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updated udf user guide based on reviews
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‎doc/source/user_guide/user_defined_functions.rst

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@@ -87,88 +87,64 @@ Methods that support User-Defined Functions
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User-Defined Functions can be applied across various pandas methods:
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+----------------------------+------------------------+--------------------------+---------------------------------------------------------------------------+
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| Method | Function Input | Function Output | Description |
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+============================+========================+==========================+===========================================================================+
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| :meth:`map` | Scalar | Scalar | Apply a function to each element |
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+----------------------------+------------------------+--------------------------+---------------------------------------------------------------------------+
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| :meth:`apply` (axis=0) | Column (Series) | Column (Series) | Apply a function to each column |
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+----------------------------+------------------------+--------------------------+---------------------------------------------------------------------------+
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| :meth:`apply` (axis=1) | Row (Series) | Row (Series) | Apply a function to each row |
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+----------------------------+------------------------+--------------------------+---------------------------------------------------------------------------+
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| :meth:`agg` | Series/DataFrame | Scalar or Series | Aggregate and summarizes values, e.g., sum or custom reducer |
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+----------------------------+------------------------+--------------------------+---------------------------------------------------------------------------+
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| :meth:`transform` | Series/DataFrame | Same shape as input | Apply a function while preserving shape; raises error if shape changes |
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+----------------------------+------------------------+--------------------------+---------------------------------------------------------------------------+
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| :meth:`filter` | - | - | Return rows that satisfy a boolean condition |
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+----------------------------+------------------------+--------------------------+---------------------------------------------------------------------------+
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| :meth:`pipe` | Series/DataFrame | Series/DataFrame | Chain functions together to apply to Series or Dataframe |
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+----------------------------+------------------------+--------------------------+---------------------------------------------------------------------------+
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+----------------------------+------------------------+--------------------------+----------------------------------------------------------------------------------------------------------------------------------------------+
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| Method | Function Input | Function Output | Description |
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+============================+========================+==========================+==============================================================================================================================================+
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| :meth:`map` | Scalar | Scalar | Apply a function to each element |
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+----------------------------+------------------------+--------------------------+----------------------------------------------------------------------------------------------------------------------------------------------+
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| :meth:`apply` (axis=0) | Column (Series) | Column (Series) | Apply a function to each column |
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+----------------------------+------------------------+--------------------------+----------------------------------------------------------------------------------------------------------------------------------------------+
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| :meth:`apply` (axis=1) | Row (Series) | Row (Series) | Apply a function to each row |
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+----------------------------+------------------------+--------------------------+----------------------------------------------------------------------------------------------------------------------------------------------+
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| :meth:`agg` | Series/DataFrame | Scalar or Series | Aggregate and summarizes values, e.g., sum or custom reducer |
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+----------------------------+------------------------+--------------------------+----------------------------------------------------------------------------------------------------------------------------------------------+
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| :meth:`transform` (axis=0) | Column (Series) | Column(Series) | Same as :meth:`apply` with (axis=0), but it raises an exception if the function changes the shape of the data |
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+----------------------------+------------------------+--------------------------+----------------------------------------------------------------------------------------------------------------------------------------------+
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| :meth:`transform` (axis=1) | Row (Series) | Row (Series) | Same as :meth:`apply` with (axis=1), but it raises an exception if the function changes the shape of the data |
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+----------------------------+------------------------+--------------------------+----------------------------------------------------------------------------------------------------------------------------------------------+
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| :meth:`filter` | Series or DataFrame | Boolean | Only accepts UDFs in group by. Function is called for each group, and the group is removed from the result if the function returns ``False`` |
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+----------------------------+------------------------+--------------------------+----------------------------------------------------------------------------------------------------------------------------------------------+
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| :meth:`pipe` | Series/DataFrame | Series/DataFrame | Chain functions together to apply to Series or Dataframe |
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+----------------------------+------------------------+--------------------------+----------------------------------------------------------------------------------------------------------------------------------------------+
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.. note::
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Some of these methods are can also be applied to groupby, resample, and various window objects.
