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.

Practice

1. What does the filter() function do in Python?

easy

A. Sorts the items in a list

B. Changes all items in a list to uppercase

C. Selects items from a list that meet a condition

D. Adds all items in a list together

Solution

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

   The filter() function takes a function and a list, then keeps only items where the function returns True.

2. Step 2: Compare options with the purpose

   Only Selects items from a list that meet a condition describes selecting items based on a condition, which matches filter()'s job.

3. Final Answer:

   Selects items from a list that meet a condition -> Option C

4. Quick Check:

   filter() selects items [OK]

Hint: Remember: filter keeps items passing the test [OK]

Common Mistakes:

• Thinking filter changes items instead of selecting
• Confusing filter with map or sort
• Assuming filter adds or combines items

2. Which of these is the correct syntax to use filter() to keep even numbers from a list nums?

easy

A. filter(lambda x: x % 2 == 0, nums)

B. filter(nums, lambda x: x % 2 == 0)

C. filter(x % 2 == 0, nums)

D. filter(lambda x: nums % 2 == 0)

Solution

1. Step 1: Recall filter() syntax

   The correct syntax is filter(function, iterable), where function tests each item.

2. Step 2: Check each option

   filter(lambda x: x % 2 == 0, nums) uses lambda x: x % 2 == 0 as function and nums as iterable, which is correct.

3. Final Answer:

   filter(lambda x: x % 2 == 0, nums) -> Option A

4. Quick Check:

   filter(function, iterable) correct order [OK]

Hint: filter(function, iterable) order matters [OK]

Common Mistakes:

• Swapping function and iterable arguments
• Using expression instead of function
• Missing lambda or function for filtering

3. What is the output of this code?

nums = [1, 2, 3, 4, 5]
even_nums = list(filter(lambda x: x % 2 == 0, nums))
print(even_nums)

medium

A. [1, 3, 5]

B. [2, 4]

C. [1, 2, 3, 4, 5]

D. []

Solution

1. Step 1: Understand the filter condition

   The lambda function keeps numbers where x % 2 == 0, meaning even numbers.

2. Step 2: Apply filter to the list

   From [1, 2, 3, 4, 5], only 2 and 4 are even, so the filtered list is [2, 4].

3. Final Answer:

   [2, 4] -> Option B

4. Quick Check:

   Filter keeps even numbers [OK]

Hint: Filter keeps items where function returns True [OK]

Common Mistakes:

• Confusing even and odd numbers
• Forgetting to convert filter to list
• Expecting original list unchanged

4. Find the error in this code snippet:

nums = [10, 15, 20]
result = filter(x % 10 == 0, nums)
print(list(result))

medium

A. Wrong variable name 'nums'

B. No error, code runs fine

C. filter() cannot be converted to list

D. Missing lambda function for filter

Solution

1. Step 1: Check filter function argument

   The first argument to filter() must be a function, but x % 10 == 0 is an expression, not a function.

2. Step 2: Identify fix

   We need to wrap the expression in a lambda: lambda x: x % 10 == 0 to make it a function.

3. Final Answer:

   Missing lambda function for filter -> Option D

4. Quick Check:

   filter needs a function as first argument [OK]

Hint: filter needs a function, use lambda for expressions [OK]

Common Mistakes:

• Passing expression instead of function
• Assuming filter returns list directly
• Ignoring syntax errors in lambda usage

5. You have a list of words: words = ['apple', '', 'banana', None, 'cherry', '']. Which code correctly filters out empty strings and None values using filter()?

hard

A. list(filter(lambda w: w, words))

B. list(filter(lambda w: w == '', words))

C. list(filter(lambda w: w is None, words))

D. list(filter(lambda w: w != None or w != '', words))

Solution

1. Step 1: Understand filtering out empty and None

   Empty strings and None are 'falsy' in Python, so lambda w: w keeps only truthy values.

2. Step 2: Check each option

   list(filter(lambda w: w, words)) keeps only truthy values, removing empty strings and None. Options B and C keep only empty or None, which is opposite. list(filter(lambda w: w != None or w != '', words)) uses wrong logic and keeps all.

3. Final Answer:

   list(filter(lambda w: w, words)) -> Option A

4. Quick Check:

   filter with lambda w: w removes falsy values [OK]

Hint: Use lambda x: x to remove falsy values [OK]

Common Mistakes:

• Using wrong condition to keep empty or None
• Using 'or' instead of 'and' in condition
• Expecting filter to remove without function