filter() function in Python - Step-by-Step Execution

Concept Flow - filter() function

Start with iterable

↓

Apply function to each item

↓

Function returns True?

No→Skip item

Yes↓

Keep item in result

↓

Repeat for all items

↓

Return filtered iterable

The filter() function checks each item in a list with a function. If the function says True, the item stays; if False, it is skipped.

Execution Sample

Python

numbers = [1, 2, 3, 4, 5]
def is_even(n):
    return n % 2 == 0
filtered = filter(is_even, numbers)
print(list(filtered))

This code keeps only even numbers from the list.

Execution Table

Step	Current Item (n)	Function is_even(n)	Result	Action	Filtered List So Far
1	1	1 % 2 == 0 -> False	False	Skip 1	[]
2	2	2 % 2 == 0 -> True	True	Keep 2	[2]
3	3	3 % 2 == 0 -> False	False	Skip 3	[2]
4	4	4 % 2 == 0 -> True	True	Keep 4	[2, 4]
5	5	5 % 2 == 0 -> False	False	Skip 5	[2, 4]
End	-	-	-	All items checked	[2, 4]

💡 All items processed, filter returns only items where function returned True.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
n	-	1	2	3	4	5	-
filtered list	[]	[]	[2]	[2]	[2, 4]	[2, 4]	[2, 4]

Key Moments - 2 Insights

Why does the filter skip some items instead of keeping all?

Because the function returns False for those items (see execution_table rows 1, 3, 5), filter only keeps items where the function returns True.

Is the filter result a list immediately?

No, filter returns an iterator. We convert it to a list with list(filtered) to see the results, as shown in the execution_sample code.

Visual Quiz - 3 Questions

Look at the execution_table, what is the filtered list after step 4?

A [2]

B [1, 2, 3, 4]

C [2, 4]

D [1, 3, 5]

💡 Hint

Check the 'Filtered List So Far' column at step 4 in the execution_table.

At which step does the function return True for the first time?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Function is_even(n)' and 'Result' columns in the execution_table.

If the function always returned True, what would the filtered list be after all steps?

A []

B [1, 2, 3, 4, 5]

C [2, 4]

D [1, 3, 5]

💡 Hint

If function returns True for every item, filter keeps all items (see concept_flow).

Concept Snapshot

filter(function, iterable)
- Applies function to each item
- Keeps items where function returns True
- Returns an iterator
- Convert to list to see results
- Useful to select items by condition

Full Transcript

The filter() function takes a list and a function. It checks each item with the function. If the function returns True, the item is kept; if False, it is skipped. The result is an iterator that can be converted to a list. For example, filtering even numbers keeps only numbers divisible by 2. Step by step, each item is tested and either added or skipped. This helps select items easily without writing loops.