Scalar lookup
Read one known cell from a labeled table without selecting a larger slice.
Pandas · Module
df.at
DataFrame.at reads or writes one cell using a row-label and column-label pair. It is the label-based scalar counterpart to positional iat.
How it works
DataFrame.at provides scalar access using a row label and a column label. It supports both retrieval and assignment, making it suitable when exactly one cell is involved rather than a multi-cell selection.
Syntax
DataFrame.at[row_label, column_label]Returns
The value at the specified row/column label pair when reading; assignment updates that cell.
Key points
- Use
DataFrame.atto address one cell through its row and column labels. - Its lookup semantics are label-based, like
loc. - Choose it for scalar reads or writes rather than multi-cell selection.
- A bracketed row/column pair returns the selected cell's value.
- Assigning to the same indexer changes the selected cell.
Common mistakes
- Reading a missing row or column label raises
KeyError. - For a DataFrame, the indexer must represent a row/column pair, and each label must be scalar.
Example: Read and update one labeled cell
import pandas as pd
df = pd.DataFrame(
[[0, 2, 3], [0, 4, 1], [10, 20, 30]],
columns=["A", "B", "C"],
)
value = df.at[1, "B"]
df.at[1, "B"] = 10
assert value == 4
assert df.at[1, "B"] == 10Use the label pair inside brackets to read one cell, or assign through the same expression to update it.
When to use it
Scalar update
Change one field identified by its row and column labels.
Series scalar access
Work with a Series obtained from a DataFrame row.
Interview questions
QWhat is DataFrame.at used for?
at performs label-based scalar access on a DataFrame. It can retrieve or assign one value identified by a row label and a column label.
Follow-up: How would you access one cell by position instead of labels? Use iat, which provides integer-position-based scalar access.
QHow does at differ from loc?
Both at and loc use labels, but at is intended for a single value. Use loc when selecting groups of rows or columns.
Follow-up: What is the positional counterpart of loc? iloc selects by integer positions, while iat is the scalar positional counterpart.
QHow do you update a single DataFrame cell with at?
Read with df.at[row_label, column_label] and assign with df.at[row_label, column_label] = value. The same labeled expression identifies the target cell in both cases.
Follow-up: What happens if the requested label is absent during a read? A missing label causes KeyError.
Related
DataFrame.iatDataFrame.locDataFrame.iloc