrapped(wrap_pyfunction!(concat_ws))?;
     m.add_wrapped(wrap_pyfunction!(concat))?;
+    m.add_wrapped(wrap_pyfunction!(contains))?;
     m.add_wrapped(wrap_pyfunction!(corr))?;
     m.add_wrapped(wrap_pyfunction!(cos))?;
     m.add_wrapped(wrap_pyfunction!(cosh))?;

python/datafusion/functions.py

@@ -116,6 +116,7 @@
     "col",
     "concat",
     "concat_ws",
+    "contains",
     "corr",
     "cos",
     "cosh",
@@ -436,6 +437,20 @@ def digest(value: Expr, method: Expr) -> Expr:
     return Expr(f.digest(value.expr, method.expr))
 
 
+def contains(string: Expr, search_str: Expr) -> Expr:
+    """Return true if ``search_str`` is found within ``string`` (case-sensitive).
+
+    Examples:
+        >>> ctx = dfn.SessionContext()
+        >>> df = ctx.from_pydict({"a": ["the quick brown fox"]})
+        >>> result = df.select(
+        ...     dfn.functions.contains(dfn.col("a"), dfn.lit("brown")).alias("c"))
+        >>> result.collect_column("c")[0].as_py()
+        True
+    """
+    return Expr(f.contains(string.expr, search_str.expr))
+
+
 def concat(*args: Expr) -> Expr:
     """Concatenates the text representations of all the arguments.