Why does Pandas GroupBy.apply run twice on the first group?

Pandas GroupBy.apply Duplicates First Group: A Detailed Explanation

The pandas GroupBy.apply method is designed to apply a function to each group in a DataFrame. However, it has been observed that the first group is applied with the function twice, causing duplication in the output.

This behavior is not an error but rather an intrinsic design feature of the apply method. It needs to determine the shape of the returned data to properly combine the results. To achieve this, the function is invoked twice as an initial probing step.

Depending on the intended operation, it's recommended to use alternative methods like aggregate, transform, or filter instead of apply. These functions expect specific return value shapes and do not require the double call.

If the function used within apply has no side effects, the duplicate call on the first group is often inconsequential. However, it's essential to be aware of this behavior to avoid confusion and ensure proper interpretation of the results.

In summary, the double call on the first group is intended to determine the shape of the returned data from the apply function and guide the result aggregation process. By understanding this design, developers can effectively leverage the GroupBy.apply method in their pandas data manipulation tasks.

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