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See :ref:`groupby`, :ref:`resample()<timeseries>`, :ref:`rolling()<window>`, :ref:`expanding()<window>`,
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and :ref:`ewm()<window>` for details.
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Choosing the Right Method
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-------------------------
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When applying UDFs in pandas, it is essential to select the appropriate method based
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on your specific task. Each method has its strengths and is designed for different use
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cases. Understanding the purpose and behavior of each method will help you make informed
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decisions, ensuring more efficient and maintainable code.
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Below is a table overview of all methods that accept UDFs:
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+------------------+--------------------------------------+---------------------------+--------------------+------------------------------------------+
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| Method | Purpose | Supports UDFs | Keeps Shape | Recommended Use Case |
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+==================+======================================+===========================+====================+==========================================+
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| :meth:`apply` | General-purpose function | Yes | Yes (when axis=1) | Custom row-wise or column-wise operations|
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+------------------+--------------------------------------+---------------------------+--------------------+------------------------------------------+
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| :meth:`agg` | Aggregation | Yes | No | Custom aggregation logic |
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+------------------+--------------------------------------+---------------------------+--------------------+------------------------------------------+
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| :meth:`transform`| Transform without reducing dimensions| Yes | Yes | Broadcast element-wise transformations |
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+------------------+--------------------------------------+---------------------------+--------------------+------------------------------------------+
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| :meth:`map` | Element-wise mapping | Yes | Yes | Simple element-wise transformations |
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+------------------+--------------------------------------+---------------------------+--------------------+------------------------------------------+
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| :meth:`pipe` | Functional chaining | Yes | Yes | Building clean operation pipelines |
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+------------------+--------------------------------------+---------------------------+--------------------+------------------------------------------+
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| :meth:`filter` | Row/Column selection | Not directly | Yes | Subsetting based on conditions |
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+------------------+--------------------------------------+---------------------------+--------------------+------------------------------------------+
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.. note::
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Some of these methods are can also be applied to groupby, resample, and various window objects.
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See :ref:`groupby`, :ref:`resample()<timeseries>`, :ref:`rolling()<window>`, :ref:`expanding()<window>`,
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and :ref:`ewm()<window>` for details.
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:meth:`DataFrame.apply`
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~~~~~~~~~~~~~~~~~~~~~~~
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The :meth:`DataFrame.apply` allows you to apply UDFs along either rows or columns. While flexible,
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The :meth:`apply` method allows you to apply UDFs along either rows or columns. While flexible,
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it is slower than vectorized operations and should be used only when you need operations
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that cannot be achieved with built-in pandas functions.
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When to use: :meth:`DataFrame.apply` is suitable when no alternative vectorized method or UDF method is available,
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When to use: :meth:`apply` is suitable when no alternative vectorized method or UDF method is available,
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but consider optimizing performance with vectorized operations wherever possible.
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Documentation can be found at :meth:`~DataFrame.apply`.
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:meth:`DataFrame.agg`
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~~~~~~~~~~~~~~~~~~~~~
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If you need to aggregate data, :meth:`DataFrame.agg` is a better choice than apply because it is
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If you need to aggregate data, :meth:`agg` is a better choice than apply because it is
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specifically designed for aggregation operations.
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When to use: Use :meth:`DataFrame.agg` for performing custom aggregations, where the operation returns
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When to use: Use :meth:`agg` for performing custom aggregations, where the operation returns
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a scalar value on each input.
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Documentation can be found at :meth:`~DataFrame.agg`.
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:meth:`DataFrame.transform`
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The transform method is ideal for performing element-wise transformations while preserving the shape of the original DataFrame.
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The :meth:`transform` method is ideal for performing element-wise transformations while preserving the shape of the original DataFrame.
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It is generally faster than apply because it can take advantage of pandas' internal optimizations.
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When to use: When you need to perform element-wise transformations that retain the original structure of the DataFrame.
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Documentation can be found at :meth:`~DataFrame.transform`.
