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{{ header }}

Copy-on-Write (CoW)

.. ipython:: python
    :suppress:

    pd.options.mode.copy_on_write = True

Copy-on-Write was first introduced in version 1.5.0. Starting from version 2.0 most of the optimizations that become possible through CoW are implemented and supported. A complete list can be found at :ref:`Copy-on-Write optimizations <copy_on_write.optimizations>`.

We expect that CoW will be enabled by default in version 3.0.

CoW will lead to more predictable behavior since it is not possible to update more than one object with one statement, e.g. methods won't have side-effects. Additionally, through delaying copies as long as possible, the average performance will improve.

Description

CoW means that any DataFrame or Series derived from another in any way always behaves as a copy. As a consequence, we can only change the values of an object through modifying the object itself. CoW disallows updating a DataFrame or a Series that shares data with another DataFrame or Series object inplace.

This avoids side-effects when modifying values and hence, most methods can avoid actually copying the data and only trigger a copy when necessary.

The following example will operate inplace with CoW:

.. ipython:: python

    df = pd.DataFrame({"foo": [1, 2, 3], "bar": [4, 5, 6]})
    df.iloc[0, 0] = 100
    df

The object df does not share any data with any other object and hence no copy is triggered when updating the values. In contrast, the following operation triggers a copy of the data under CoW:

.. ipython:: python

    df = pd.DataFrame({"foo": [1, 2, 3], "bar": [4, 5, 6]})
    df2 = df.reset_index(drop=True)
    df2.iloc[0, 0] = 100

    df
    df2

reset_index returns a lazy copy with CoW while it copies the data without CoW. Since both objects, df and df2 share the same data, a copy is triggered when modifying df2. The object df still has the same values as initially while df2 was modified.

If the object df isn't needed anymore after performing the reset_index operation, you can emulate an inplace-like operation through assigning the output of reset_index to the same variable:

.. ipython:: python

    df = pd.DataFrame({"foo": [1, 2, 3], "bar": [4, 5, 6]})
    df = df.reset_index(drop=True)
    df.iloc[0, 0] = 100
    df

The initial object gets out of scope as soon as the result of reset_index is reassigned and hence df does not share data with any other object. No copy is necessary when modifying the object. This is generally true for all methods listed in :ref:`Copy-on-Write optimizations <copy_on_write.optimizations>`.

Previously, when operating on views, the view and the parent object was modified:

.. ipython:: python

    with pd.option_context("mode.copy_on_write", False):
        df = pd.DataFrame({"foo": [1, 2, 3], "bar": [4, 5, 6]})
        view = df[:]
        df.iloc[0, 0] = 100

        df
        view

CoW triggers a copy when df is changed to avoid mutating view as well:

.. ipython:: python

    df = pd.DataFrame({"foo": [1, 2, 3], "bar": [4, 5, 6]})
    view = df[:]
    df.iloc[0, 0] = 100

    df
    view

Chained Assignment

Chained assignment references a technique where an object is updated through two subsequent indexing operations, e.g.

.. ipython:: python

    with pd.option_context("mode.copy_on_write", False):
        df = pd.DataFrame({"foo": [1, 2, 3], "bar": [4, 5, 6]})
        df["foo"][df["bar"] > 5] = 100
        df

The column foo is updated where the column bar is greater than 5. This violates the CoW principles though, because it would have to modify the view df["foo"] and df in one step. Hence, chained assignment will consistently never work and raise a ChainedAssignmentError with CoW enabled:

.. ipython:: python
    :okexcept:

    df = pd.DataFrame({"foo": [1, 2, 3], "bar": [4, 5, 6]})
    df["foo"][df["bar"] > 5] = 100

With copy on write this can either be done by using loc or doing this in multiple steps.

Copy-on-Write optimizations

A new lazy copy mechanism that defers the copy until the object in question is modified and only if this object shares data with another object. This mechanism was added to following methods:

These methods return views when Copy-on-Write is enabled, which provides a significant performance improvement compared to the regular execution.

How to enable CoW

Copy-on-Write can be enabled through the configuration option copy_on_write. The option can be turned on __globally__ through either of the following:

.. ipython:: python

    pd.set_option("mode.copy_on_write", True)

    pd.options.mode.copy_on_write = True

Alternatively, CoW can be enabled locally for testing purposes through:

.. ipython:: python

    with pd.option_context("mode.copy_on_write", True):
        ...


.. ipython:: python
    :suppress:

    pd.options.mode.copy_on_write = False