SnipBits

How to filter Lists in Python

In this post, we will discuss three methods for list filtering in Python. Specifically, we will walk through how to use list comprehension, generator expressions and the built-in filter() method to filter lists in python.

Filtering Lists using list comprehension

Let's assume that we have Data in a List and we want to extract values or reduce the list based on some Criteria. For example . let's consider the Following List

not_filtered_values = ["22", "21". None, "89"]

to start we can use list comprehension to filter the None values.

only_numbers_array = [ n for n in not_filtered_values if n != None]
# result= ["22", "21", "89"]

we can convert all elements of the List to Integers with a slight change to the list comprehension

only_numbers_array = [int(n) for n in not_filtered_values if n != None]
# result= [22, 21, 89]

we can also filter the List, this can happen when we change the if condition

Filtering using list() and filter() methods

Sometimes the Filtering Criterias are not so easy to express using list comprehension or generators. For those cases we can define a function that takes a list and tries to convert each element to an Integer . When the conversion throws no errors , we return true. When the conversion throws a value Error, we use an except Statement to catch the Error .

def convert_and_filter(input_list):
  try:
    int(input_list)
    return true
  except ValueError :
    return false

The filter() function creates an Iterator and the list() method allows us to create a list of Results:

filtered_numbers = list(filter(convert_and_filter, not_filtered_values))

Filtering using Generators

When we are dealing with a lot of Data, which is often the case m we can filter using a generator expression. we can do the following :

not_filtered_values = ["22", "21". None, "89"]

numbers = (int(n) for n in not_filtered_values if n != None)

We can now iterate over the generator

you can check here to see how to remove Duplicates from a list in Python


Maher Gallaoui

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