Pandas ValueError: You are trying to merge on int64 and object columns

Updated: February 23, 2024 By: Guest Contributor Post a comment

The Problem

Encountering a ValueError while merging DataFrames in Pandas due to column data type mismatches is a common issue that many data scientists and analysts come across. This error typically arises when attempting to merge two DataFrames on columns that have differing data types: one being an integer type (int64) and the other being a string or object type (object). Understanding and resolving this type of error is crucial for smooth data manipulation tasks in Pandas. In this tutorial, we’ll explore the reasons behind this error and dive into multiple solutions to fix it efficiently.

Solution 1: Explicitly Convert Column Data Types

Changing the data types of the problematic columns to match before merging is the most straightforward approach to addressing this error.

  1. Identify the columns causing the issue by verifying their data types using df.dtypes.
  2. Convert the object column in one DataFrame to int64 using pd.to_numeric(), or conversely, convert the int64 column in the other DataFrame to object using df.astype().
  3. Proceed with the merge operation.

Code Example:

import pandas as pd
df1 = pd.DataFrame({'key': [1, 2, 3], 'value_df1': ['A', 'B', 'C']})
df2 = pd.DataFrame({'key': ['1', '2', '3'], 'value_df2': ['D', 'E', 'F']})
# Convert the key column in df2 to int64
df2['key'] = pd.to_numeric(df2['key'])
# Now merge
df_merged = pd.merge(df1, df2, on='key')


   key value_df1 value_df2
0    1         A        D
1    2         B        E
2    3         C        F

Pros and Cons:

This method is straightforward and directly addresses the type mismatch issue. However, it requires a clear understanding of the data and might not be feasible if numeric conversion is not suitable (e.g., strings that cannot be converted to numbers).

Solution 2: Use merge() with validate option

An alternative approach is utilizing the validate option in pd.merge() function to ensure the keys from both DataFrames are compatible before performing the merge, thus, indirectly dealing with the type mismatch error.

  1. First, ensure that the data types are suitable for the desired type of merge. If not, refer to Solution 1.
  2. Use pd.merge() with the validate argument to specify the expectations (e.g., ‘1:1’, ‘1:m’, ‘m:1’, ‘m:m’).
  3. If the data types mismatch but are logically mergeable, the validate step will help catch this before attempting the merge.

Please note that in some scenarios, you may still need to adjust the data types as in Solution 1.

While there is no direct code example for this as it pertains more to methodological rigor, employing the validate option ensures accuracy in the merge operation and helps to avoid potential errors in logic or structure, including but not limited to type mismatches.

Pros and Cons:

This solution offers an additional layer of protection by verifying the integrity of the merge operation, but it might not resolve the data type mismatch on its own. It is best used in conjunction with other approaches like explicitly changing the data types.