PythonCheat Sheet

Python File I/O Cheat Sheet

Quick-reference for file reading, writing, and path manipulation. Each section includes copy-ready snippets with inline output comments.

open() Modes

The mode argument controls read/write/append and text/binary.

Common modes

open('f.txt', 'r')    # Read text (default)
open('f.txt', 'w')    # Write text (truncates!)
open('f.txt', 'a')    # Append text
open('f.txt', 'x')    # Create (fails if exists)
open('f.png', 'rb')   # Read binary
open('f.png', 'wb')   # Write binary

Mode summary table

# 'r'  read        (file must exist)
# 'w'  write       (creates or truncates)
# 'a'  append      (creates or appends)
# 'x'  exclusive   (fails if file exists)
# 'b'  binary mode (add to above: 'rb', 'wb')
# '+'  read+write  (add to above: 'r+', 'w+')

'w' destroys existing content. Use 'a' to append or 'x' to prevent overwriting.

Reading Files

Read entire file as string

with open('data.txt') as f:
    content = f.read()
len(content)   # => number of characters

Read all lines into a list

with open('data.txt') as f:
    lines = f.readlines()
lines[0]   # => 'first line\n'

readlines() keeps the trailing newline. Use line.strip() to remove it.

Iterate line by line (memory efficient)

with open('data.txt') as f:
    for line in f:
        print(line.strip())

Read with encoding

with open('data.txt', encoding='utf-8') as f:
    content = f.read()

Read first N characters

with open('data.txt') as f:
    head = f.read(100)   # first 100 characters

Writing and Appending

Write (creates or truncates)

with open('output.txt', 'w') as f:
    f.write('Hello, world!\n')
    f.write('Second line\n')

Write multiple lines

lines = ['line 1\n', 'line 2\n', 'line 3\n']
with open('output.txt', 'w') as f:
    f.writelines(lines)

writelines() does NOT add newlines automatically. Include \n in each string.

Append to existing file

with open('log.txt', 'a') as f:
    f.write('New log entry\n')

print() to a file

with open('output.txt', 'w') as f:
    print('Hello', 'world', sep=', ', file=f)
    # writes: 'Hello, world\n'

Context Managers (with statement)

The with statement ensures the file is closed automatically, even if an exception occurs.

Basic with statement

with open('data.txt') as f:
    data = f.read()
# f is closed here, even if an exception occurred

Multiple files at once

with open('input.txt') as src, open('output.txt', 'w') as dst:
    dst.write(src.read())

Without with (manual close)

f = open('data.txt')
try:
    data = f.read()
finally:
    f.close()   # Must close manually!

Always prefer the with statement. Manual close is error-prone.

pathlib.Path (Modern Path Handling)

pathlib provides an object-oriented interface for filesystem paths. Prefer it over os.path for new code.

Create and join paths

from pathlib import Path
p = Path('src') / 'lib' / 'utils.py'
str(p)   # => 'src/lib/utils.py'

Read and write with Path

p = Path('data.txt')
content = p.read_text(encoding='utf-8')
p.write_text('new content', encoding='utf-8')

Path inspection

p = Path('/home/user/project/main.py')
p.name       # => 'main.py'
p.stem       # => 'main'
p.suffix     # => '.py'
p.parent     # => PosixPath('/home/user/project')
p.exists()   # => True or False
p.is_file()  # => True or False
p.is_dir()   # => False

List directory contents

p = Path('src')
list(p.iterdir())           # all items
list(p.glob('*.py'))        # Python files
list(p.rglob('*.py'))       # recursive Python files

Create directories

Path('output/data').mkdir(parents=True, exist_ok=True)

Reading and Writing CSV

Read CSV rows

import csv
with open('data.csv') as f:
    reader = csv.reader(f)
    header = next(reader)   # first row as list
    for row in reader:
        print(row)          # each row is a list

Read CSV as dicts

import csv
with open('data.csv') as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row['name'], row['age'])

Write CSV

import csv
rows = [['name', 'age'], ['Alice', 30], ['Bob', 25]]
with open('out.csv', 'w', newline='') as f:
    writer = csv.writer(f)
    writer.writerows(rows)

Pass newline='' to open() when writing CSV to prevent double line endings on Windows.

Reading and Writing JSON

Read JSON file

import json
with open('config.json') as f:
    data = json.load(f)
data['name']   # => access like a dict

Write JSON file

import json
data = {'name': 'Alice', 'scores': [85, 92, 88]}
with open('output.json', 'w') as f:
    json.dump(data, f, indent=2)

JSON string (not file)

import json
s = json.dumps({'a': 1})   # => '{"a": 1}'
d = json.loads(s)           # => {'a': 1}

Exception Handling for File Operations

Handle the specific exceptions that file operations raise.

FileNotFoundError

try:
    with open('missing.txt') as f:
        data = f.read()
except FileNotFoundError:
    data = ''   # graceful fallback

PermissionError

try:
    with open('/etc/shadow') as f:
        data = f.read()
except PermissionError:
    print('Access denied')

JSON decode error

import json
try:
    with open('config.json') as f:
        config = json.load(f)
except FileNotFoundError:
    config = {}
except json.JSONDecodeError as e:
    print(f'Invalid JSON: {e}')
    config = {}

Check before opening with pathlib

from pathlib import Path
p = Path('data.txt')
if p.exists() and p.is_file():
    content = p.read_text()

Prefer try/except over existence checks (EAFP). The file can be deleted between check and read.

os.path Utilities (Legacy)

os.path still works and appears in older codebases. Prefer pathlib for new code.

Join paths

import os
os.path.join('src', 'lib', 'utils.py')
# => 'src/lib/utils.py'

Split path components

import os
os.path.basename('/home/user/file.txt')   # => 'file.txt'
os.path.dirname('/home/user/file.txt')    # => '/home/user'
os.path.splitext('main.py')               # => ('main', '.py')

Check existence

import os
os.path.exists('data.txt')   # => True or False
os.path.isfile('data.txt')   # => True if regular file
os.path.isdir('src')         # => True if directory

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