How to Write a List to a File in Python

Aug 23, 2021 • 2 min read

TLDR: Print items line by line

Use this method to write items of a Python list one by one to a text file.

Python 3

cities = ['Atlanta', 'Boston', 'Chicago', 'Los Angeles', 'New York', 'San Francisco', 'Seattle']

FILE_PATH = './cities.txt'

with open(FILE_PATH, 'w') as output_file:
	for city in cities:
		print(city, file=output_file)

Python 2

cities = ['Atlanta', 'Boston', 'Chicago', 'Los Angeles', 'New York', 'San Francisco', 'Seattle']

FILE_PATH = './cities.txt'

with open(FILE_PATH, 'w') as output_file:
	for city in cities:
		print >> output_file, city

The output in ./cities.txt will look like this:

Atlanta
Boston
Chicago
Los Angeles
New York
San Francisco
Seattle

Write a Python list to a JSON file

Say you have a list of complex objects, or you need a simple way to read the list back into another program. JSON is the way to go, and doing this in Python is as simple as calling a function.

import json

cities = [
	{
		'city': 'Atlanta',
		'state': 'Georgia'
	},
	{
		'city': 'Boston',
		'state': 'Massachusetts'
	},
	{
		'city': 'Los Angeles',
		'state': 'California'
	},
	{
		'city': 'Seattle',
		'state': 'Washington'
	}
]

FILE_PATH = './cities.json'

with open(FILE_PATH, 'w') as output_file:
	json.dump(cities, output_file, indent=2)

As you might guess, here is our output in ./cities.json:

[
  {
    "city": "Atlanta",
    "state": "Georgia"
  },
  {
    "city": "Boston",
    "state": "Massachusetts"
  },
  {
    "city": "Los Angeles",
    "state": "California"
  },
  {
    "city": "Seattle",
    "state": "Washington"
  }
]

How it works

First off, we need to import the json module from Python's standard library.

import json

Once we have our list, we open a file for writing.

FILE_PATH = './cities.json'

with open(FILE_PATH, 'w') as output_file:
	...

Using with statement to open files is a good practice because it ensures that the resource (file) opened with open(FILE_PATH, 'w') is closed after the execution flow leaves the with block.

We then call json.dump(cities, output_file, indent=2) to actually write the list to the file.

The json.dump function has a number of useful parameters - here we've used indent=2 . It pretty-prints our json using 2 spaces as unit of indentation. Without it, the output would look like this:

[{"city": "Atlanta", "state": "Georgia"}, {"city": "Boston", "state": "Massachusetts"}, {"city": "Los Angeles", "state": "California"}, {"city": "Seattle", "state": "Washington"}]

More compact, but hardly human-readable.

Another useful parameter to json.dump is sort_keys=True . It ensures that all keys of every dictionary of our object are output in sorted order.

Write a Python list to a JSON string

We've seen how json.dump(list, file) writes a list to a file. It's often useful to dump an object or a list to a string, eg. for sending as a request payload to an API.

We can achieve this by simply calling the json.dumps(object) method, which returns a JSON representation of the object as a plain Python string.

Combining this with our first method of writing lists to files using print , we can also write lists to JSON files in the following way:

import json

cities = ['Atlanta', 'Boston', 'Chicago', 'Los Angeles', 'New York', 'San Francisco', 'Seattle']

FILE_PATH = './cities.json'

with open(FILE_PATH, 'w') as output_file:
	print(json.dumps(cities, indent=2), file=output_file)