Filter List/Tuple elements in Python

Python offers an incredibly powerful set of tools for working with lists and tuples. Filtering elements from these objects is a common task that you may find yourself needing to perform from time to time. Fortunately, Python provides a simple and efficient way to filter elements from lists and tuples using built-in functions like  filter() , lambda functions, and list comprehension. In this tutorial, we will explore how you can filter elements from your Python lists and tuples.

Filter List/Tuple elements in Python : 

• Sometimes we need to select some of the elements of the List (or tuple) based on some condition.
• This process of shortlisting elements based on some criteria is called Filtering the elements.
• We can filter the elements of a list in Python in following ways :
  • Using loops
  • Using filter() function
  • using List comprehensions

Filtering elements using loops :

• We can iterate over all the elements using a for loop or a while loop.
• The control flow for filtering using loops :
  • Check each element of the list for the condition.
  • If the element satisfies the condition add the element to a new list.
  • return the new list as result.
• Loops are slower than the  filter()  function discussed ahead.
• Code Sample : 

my_list = [10, 20, 30, 40]
print("my_list = ", my_list)

new_list = []
for item in my_list:
    if 15 < item < 35:
        new_list.append(item)

print("new_list = ", new_list)

# Output
# my_list =  [10, 20, 30, 40]
# new_list =  [20, 30]

Filtering elements using filter() function :

•  filter()  is a built-in function that filters list or tuple elements more smoothly.
•  filter()  function can be used to filter elements of any iterable object - not just List or Tuple.
• The syntax for filter() function is :  new_list = filter(logic_function, orig_list) 
• Here :
  • logic_function is the function that contains the condition on which we need to filter the elements.
  • orig_list is the original list that we need to filter out.
  • new_list is the list of filtered out elements.
• Control flow of filter function - 
  • filter() function iterates all the elements of the list (or tuple).
  • It calls the logic_function() for each element.
  • It creates an iterator containing the elements that satisfy the condition in the logic_function.
  • filter() function then returns this new iterator as a result.
• Code Sample : 

def filter_function(item):
    if 15 < item < 35:
        return item


my_list = [10, 20, 30, 40]
print("my_list = ", my_list)

new_list_iterator = filter(filter_function, my_list)
new_list = list(new_list_iterator)

print("new_list = ", new_list)

# Output
# my_list =  [10, 20, 30, 40]
# new_list =  [20, 30]

Filtering elements using lambda expressions with filter() function :

• If our filtering logic is of one line, we can use anonymous functions instead of normal function.
• Code Sample : 

my_list = [10, 20, 30, 40]
print("my_list = ", my_list)

new_list_iterator = filter(lambda x: 15 < x < 35, my_list)
new_list = list(new_list_iterator)

print("new_list = ", new_list)

# Output
# my_list =  [10, 20, 30, 40]
# new_list =  [20, 30]

Filtering elements using List comprehensions : 

• List comprehension is another method of filtering elements of a list.
• List comprehensions is a very important topic in Python. 
• It is recommended that readers should go through our tutorial on List comprehensions to get a good hold of the topic.
• For the sake of completeness, below we have a simple example of filtering elements using list comprehension in python.
• Code Sample : 

my_list = [10, 20, 30, 40]
print("my_list = ", my_list)

new_list = [x for x in my_list if 15 < x < 35]
print("new_list = ", new_list)

# Output
# my_list =  [10, 20, 30, 40]
# new_list =  [20, 30]