[GitHub] [spark] zhengruifeng commented on a diff in pull request #39068: [SPARK-41434][CONNECT][PYTHON] Initial `LambdaFunction` implementation

[GitBox <gi...@apache.org>]

zhengruifeng commented on code in PR #39068:
URL: https://github.com/apache/spark/pull/39068#discussion_r1049238906


##########
python/pyspark/sql/connect/functions.py:
##########
@@ -79,6 +85,84 @@ def _invoke_binary_math_function(name: str, col1: Any, col2: Any) -> Column:
     return _invoke_function(name, *_cols)
 
 
+def _get_lambda_parameters(f: Callable) -> ValuesView[inspect.Parameter]:
+    signature = inspect.signature(f)
+    parameters = signature.parameters.values()
+
+    # We should exclude functions that use
+    # variable args and keyword argnames
+    # as well as keyword only args
+    supported_parameter_types = {
+        inspect.Parameter.POSITIONAL_OR_KEYWORD,
+        inspect.Parameter.POSITIONAL_ONLY,
+    }
+
+    # Validate that
+    # function arity is between 1 and 3
+    if not (1 <= len(parameters) <= 3):
+        raise ValueError(
+            "f should take between 1 and 3 arguments, but provided function takes {}".format(
+                len(parameters)
+            )
+        )
+
+    # and all arguments can be used as positional
+    if not all(p.kind in supported_parameter_types for p in parameters):
+        raise ValueError("f should use only POSITIONAL or POSITIONAL OR KEYWORD arguments")
+
+    return parameters
+
+
+def _create_lambda(f: Callable) -> LambdaFunction:
+    """
+    Create `o.a.s.sql.expressions.LambdaFunction` corresponding
+    to transformation described by f
+
+    :param f: A Python of one of the following forms:
+            - (Column) -> Column: ...
+            - (Column, Column) -> Column: ...
+            - (Column, Column, Column) -> Column: ...
+    """
+    parameters = _get_lambda_parameters(f)
+
+    arg_names = ["x", "y", "z"]
+
+    arg_cols: List[Column] = []
+    for arg in arg_names[: len(parameters)]:
+        # TODO: How to make sure lambda variable names are unique? RPC for increasing ID?

Review Comment:
   PySpark invokes `UnresolvedNamedLambdaVariable.freshVarName` in JVM to get a unique variable name
   
    https://github.com/apache/spark/blob/master/python/pyspark/sql/functions.py#L8157
   
   ```
   object UnresolvedNamedLambdaVariable {
   
     // Counter to ensure lambda variable names are unique
     private val nextVarNameId = new AtomicInteger(0)
   
     def freshVarName(name: String): String = {
       s"${name}_${nextVarNameId.getAndIncrement()}"
     }
   }
   ```



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