Detecting missing values with isna() in Pandas - Step-by-Step Execution

Concept Flow - Detecting missing values with isna()

Start with DataFrame

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Call isna() method

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Check each cell for missing value

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Return DataFrame of True/False

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Use result to find or handle missing data

We start with a DataFrame, call isna() to check each cell for missing values, and get back a DataFrame showing True where data is missing and False otherwise.

Execution Sample

Pandas

import pandas as pd

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

This code creates a DataFrame with some missing values and uses isna() to detect them, printing a DataFrame of True/False.

Execution Table

Step	DataFrame Cell	Value	isna() Result	Explanation
1	df['A'][0]	1	False	Value is 1, not missing
2	df['A'][1]	None	True	Value is None, missing detected
3	df['A'][2]	3	False	Value is 3, not missing
4	df['B'][0]	4	False	Value is 4, not missing
5	df['B'][1]	5	False	Value is 5, not missing
6	df['B'][2]	None	True	Value is None, missing detected
7	End	-	-	All cells checked, isna() returns DataFrame of True/False

💡 All cells checked, isna() returns DataFrame showing True where values are missing.

Variable Tracker

Variable	Start	After isna() call
df	{'A':[1,None,3],'B':[4,5,None]}	Same DataFrame, unchanged
missing	Not defined	{'A':[False,True,False],'B':[False,False,True]}

Key Moments - 2 Insights

Why does isna() return True for None values?

Because None represents missing data in pandas, isna() detects it and returns True as shown in rows 2 and 6 of the execution_table.

Does isna() change the original DataFrame?

No, isna() returns a new DataFrame of booleans without modifying the original, as shown in variable_tracker where df stays the same.

Visual Quiz - 3 Questions

Look at the execution_table, what is the isna() result for df['B'][1]?

A True

B False

C None

D Error

💡 Hint

Check row 5 in the execution_table where df['B'][1] is 5 and isna() returns False.

At which step does isna() detect the first missing value?

A Step 1

B Step 4

C Step 2

D Step 6

💡 Hint

Look at execution_table row 2 where df['A'][1] is None and isna() returns True.

If the DataFrame had no None values, what would the missing DataFrame contain?

A All False

B All True

C Mixed True and False

D Error

💡 Hint

Refer to variable_tracker and execution_table showing True only where values are None.

Concept Snapshot

pandas.DataFrame.isna() detects missing values.
Returns a DataFrame of booleans: True if missing, False if not.
Does not change original data.
Use to find or filter missing data easily.

Full Transcript

We start with a pandas DataFrame containing some missing values represented by None. Calling the isna() method checks each cell and returns a new DataFrame of the same shape with True where values are missing and False otherwise. This helps us quickly see where data is missing without changing the original DataFrame. For example, in the sample code, isna() returns True for cells with None and False for others. This method is useful for detecting missing data before cleaning or analysis.