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Read a file line by line into a list Python

David Y.

The Problem

In Python, how do I read a file and store each line in a list?

The Solution

We can read the lines of a file into a list by opening the file and looping through it using a list comprehension:

with open('myfile.txt') as f:
    lines = [line.rstrip() for line in f]

The with syntax ensures that the file will be closed once our list is assembled. rstrip() is a string method that removes trailing whitespace and newlines.

Note that for sufficiently large files, this operation could become slow and memory intensive, and, depending on the purpose of our script, it may be more efficient to apply a function to each line as it’s read in rather than storing each line in a list for later manipulation. For example:

with open('myfile.txt') as f:
    for line in f:
        process(line)

def process(line):
    # insert line processing code here

Get Started With Sentry

Get actionable, code-level insights to resolve Python performance bottlenecks and errors.

  1. Create a free Sentry account

  2. Create a Python project and note your DSN

  3. Grab the Sentry Python SDK

pip install --upgrade sentry-sdk
  1. Configure your DSN
import sentry_sdk

sentry_sdk.init(
    "https://<key>@sentry.io/<project>",

    # Set traces_sample_rate to 1.0 to capture 100%
    # of transactions for performance monitoring.
    # We recommend adjusting this value in production.
    traces_sample_rate=1.0,
)

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