How to Efficiently Set Values in Pandas DataFrame Cells by Index?

Setting Values in Pandas DataFrame Cells by Index

To set a specific cell value in a Pandas DataFrame based on its index, consider the following:

Using df.set_value(index, column, value)

df.set_value() allows you to directly assign a value to a cell using its index. However, note that this method is slated for deprecation.

# Correct way:
df.set_value('C', 'x', 10)

Using df.at[index, column] = value

The preferred alternative is to use df.at[] to update cell values directly. This method is more efficient and provides a more concise syntax.

df.at['C', 'x'] = 10

Using Chained Indexing

Caution: While it might seem intuitive to use chained indexing to set cell values (e.g., df.xs('C')['x'] = 10), this method only modifies a copy of the row or column. To modify the original DataFrame directly, use either df.set_value() or df.at[].

Why Chained Indexing Fails

Chained indexing (e.g., df.xs('C')['x'] = 10) creates a new DataFrame object with a reference to the original data. Assignments made to this new object are not propagated to the original DataFrame.

Performance

Benchmarks show that df.set_value() is the fastest option, followed by df'x' = 10 and df.at['C', 'x'] = 10. However, performance differences can vary depending on DataFrame size and complexity.

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