Pandas

ValueError: Length of values does not match length of index - How to Fix it

Answer

This error means you're trying to assign a list or array to a dataframe column, but the number of values doesn't match the number of rows. Fix it by ensuring your new data has exactly the same length as the dataframe, or use .reset_index() if index misalignment is the issue.

Why This Happens

When you assign values to a column, pandas expects one value per row. If your dataframe has 100 rows but you're assigning a list of 95 values, it fails. This also happens when you've filtered a dataframe and try to assign values from the original, or when indexes don't align after a merge or concat.

Solution

The rule: always check len(df) against len(your_values) before assignment. If you've filtered, reset the index.

import pandas as pd

df = pd.DataFrame({'a': [1, 2, 3, 4, 5]})

# ❌ Problematic: assigning wrong length
df['b'] = [10, 20, 30]
# ValueError: Length of values (3) does not match length of index (5)

# ✅ Fixed: ensure same length
df['b'] = [10, 20, 30, 40, 50]

# ✅ Fixed: if working with filtered data, reset index
filtered = df[df['a'] > 2].reset_index(drop=True)
filtered['b'] = [30, 40, 50]  # now lengths match

# ✅ Debug: check lengths before assigning
print(len(df))          # 5
print(len(new_values))  # should also be 5