Reading and Writing Files in Python

Analogy categoryAnalogies

1. Introduction

Once a file is open, Python gives you several methods to read or write its contents depending on whether you want the whole file at once, one line at a time, or all lines as a list. Choosing the right method affects both memory usage and how easy the resulting code is to work with.

🏏

Cricket analogy: Choosing read() versus readline() versus readlines() is like choosing to read a full match scorecard at once, ball-by-ball commentary one line at a time, or every over's summary as a list -- each suits a different review need.

For writing, Python offers write() for a single string and writelines() for a sequence of strings, giving you flexibility over how data is produced.

🏏

Cricket analogy: write() is like scribbling a single note on the scorecard, while writelines() is like pasting an entire pre-written list of over summaries onto the card in one motion, giving the scorer flexibility in how entries are added.

2. Syntax

python

# Reading
f.read()          # entire file as one string
f.read(n)         # up to n characters/bytes
f.readline()       # a single line, including the trailing newline
f.readlines()      # a list of all lines

# Writing
f.write('text')          # write a single string
f.writelines(['a\n', 'b\n'])  # write a list of strings (no automatic newlines added)

# Iterating line by line
for line in f:
    print(line)

3. Explanation

read() loads the entire file into memory as a single string, which is convenient for small files but wasteful for very large ones. readline() reads just the next line (including its trailing '\n'), while readlines() returns every line as a list of strings, each still containing its newline character.

🏏

Cricket analogy: read() is like reading the entire match scorecard into your head at once, fine for a single T20 but overwhelming for a five-day Test archive; readline() reads just the next over's line; readlines() lists every over's line including its line break.

writelines() writes each string in the given iterable one after another; unlike print(), it does not add newline characters automatically, so your input strings must already include '\n' where line breaks are needed.

🏏

Cricket analogy: writelines() pastes each over-summary string one after another exactly as given, unlike a scoreboard announcer who automatically adds a pause between overs -- so each string must already include its own line-break marker.

Both readline() and readlines() keep the trailing '\n' on each line. A common pattern is to call line.strip() (or .rstrip('\n')) when printing or processing lines to remove that trailing newline.

4. Example

python

import tempfile, os

path = os.path.join(tempfile.gettempdir(), 'demo_read_write.txt')

f = open(path, 'w')
f.writelines(['apple\n', 'banana\n', 'cherry\n'])
f.close()

f = open(path, 'r')
line1 = f.readline()
rest = f.readlines()
f.close()

print(line1.strip())
for line in rest:
    print(line.strip())

os.remove(path)

5. Output

text

apple
banana
cherry

6. Key Takeaways

read() returns the whole file as one string; readlines() returns a list of lines; readline() returns one line at a time.
writelines() writes an iterable of strings without adding newlines automatically.
Lines from readline()/readlines() retain their trailing '\n'; use strip() to remove it.
Iterating directly over a file object ('for line in f') reads it lazily, line by line, which is memory-efficient for large files.
Reading and writing should be paired with proper closing (or 'with') to avoid resource leaks.