Detecting missing values with isna() in Pandas

Introduction

We use isna() to find missing or empty data in tables. This helps us clean and understand our data better.

When you want to check if any data is missing in a survey results table.

Before analyzing sales data, to find where values are not recorded.

To identify empty cells in a dataset before filling or removing them.

When cleaning data from sensors that sometimes fail to record values.

Syntax

Pandas

DataFrame.isna()

This returns a table of the same shape with True where data is missing and False where data is present.

You can use it on a whole DataFrame or on a single column (Series).

Examples

Check missing values in the entire DataFrame df.

Pandas

df.isna()

Check missing values only in the 'Age' column.

Pandas

df['Age'].isna()

Count how many missing values are in each column.

Pandas

df.isna().sum()

Sample Program

This code creates a small table with some missing values. Then it shows where the missing values are with True and counts how many missing values each column has.

Pandas

import pandas as pd

data = {'Name': ['Alice', 'Bob', None, 'David'],
        'Age': [25, None, 22, 23],
        'City': ['New York', 'Los Angeles', 'Chicago', None]}

df = pd.DataFrame(data)

missing = df.isna()
print(missing)

missing_count = df.isna().sum()
print(missing_count)

Important Notes

isna() treats None and NaN as missing values.

You can also use isnull() which works the same as isna().

Summary

isna() helps find missing data easily.

It returns a table of True/False showing missing spots.

Counting missing values helps decide how to clean data.

Practice

1. What does the pandas.DataFrame.isna() function do?

easy

A. It converts all values to strings.

B. It fills missing values with zeros.

C. It removes rows with missing values.

D. It shows which values in the DataFrame are missing (NaN).

Solution

1. Step 1: Understand the purpose of isna()

   The isna() function checks each cell in a DataFrame and marks True if the value is missing (NaN), otherwise False.

2. Step 2: Compare options with function behavior

   Only It shows which values in the DataFrame are missing (NaN). correctly describes this behavior. Options B, C, and D describe other functions or actions.

3. Final Answer:

   It shows which values in the DataFrame are missing (NaN). -> Option D

4. Quick Check:

   isna() = marks missing values True [OK]

Hint: Remember: isna() marks missing spots True [OK]

Common Mistakes:

• Confusing isna() with fillna()
• Thinking isna() removes data
• Assuming isna() changes data types

2. Which of the following is the correct syntax to check missing values in a DataFrame named df?

easy

A. isna(df)

B. df.isna

C. df.isna()

D. df.isna[]

Solution

1. Step 1: Recall method call syntax in pandas

   Methods like isna() must be called with parentheses on the DataFrame object, so df.isna() is correct.

2. Step 2: Evaluate each option

   df.isna() uses correct syntax. df.isna misses parentheses, so it returns the method object, not the result. isna(df) is incorrect because isna is a method, not a standalone function. df.isna[] uses invalid bracket syntax.

3. Final Answer:

   df.isna() -> Option C

4. Quick Check:

   Method call needs parentheses () [OK]

Hint: Use parentheses to call isna(): df.isna() [OK]

Common Mistakes:

• Forgetting parentheses after isna
• Trying to call isna as a standalone function
• Using brackets instead of parentheses

3. Given the DataFrame:

import pandas as pd
data = {'A': [1, None, 3], 'B': [4, 5, None]}
df = pd.DataFrame(data)
print(df.isna())

What is the output?

medium

A. A B
0 False False
1 True False
2 False True

B. A B
0 True False
1 False True
2 False False

C. A B
0 False True
1 False False
2 True False

D. A B
0 False False
1 False True
2 True False

Solution

1. Step 1: Identify missing values in each column

   Column 'A' has None at index 1, so True there. Column 'B' has None at index 2, so True there. All other values are present (False).

2. Step 2: Match the True/False pattern to output

   A B 0 False False 1 True False 2 False True matches the pattern: index 1 in 'A' is True, index 2 in 'B' is True, others False.

3. Final Answer:

   A B 0 False False 1 True False 2 False True -> Option A

4. Quick Check:

   None values marked True by isna() [OK]

Hint: Look for None positions; isna() marks them True [OK]

Common Mistakes:

• Mixing row and column positions
• Confusing True and False values
• Assuming isna() replaces values

4. You run this code:

import pandas as pd
data = {'X': [10, 20, None], 'Y': [None, 40, 50]}
df = pd.DataFrame(data)
missing = df.isna

Why does missing.sum() cause an error?

medium

A. Because missing is a DataFrame, not a function.

B. Because isna is not called with parentheses when assigned.

C. Because isna() does not exist in pandas.

D. Because df is not defined.

Solution

1. Step 1: Understand method assignment without parentheses

   Assigning missing = df.isna stores the method itself, not the result. To get the result, you must call it with parentheses.

2. Step 2: Explain why missing.sum() causes error

   Assigning missing = df.isna stores the method object, not the DataFrame result. Method objects do not have a .sum() attribute--only DataFrames do. Thus, missing.sum() raises AttributeError.

3. Final Answer:

   Because isna is not called with parentheses when assigned. -> Option B

4. Quick Check:

   Assign method with () to get result [OK]

Hint: Assign with parentheses to get result, not method [OK]

Common Mistakes:

• Assigning method without parentheses and expecting result
• Confusing method object with method call
• Assuming isna() is not a pandas method

5. You have a DataFrame df with columns 'Age' and 'Salary'. You want to count how many missing values are in each column using isna(). Which code correctly does this?

hard

A. df.isna().sum()

B. df.isna.sum()

C. df.sum().isna()

D. df.isna().count()

Solution

1. Step 1: Use isna() to mark missing values

   The isna() method returns True for missing values and False otherwise.

2. Step 2: Sum True values per column to count missing

   Since True is treated as 1 and False as 0, summing over rows counts missing values per column. So df.isna().sum() gives the count per column.

3. Final Answer:

   df.isna().sum() -> Option A

4. Quick Check:

   Sum True from isna() to count missing [OK]

Hint: Sum isna() results to count missing per column [OK]

Common Mistakes:

• Forgetting parentheses after isna
• Using count() which counts all rows, not missing
• Applying sum() before isna()