Pandas Error: NDFrame.asof() got an unexpected keyword argument ‘columns’

Updated: February 24, 2024 By: Guest Contributor Post a comment

Understanding the Error

Encountering an error while coding can be quite challenging, especially if it’s related to a popular library like Pandas in Python, which is widely used for data manipulation and analysis. One such error that often confounds users is NDFrame.asof() got an unexpected keyword argument ‘columns’. This error typically surfaces when attempting to use the asof() method incorrectly. This tutorial aims to decode this error, discuss its causes, and explore actionable solutions.

Why the Error Occurs?

Before diving into the solutions, it’s essential to understand what the error message entails. The asof() method in Pandas is designed to perform last observation carried forward (LOCF) operations, primarily used for time series data. The error message indicates that an unsupported keyword argument ‘columns’ was passed to the method. This misuse happens due to a misconception about the method’s allowed parameters or overlooking the method’s documentation.

Solution 1: Use the ‘Subset’ Argument

The correct way to specify columns for the asof() method is through the ‘subset’ argument, rather than ‘columns’. This approach aligns with the method’s documentation and expected parameters.


  1. Identify the column(s) you want to apply the LOCF operation to.
  2. Use the ‘subset’ keyword argument to specify these columns when calling asof().

Code Example:

import pandas as pd

df = pd.DataFrame({
    'time': pd.date_range('2020-01-01', periods=5),
    'value': [None, 2, None, 4, 5],
    'group': [1, 1, 2, 2, 2]

result = df.asof(subset=['time'], where=pd.Index([3, 4]))

Notes: Using the ‘subset’ keyword enhances the method’s precision by allowing a specific focus on relevant columns. However, it’s vital to cross-check the column types since asof() primarily works with time series and numeric data which are index-like.

Solution 2: Inspection and Correction of Method Usage

At times, the core issue stems from misunderstanding how the asof() method is supposed to work or its purpose. A detailed inspection and revisiting the method’s documentation might be necessary.


  1. Review the asof() method documentation on the official Pandas website.
  2. Identify the correct way to implement the method as per your requirement.

No code example for this solution since the focus is on gaining a correct understanding of the asof() method and its parameters.

Notes: This approach may seem indirect but preventing further errors due to misconceptions or incorrect assumptions about the method’s functionality saves time in the long run. Understanding the documentation could unveil better or more suited methods for the task at hand.