What Causes the Pandas Error \'ValueError: cannot reindex from a duplicate axis\'?

Understanding "ValueError: cannot reindex from a duplicate axis"

In Python's Pandas library, this error message indicates that an operation cannot be performed because the data structure contains duplicate values in the axis being referenced.

Cause of the Error

Specifically, the error occurs when attempting to reindex a DataFrame or Series by assigning a new row or column using an index that already exists. Reindexing involves changing the index labels, and if duplicate labels are present, it becomes impossible to uniquely map the operations to the corresponding data.

Example Scenario

To illustrate this error, consider a Pandas DataFrame with duplicate column names. Here's a Python snippet that demonstrates how this can occur:

<code class="python">import pandas as pd

df = pd.DataFrame({
    "ID": [1, 2, 3],
    "Name": ["Alice", "Bob", "Charlie"],
    "Score": [90, 80, 70],
    "Age": [25, 26, 27]
})

df["Score"] = df["Score"] * 2
df["Age"] = df["Age"] + 1

# Attempt to create a duplicate column
df["Score"] = df["Score"] * 1.1</code>

When you execute this snippet, you'll encounter the following error:

ValueError: cannot reindex from a duplicate axis

This is because the DataFrame already has a column named "Score," and you're attempting to assign a new value to it using the same name. The duplicate column prevents Pandas from reindexing the column successfully.

Solution

To resolve this error, you must ensure that the index values used for reindexing are unique. In the case of assignment to existing rows or columns, it means avoiding duplicate labels. If the duplicate values were introduced unintentionally, you can check the index using the .duplicated() method and remove the duplicates accordingly.

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