Python Filter Data from a List Example

Filtering a list means creating a new list that keeps only the items you want.

This example shows simple beginner-friendly ways to filter data from a Python list using:

• a for loop
• an if statement
• a list comprehension

If you are new to loops and conditions, this is a good place to start before reading more detailed guides on Python for loops and Python if statements.

Quick example #

numbers = [1, 2, 3, 4, 5, 6]
even_numbers = [n for n in numbers if n % 2 == 0]
print(even_numbers)

Output:

[2, 4, 6]

Use a list comprehension when you want to create a new list that keeps only matching items.

What this example shows #

• How to create a new list from an existing list
• How to keep only items that match a condition
• How filtering does not change the original list unless you reassign it
• When to use a loop vs a list comprehension

Example 1: Filter with a for loop #

A for loop is often the easiest way to understand filtering.

The basic idea is:

1. Start with an empty list
2. Loop through the original list
3. Check each item with an if statement
4. Add matching items to the new list

numbers = [1, 2, 3, 4, 5, 6]
even_numbers = []

for number in numbers:
    if number % 2 == 0:
        even_numbers.append(number)

print("Original list:", numbers)
print("Filtered list:", even_numbers)

Output:

Original list: [1, 2, 3, 4, 5, 6]
Filtered list: [2, 4, 6]

Why this works #

• numbers is the original list
• even_numbers = [] creates an empty result list
• for number in numbers: checks each item one by one
• if number % 2 == 0: keeps only even numbers
• even_numbers.append(number) adds matching items to the result

The key line is:

if number % 2 == 0:

That condition returns True only for even numbers.

If you want to learn more about adding items to a list, see Python list.append().

Example 2: Filter with a list comprehension #

After you understand the loop version, you can write the same logic in one line with a list comprehension.

numbers = [1, 2, 3, 4, 5, 6]
even_numbers = [number for number in numbers if number % 2 == 0]

print("Original list:", numbers)
print("Filtered list:", even_numbers)

Output:

Original list: [1, 2, 3, 4, 5, 6]
Filtered list: [2, 4, 6]

This version is shorter, but it does the same job:

• loop through numbers
• test each item
• keep only matching items

Use a list comprehension when the condition is short and easy to read.

For more practice, read how to use list comprehensions in Python.

Common filtering conditions #

You can filter lists in many ways. Here are a few simple patterns.

Numbers greater than a value #

numbers = [3, 8, 12, 1, 20]
result = [n for n in numbers if n > 10]
print(result)

Output:

[12, 20]

Even or odd numbers #

numbers = [1, 2, 3, 4, 5, 6]
odd_numbers = [n for n in numbers if n % 2 != 0]
print(odd_numbers)

Output:

[1, 3, 5]

Strings that contain a word #

words = ["apple pie", "banana", "green apple", "orange"]
result = [word for word in words if "apple" in word]
print(result)

Output:

['apple pie', 'green apple']

Strings that start with a letter #

names = ["Alice", "Bob", "Anna", "Charlie"]
result = [name for name in names if name.startswith("A")]
print(result)

Output:

['Alice', 'Anna']

Items that are not empty #

items = ["book", "", "pen", "", "paper"]
result = [item for item in items if item != ""]
print(result)

Output:

['book', 'pen', 'paper']

Expected output and why it works #

When you filter a list, it helps to print both the original list and the new list.

names = ["Anna", "Ben", "Alex", "Cara"]
filtered_names = []

for name in names:
    if name.startswith("A"):
        filtered_names.append(name)

print("Original list:", names)
print("Filtered list:", filtered_names)

Output:

Original list: ['Anna', 'Ben', 'Alex', 'Cara']
Filtered list: ['Anna', 'Alex']

Why it works:

• The original list stays the same
• Only matching items are added to filtered_names
• The condition is the part that decides what stays

In this example, the important line is:

if name.startswith("A"):

Only names that start with "A" are added to the new list.

If you want a step-by-step guide for this task, see how to filter a list in Python.

Beginner mistakes to avoid #

Here are some common problems when filtering lists.

Forgetting to create a new list #

This will fail because result does not exist yet:

numbers = [1, 2, 3]

for number in numbers:
    if number > 1:
        result.append(number)

Create the list first:

numbers = [1, 2, 3]
result = []

for number in numbers:
    if number > 1:
        result.append(number)

print(result)

Using = instead of == #

This is wrong inside a condition:

if number = 2:

Use == to compare values:

if number == 2:

Expecting the original list to change automatically #

Filtering usually creates a new list.

numbers = [1, 2, 3, 4]
result = [n for n in numbers if n > 2]

print(numbers)
print(result)

Output:

[1, 2, 3, 4]
[3, 4]

The original list is still [1, 2, 3, 4].

Writing a condition that returns the wrong type #

Conditions should evaluate to True or False.

Good:

if number > 3:

Less useful for filtering logic:

if number + 3:

That second example produces a value, but it does not clearly express the rule you want.

Confusing filtering with sorting #

Filtering removes items that do not match a condition.

Sorting changes the order of items.

These are different tasks.

Common causes of filtering problems #

Many beginner errors come from a small number of causes:

• Trying to remove items from the same list while looping over it
• Using append() on a variable that was not created as a list
• Writing an invalid condition inside the loop or comprehension
• Mixing numbers and strings in a comparison without conversion

For example, this can cause problems:

items = [1, "2", 3]

for item in items:
    if item > 1:
        print(item)

In this list, some items are numbers and one is a string. Python cannot always compare them directly in a useful way.

Debugging tips #

If your filtering code does not work, print values as the loop runs.

Useful debug lines:

print(items)
print(item)
print(result)
print(type(item))
print(item % 2 == 0)

Example:

numbers = [1, 2, 3, 4]
result = []

for item in numbers:
    print("Current item:", item)
    print("Is even?", item % 2 == 0)

    if item % 2 == 0:
        result.append(item)

print("Final result:", result)

This helps you see:

• what each item is
• whether the condition is True or False
• what gets added to the result list

FAQ #

What is the easiest way to filter a list in Python? #

For beginners, use a for loop with an if statement. After that, learn the list comprehension version.

Does filtering change the original list? #

Not unless you assign the result back to the same variable. Usually filtering creates a new list.

Should I use filter() for this page? #

No. This example focuses on loops and list comprehensions because they are easier for beginners to read.

Can I filter a list of strings? #

Yes. You can keep only strings that match a condition, such as containing a word or starting with a letter.

See also #

• How to filter a list in Python
• How to use list comprehensions in Python
• Python if statements explained
• Python for loops explained
• Python list append() method

Try changing the examples in this page to make your own filters:

• keep numbers greater than 10
• keep only odd numbers
• keep strings that contain "cat"
• keep names that start with "B"

That is the main skill: write a condition, test each item, and keep only the ones that match.