filter() function in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the filter() function do in Python?

The filter() function takes a function and a sequence (like a list) and returns a new sequence with only the items for which the function returns True.

beginner

How do you use filter() with a lambda function to keep only even numbers from a list?

You write:
filter(lambda x: x % 2 == 0, my_list)
This keeps only numbers divisible by 2 (even numbers).

intermediate

What type of object does filter() return in Python 3?

It returns a filter object, which is an iterator. You can convert it to a list using list() to see the filtered items.

intermediate

What happens if the function passed to filter() is None?

If the function is None, filter() removes all items that are false in a Boolean context (like False, 0, '', None).

beginner

Can filter() be used with any iterable, or only lists?

filter() works with any iterable, such as lists, tuples, sets, or even strings.

What does filter() return in Python 3?

A A dictionary

B A list

C A tuple

D A filter object (an iterator)

filter() returns a filter object, which is an iterator. You can convert it to a list if needed.

What happens if you pass None as the function to filter()?

A It returns the original iterable unchanged

B It raises an error

C It filters out all false values from the iterable

D It returns an empty list

Passing None makes filter() remove all items that are false in a Boolean context.

Which of these is a correct way to use filter() to keep only positive numbers from a list nums?

A filter(lambda x: x > 0, nums)

B filter(lambda x: x < 0, nums)

C filter(lambda x: x == 0, nums)

D filter(lambda x: x != 0, nums)

The function lambda x: x > 0 keeps only numbers greater than zero.

How can you see the filtered results from filter()?

A Print the filter object directly

B Convert the filter object to a list using <code>list()</code>

C Use <code>str()</code> on the filter object

D Use <code>dict()</code> on the filter object

The filter object is an iterator, so converting it to a list shows the filtered items.

Can filter() be used on a string?

A Yes, because strings are iterable

B No, strings are not iterable

C Only if converted to a list first

D Only with a special function

Strings are iterable, so filter() can be used directly on them.

Explain how the filter() function works and give an example using a lambda function to filter even numbers.

Think about how you pick only certain items from a list based on a test.

What happens when you pass None as the function to filter()? Why might this be useful?

Consider what values are treated as false in Python.

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