In this article, you’ll learn various methods to write a list to a file in Python. Whether you want to save a list to a text file or organize it in a CSV file, we’ll cover all the techniques step by step.
Writing a List to a Text File
Let’s look at the common ways to write a list in a plain text file.
Method 1: Writing Line by Line
Sometimes you need to write each list element as a new line in the file. For example, if you have a shopping list and want to store each item on its own line, this method is ideal.
Example Code
In this example code, the open function opens the file in write mode. The with statement ensures the file is closed properly, even if an error occurs. Inside the with block, a loop writes each item from my_list to the file, appending \n to place each item on a new line.
# List of items
my_list = ["apple", "banana", "cherry"]
# Open a file in write mode
with open("output.txt", "w") as file:
for item in my_list:
file.write(item + "\n") # Add a newline after each item
print("List written to output.txt successfully!")Output
After running the code, if you open the output.txt file, it will contain:
apple banana cherry
Method 2: Writing the Entire List at Once
If you prefer to write the whole list to the file in a single step, then this method is for you. Instead of writing line by line, the list is joined into a single string where each element is separated by a delimiter, such as a newline character.
Example Code
Here, the join method combines all elements of the list into a single string, with each element separated by a newline (\n). This string is then written in one go into the file. The open function, along with the with statement, handles the file operation just like in the previous method.
# List of items
my_list = ["apple", "banana", "cherry"]
# Open a file in write mode
with open("output.txt", "w") as file:
file.write("\n".join(my_list))
print("List written to output.txt successfully!")Output
After executing the code, the output.txt file will contain the same result:
apple banana cherry
Method 3: Appending to a Text File
There are cases when you don’t want to overwrite a file but instead add new data to it. For example, if you’re maintaining a daily journal or a running log, appending new entries is more practical.
Example Code
In the code below, When the file is opened in append mode (a), Python ensures that any new data is added to the end of the existing content without overwriting it. The for loop works just like in the first method.
# List of items
my_list = ["date", "elderberry", "fig"]
# Open a file in append mode
with open("output.txt", "a") as file:
for item in my_list:
file.write(item + "\n")
print("List appended to output.txt successfully!")Output
If the file output.txt initially contained:
apple banana cherry
After running the code, it will look like this:
apple banana cherry date elderberry fig
Writing a List to a CSV File
You can use Python’s built-in module csv to save a list to a CSV file. Let’s explore the techniques to store your lists in a CSV format.
If you’re managing tabular data across files, combining multiple CSV files can simplify your workflow.
Method 1: Writing a Single Row
Writing a list to a CSV file is similar to writing to a text file, but it’s more structured. Each element in the list becomes a cell in a single row of the CSV file.
Example Code
In this example code, the csv.writer object prepares the file for CSV writing. Then, the writer.writerow method writes the entire list as a single row in the CSV file.
Here, the newline="" argument ensures that no extra blank lines are added, a common issue on Windows systems.
import csv
# List of items
my_list = ["apple", "banana", "cherry"]
# Open a CSV file in write mode
with open("output.csv", "w", newline="") as file:
writer = csv.writer(file)
writer.writerow(my_list)
print("List written to output.csv successfully!")Output
The resulting output.csv file will contain:
apple,banana,cherry
Method 2: Writing Multiple Rows from a Nested List
If you have a nested list (a list of lists), each sublist can represent a row in a CSV file. For instance, this is useful when you’re working with structured data (tabular data).
Example Code
As seen in the code below, the writer.writerows method writes all the rows at once. Each sublist in nested_list becomes a row in the CSV file. The purpose of the newline="" argument is as same as in the first method.
import csv
# Nested list
nested_list = [
["Name", "Age", "City"],
["Alice", "25", "New York"],
["Bob", "30", "San Francisco"],
["Charlie", "35", "Chicago"]
]
# Open a CSV file in write mode
with open("output.csv", "w", newline="") as file:
writer = csv.writer(file)
writer.writerows(nested_list) # Write all rows at once
print("Nested list written to output.csv successfully!")Output
After running the code, the output.csv file will look like this:
Name,Age,City Alice,25,New York Bob,30,San Francisco Charlie,35,Chicago
For handling structured data, understanding scientific notation in Python can be useful when working with numerical datasets.
Method 3: Writing with a Custom Delimiter
By default, a CSV file uses commas to separate values. However, sometimes you might want to use a different delimiter, such as tabs or semicolons. This can be helpful when dealing with non-standard CSV formats.
Example Code
Here, the csv.writer function uses the delimiter argument to set the character that separates values. The writer.writerow method saves the list as one row, with values separated by semicolons (;).
import csv
# List of items
my_list = ["apple", "banana", "cherry"]
# Open a CSV file with a semicolon delimiter
with open("output_semicolon.csv", "w", newline="") as file:
writer = csv.writer(file, delimiter=";")
writer.writerow(my_list)
print("List written to output_semicolon.csv successfully!")Output
The resulting output_semicolon.csv file will contain:
apple;banana;cherry
Method 4: Appending to a CSV File
Similar to text files, you can also append data to an existing CSV file. This is helpful when you want to add new rows without overwriting the existing content.
Example Code
In this code, the file is opened in append mode (a) to ensure existing content isn’t overwritten. Then, the writer.writerows method adds the new rows to the end of the file.
import csv
# List of items to append
new_rows = [
["David", "40", "Los Angeles"],
["Eve", "28", "Boston"]
]
# Open the file in append mode
with open("output.csv", "a", newline="") as file:
writer = csv.writer(file)
writer.writerows(new_rows) # Append rows to the existing file
print("Rows appended to output.csv successfully!")Output
If the initial output.csv file contained:
Name,Age,City Alice,25,New York Bob,30,San Francisco Charlie,35,Chicago
After appending, the file will look like this:
Name,Age,City Alice,25,New York Bob,30,San Francisco Charlie,35,Chicago David,40,Los Angeles Eve,28, Boston
Why Do We Need to Write a List to a File?
There are several reasons to save a Python list to files. Common ones include:
- Data Persistence: Even after the program stops running, the data is saved permanently.
- Sharing Information: Files make it easy to share data with others or transfer it between systems.
- Backup and Recovery: Storing data in files provides a reliable way to back up important information for future use.
- Cross-Application Compatibility: Files, especially CSVs, allow data to be used in other applications like spreadsheets or databases.
- Organization: Writing data to files helps structure information, making it easier to retrieve, analyze, or modify later.
Conclusion
You’ve now learned several methods to write a list to a file in Python. Whether it’s a plain text file or a CSV file, you can choose the method that best suits your needs. For simple tasks, writing line-by-line works well. For structured data, the CSV format provides flexibility and scalability. With these techniques, you’re well-equipped to handle data-saving tasks in your Python projects.
If you’re looking for more beginner-friendly articles on Python programming with easy-to-understand analogies, be sure to explore our Python tutorial series at Syntax Scenarios.