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.. code-block:: python
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from sklearn.linear_model import LinearRegression
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:meth:`DataFrame.filter`
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~~~~~~~~~~~~~~~~~~~~~~~~
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The :meth:`DataFrame.filter` method is used to select subsets of the DataFrame’s
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The :meth:`filter` method is used to select subsets of the DataFrame’s
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columns or row. It is useful when you want to extract specific columns or rows that
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match particular conditions.
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When to use: Use :meth:`DataFrame.filter` when you want to use a UDF to create a subset of a DataFrame or Series
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When to use: Use :meth:`filter` when you want to use a UDF to create a subset of a DataFrame or Series
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.. note::
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:meth:`DataFrame.filter` does not accept UDFs, but can accept
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Since filter does not directly accept a UDF, you have to apply the UDF indirectly,
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for example, by using list comprehensions.
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Documentation can be found at :meth:`~DataFrame.filter`.
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:meth:`DataFrame.map`
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~~~~~~~~~~~~~~~~~~~~~
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:meth:`DataFrame.map` is used specifically to apply element-wise UDFs and is better
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for this purpose compared to :meth:`DataFrame.apply` because of its better performance.
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The :meth:`map` method is used specifically to apply element-wise UDFs.
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When to use: Use map for applying element-wise UDFs to DataFrames or Series.
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Documentation can be found at :meth:`~DataFrame.map`.
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When to use: Use :meth:`map` for applying element-wise UDFs to DataFrames or Series.
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:meth:`DataFrame.pipe`
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~~~~~~~~~~~~~~~~~~~~~~
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The pipe method is useful for chaining operations together into a clean and readable pipeline.
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The :meth:`pipe` method is useful for chaining operations together into a clean and readable pipeline.
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It is a helpful tool for organizing complex data processing workflows.
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When to use: Use pipe when you need to create a pipeline of operations and want to keep the code readable and maintainable.
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Documentation can be found at :meth:`~DataFrame.pipe`.
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When to use: Use :meth:`pipe` when you need to create a pipeline of operations and want to keep the code readable and maintainable.
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Performance
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for common operations.
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.. note::
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If performance is critical, explore **vectorizated operations** before resorting
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If performance is critical, explore **vectorized operations** before resorting
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to UDFs.
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Vectorized Operations
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Vectorized operations in pandas are significantly faster than using :meth:`DataFrame.apply`
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with UDFs because they leverage highly optimized C functions
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via NumPy to process entire arrays at once. This approach avoids the overhead of looping
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via ``NumPy`` to process entire arrays at once. This approach avoids the overhead of looping
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through rows in Python and making separate function calls for each row, which is slow and
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inefficient. Additionally, NumPy arrays benefit from memory efficiency and CPU-level
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inefficient. Additionally, ``NumPy`` arrays benefit from memory efficiency and CPU-level
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optimizations, making vectorized operations the preferred choice whenever possible.
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Using :meth:`DataFrame.pipe` for Composable Logic
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Another useful pattern for improving readability and composability—especially when mixing
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vectorized logic with UDFs—is to use the :meth:`DataFrame.pipe` method.
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Another useful pattern for improving readability and composability, especially when mixing
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vectorized logic with UDFs, is to use the :meth:`DataFrame.pipe` method.
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The ``.pipe`` method doesn't improve performance directly, but it enables cleaner
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:meth:`DataFrame.pipe` doesn't improve performance directly, but it enables cleaner
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method chaining by passing the entire object into a function. This is especially helpful
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when chaining custom transformations:
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)
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This is functionally equivalent to calling ``add_ratio_column(df)``, but keeps your code
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clean and composable. The function you pass to ``.pipe`` can use vectorized operations,
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row-wise UDFs, or any other logic—``.pipe`` is agnostic.
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clean and composable. The function you pass to :meth:`DataFrame.pipe` can use vectorized operations,
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row-wise UDFs, or any other logic; :meth:`DataFrame.pipe` is agnostic.
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.. note::
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While :meth:`DataFrame.pipe` does not improve performance on its own,

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