How to Check If a List Contains Only Strings in Python

Introduction

In this lab, you will learn how to check if a list contains only strings in Python. You'll start by exploring string lists, understanding how to create them, access elements by index, modify elements, and add new elements using the append() method.

The lab will then guide you through applying the all() function with isinstance() to efficiently verify if all elements in a list are strings. Finally, you'll learn how to handle empty lists as a special case when checking for string-only lists.

Skills Graph

Explore String Lists

In this step, you will learn about string lists in Python. A string list is simply a list where each element is a string. Lists are a fundamental data structure in Python, used to store an ordered collection of items. Understanding how to work with lists of strings is crucial for many programming tasks, such as processing text data, manipulating filenames, and more.

First, let's create a simple string list. Open the VS Code editor in the LabEx environment. Create a new file named string_list.py in the ~/project directory.

## Create a list of strings
my_list = ["apple", "banana", "cherry"]

## Print the list
print(my_list)

Save the file. Now, open a terminal in the ~/project directory and run the script:

python string_list.py

You should see the following output:

['apple', 'banana', 'cherry']

Now, let's explore some common operations you can perform on string lists. You can access individual elements of the list using their index. Remember that Python uses zero-based indexing, meaning the first element has an index of 0.

Add the following code to your string_list.py file:

## Accessing elements by index
first_element = my_list[0]
second_element = my_list[1]

print("First element:", first_element)
print("Second element:", second_element)

Run the script again:

python string_list.py

You should see the following output:

['apple', 'banana', 'cherry']
First element: apple
Second element: banana

You can also modify elements in the list:

## Modifying an element
my_list[1] = "grape"
print(my_list)

Run the script again:

python string_list.py

You should see the following output:

['apple', 'banana', 'cherry']
First element: apple
Second element: banana
['apple', 'grape', 'cherry']

Finally, you can add new elements to the list using the append() method:

## Adding an element
my_list.append("orange")
print(my_list)

Run the script one last time:

python string_list.py

You should see the following output:

['apple', 'banana', 'cherry']
First element: apple
Second element: banana
['apple', 'grape', 'cherry']
['apple', 'grape', 'cherry', 'orange']

This demonstrates the basic operations you can perform on string lists in Python. In the next steps, you will learn more advanced techniques for working with lists.

Apply all() with isinstance()

In this step, you will learn how to use the all() function in combination with the isinstance() function to check if all elements in a list are strings. This is a common task when you need to validate the contents of a list before processing it further.

The all() function returns True if all elements in an iterable (like a list) are true. Otherwise, it returns False. The isinstance() function checks if an object is an instance of a particular class or type.

Let's start by creating a new Python file named check_string_list.py in the ~/project directory using the VS Code editor.

## Create a list with strings and non-strings
my_list = ["apple", "banana", "cherry", 123]

## Check if all elements are strings
all_strings = all(isinstance(item, str) for item in my_list)

## Print the result
print(all_strings)

Save the file. Now, open a terminal in the ~/project directory and run the script:

python check_string_list.py

You should see the following output:

False

This is because the list contains an integer (123), which is not a string.

Now, let's modify the list to contain only strings:

## Create a list with only strings
my_list = ["apple", "banana", "cherry"]

## Check if all elements are strings
all_strings = all(isinstance(item, str) for item in my_list)

## Print the result
print(all_strings)

Save the file and run the script again:

python check_string_list.py

You should now see the following output:

True

Here's a breakdown of what's happening in the code:

• isinstance(item, str): This checks if each item in the list is a string. It returns True if it is, and False otherwise.
• (isinstance(item, str) for item in my_list): This is a generator expression that yields a sequence of boolean values (True or False) indicating whether each element in the list is a string.
• all(...): This function takes the sequence of boolean values from the generator expression and returns True only if all the values are True. If any value is False, it returns False.

This combination of all() and isinstance() provides a concise and efficient way to validate that all elements in a list are of a specific type.

Check for Empty Lists

In this step, you will learn how to check if a list is empty in Python. Checking for empty lists is a common task, especially when processing data or handling user input. An empty list is a list that contains no elements.

Python provides a simple way to check if a list is empty: you can directly use the list in a boolean context. An empty list evaluates to False, while a non-empty list evaluates to True.

Let's create a new Python file named empty_list.py in the ~/project directory using the VS Code editor.

## Create an empty list
my_list = []

## Check if the list is empty
if not my_list:
    print("The list is empty")
else:
    print("The list is not empty")

Save the file. Now, open a terminal in the ~/project directory and run the script:

python empty_list.py

You should see the following output:

The list is empty

Now, let's modify the list to contain some elements:

## Create a list with elements
my_list = ["apple", "banana", "cherry"]

## Check if the list is empty
if not my_list:
    print("The list is empty")
else:
    print("The list is not empty")

Save the file and run the script again:

python empty_list.py

You should now see the following output:

The list is not empty

You can also use the len() function to check the length of the list. If the length is 0, the list is empty.

Modify the empty_list.py file to use the len() function:

## Create an empty list
my_list = []

## Check if the list is empty using len()
if len(my_list) == 0:
    print("The list is empty")
else:
    print("The list is not empty")

Save the file and run the script:

python empty_list.py

You should see the following output:

The list is empty

Both methods are commonly used to check for empty lists in Python. The first method (using the list directly in a boolean context) is often considered more Pythonic and readable.

Summary

In this lab, the initial step focuses on understanding string lists in Python, which are lists where each element is a string. The lab guides you through creating a string list, accessing elements by index (remembering Python's zero-based indexing), modifying elements, and adding new elements using the append() method.

This section provides a foundation for working with string lists, which are essential for tasks like processing text data and manipulating filenames. The examples demonstrate how to create, access, and modify string lists, providing a practical introduction to this fundamental data structure.