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Lambda with filter() in Python - Cheat Sheet & Quick Revision

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Recall & Review
beginner
What is a lambda function in Python?
A lambda function is a small anonymous function defined with the keyword lambda. It can take any number of arguments but has only one expression, which is returned.
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beginner
What does the filter() function do in Python?
filter() takes a function and a list (or any iterable) and returns a new iterator with only the items for which the function returns True.
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beginner
How do you combine lambda and filter() to get even numbers from a list?
Use filter() with a lambda that checks if a number is even:
filter(lambda x: x % 2 == 0, numbers)
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intermediate
What type of object does filter() return in Python 3?
filter() returns an iterator, which can be converted to a list using list().
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intermediate
Why use lambda with filter() instead of a named function?
Using lambda keeps the code short and readable when the filtering logic is simple and used only once.
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What does this code return?
list(filter(lambda x: x > 3, [1, 4, 2, 5]))
A[1, 2]
B[4, 5]
C[1, 4, 2, 5]
D[3, 4, 5]
Which of these is a correct use of filter() with a lambda to get odd numbers?
Afilter(lambda x: x % 2 == 1, numbers)
Bfilter(lambda x: x % 2 == 0, numbers)
Cfilter(lambda x: x > 2, numbers)
Dfilter(lambda x: x < 0, numbers)
What type of argument does filter() expect as its first parameter?
AA number
BA list
CA string
DA function that returns True or False
What will list(filter(lambda x: x, [0, 1, '', 'hello'])) return?
A[1, 'hello']
B[0, 1]
C[0, '', 'hello']
D[]
Why might you convert the result of filter() to a list?
ABecause <code>filter()</code> returns a dictionary
BBecause <code>filter()</code> returns a string
CBecause <code>filter()</code> returns an iterator, and lists are easier to use and print
DBecause <code>filter()</code> returns a number
Explain how to use a lambda function with filter() to select items from a list.
Think about how lambda defines a small function and filter uses it to keep certain items.
You got /3 concepts.
    Describe the difference between the output of filter() in Python 2 and Python 3.
    Consider what you get directly from filter() and how you can use it.
    You got /3 concepts.

      Practice

      (1/5)
      1. What does the following code do?
      filter(lambda x: x > 5, [2, 7, 4, 10])
      easy
      A. Selects numbers greater than 5 from the list
      B. Selects numbers less than 5 from the list
      C. Returns the sum of numbers greater than 5
      D. Sorts the list in ascending order

      Solution

      1. Step 1: Understand the lambda condition

        The lambda function checks if each number is greater than 5.

      2. Step 2: Apply filter with the lambda

        filter() keeps only numbers where the lambda returns True, so numbers > 5.

      3. Final Answer:

        Selects numbers greater than 5 from the list -> Option A

      4. Quick Check:

        filter with lambda x > 5 = select numbers > 5 [OK]
      Hint: filter + lambda picks items where condition is True [OK]
      Common Mistakes:
      • Thinking filter sums or sorts the list
      • Confusing greater than with less than
      • Assuming filter changes the original list
      2. Which of these is the correct syntax to filter even numbers from a list nums using lambda and filter?
      easy
      A. filter(lambda x: x % 2 == 0, nums)
      B. filter(lambda x: x // 2 == 0, nums)
      C. filter(lambda x: x % 2, nums)
      D. filter(lambda x: x == 2, nums)

      Solution

      1. Step 1: Identify condition for even numbers

        Even numbers have zero remainder when divided by 2, so x % 2 == 0.

      2. Step 2: Check filter syntax

        filter(lambda x: x % 2 == 0, nums) correctly filters even numbers.

      3. Final Answer:

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

      4. Quick Check:

        Even check = x % 2 == 0 [OK]
      Hint: Use x % 2 == 0 to check even numbers [OK]
      Common Mistakes:
      • Using floor division (//) instead of modulo (%)
      • Using x % 2 without comparison (filters odd numbers)
      • Checking equality to 2 instead of remainder zero
      3. What is the output of this code?
      nums = [1, 3, 6, 8, 11]
result = list(filter(lambda x: x < 7, nums))
print(result)
      medium
      A. [7, 8, 11]
      B. [6, 8, 11]
      C. [1, 3, 6, 8]
      D. [1, 3, 6]

      Solution

      1. Step 1: Understand the filter condition

        The lambda keeps numbers less than 7.

      2. Step 2: Apply condition to each list item

        1 < 7 (keep), 3 < 7 (keep), 6 < 7 (keep), 8 < 7 (no), 11 < 7 (no).

      3. Final Answer:

        [1, 3, 6] -> Option D

      4. Quick Check:

        Filter x < 7 = [1, 3, 6] [OK]
      Hint: Filter keeps items where lambda returns True [OK]
      Common Mistakes:
      • Including numbers not meeting condition
      • Confusing less than with greater than
      • Not converting filter object to list before printing
      4. Find the error in this code snippet:
      nums = [10, 15, 20]
filtered = filter(lambda x: x % 2 = 0, nums)
print(list(filtered))
      medium
      A. Incorrect list conversion syntax
      B. Missing parentheses after filter
      C. Using '=' instead of '==' in lambda condition
      D. Lambda function missing argument

      Solution

      1. Step 1: Check lambda condition syntax

        The condition uses '=' which is assignment, not comparison.

      2. Step 2: Correct operator usage

        It should be '==' to compare if x % 2 equals 0.

      3. Final Answer:

        Using '=' instead of '==' in lambda condition -> Option C

      4. Quick Check:

        Comparison needs '==' not '=' [OK]
      Hint: Use '==' for comparison inside lambda [OK]
      Common Mistakes:
      • Using '=' instead of '==' in conditions
      • Forgetting to convert filter result to list
      • Missing lambda argument
      5. You have a list of words: words = ['apple', '', 'banana', ' ', 'cherry', None]. Which code correctly filters out empty strings, strings with only spaces, and None values using lambda and filter?
      hard
      A. list(filter(lambda x: x == '', words))
      B. list(filter(lambda x: x and x.strip(), words))
      C. list(filter(lambda x: x is None, words))
      D. list(filter(lambda x: x.strip() == '', words))

      Solution

      1. Step 1: Understand filtering criteria

        We want to remove empty strings (''), strings with only spaces (' '), and None values.

      2. Step 2: Analyze each option

        list(filter(lambda x: x and x.strip(), words)) checks x is truthy (not None or empty) and x.strip() removes spaces; if non-empty after strip, it keeps the word.

      3. Final Answer:

        list(filter(lambda x: x and x.strip(), words)) -> Option B

      4. Quick Check:

        Filter removes empty, spaces, None with x and x.strip() [OK]
      Hint: Use x and x.strip() to remove empty/space/None [OK]
      Common Mistakes:
      • Filtering only empty strings but not spaces or None
      • Checking equality to None incorrectly
      • Calling strip() on None causing error