Check For NaN Values in Python

Last Updated : 23 Jul, 2025

In data analysis and machine learning, missing or NaN (Not a Number) values can often lead to inaccurate results or errors. Identifying and handling these NaN values is crucial for data preprocessing. Here are five methods to check for NaN values in Python.

What are Nan Values In In Python?

In Python, NaN stands for "Not a Number". It is a special floating-point value defined in the IEEE 754 floating-point standard, used to represent undefined or unrepresentable numerical results. NaN values are commonly encountered in data science and numerical computing when working with datasets that contain missing, undefined, or invalid values.

Methods To Check For NaN Values

Using isnan() from NumPy

The numpy.isnan() function is a simple way to check for NaN values in a NumPy array.

Python

# code
import numpy as np

# Example array
array = np.array([1, 2, np.nan, 4, 5])

# Check for NaN values
nan_check = np.isnan(array)
print(nan_check)

Output:

[False False  True False False]

Using isnull() from Pandas

Pandas provides the isnull() function, which is useful for checking NaN values in DataFrames and Series.

Python

# code
import pandas as pd

# Example DataFrame
data = {'A': [1, 2, np.nan, 4], 'B': [5, np.nan, 7, 8]}
df = pd.DataFrame(data)

# Check for NaN values
nan_check_df = df.isnull()
print(nan_check_df)

Output:

       A      B
0  False  False
1  False   True
2   True  False
3  False  False

Using isna() from Pandas

The isna() function in Pandas is an alias for isnull(). It works in the same way.

Python

# code
# Check for NaN values
nan_check_df = df.isna()
print(nan_check_df)

Output:

       A      B
0  False  False
1  False   True
2   True  False
3  False  False

Using pd.isna()

Pandas also provides a top-level pd.isna() function, which can be used on both DataFrames and Series.

Python

# code
# Check for NaN values in a Series
nan_check_series = pd.isna(df['A'])
print(nan_check_series)

Output:

0    False
1    False
2     True
3    False
Name: A, dtype: bool

Conclusion

These methods provide a robust toolkit for identifying NaN values in Python. Whether you are working with NumPy arrays or Pandas DataFrames, you can efficiently check for missing values and take appropriate actions to handle them. This step is crucial in ensuring the accuracy and reliability of your data analysis and machine learning models.