How to Flatten a Hierarchical Column Index in Pandas DataFrames?

Flattening Hierarchical Column Index in DataFrames

When working with data frames, a hierarchical index in the columns (axis 1) can arise from a groupby operation. The data frame will have sub-headers with multiple levels. However, flattening the index to create a single-level header is often desirable.

To flatten a hierarchical column index, there are two options:

1. Setting Columns to Top Level:

This option moves the current top-level headers to the column names directly. Using the get_level_values method along with 0, which represents the first level, we can set the columns as follows:

df.columns = df.columns.get_level_values(0)

2. Combining MultiIndex into One Index:

This option joins the multi-index column headers into a single index. Assuming the columns contain strings, the following code can be used:

df.columns = [' '.join(col).strip() for col in df.columns.values]

The strip() function ensures that any whitespace is removed from the combined header. After executing this code, the hierarchical column index will be flattened.

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