How to read and write CSV files
Using the csv module with clear, copy-paste examples.
How-To Guides
Step-by-step solutions for data handling, file operations, HTTP, databases, testing, and automation. Problem-focused and easy to reuse.
How-To guides are short, task-focused articles: you have a concrete goal (read a CSV, call an API, loop with an index) and you want working code and clear steps. Each guide assumes you know basic Python—Getting Started, data types, functions, and lists or dictionaries when relevant—and focuses on one problem at a time.
We cover files and data: read and write CSV, read and write JSON, read a text file, write to a text file, read YAML. HTTP and APIs: HTTP GET, HTTP POST, timeouts, retries. Databases: connect to SQLite, query SQLite safely. Lists and strings: loop with index (enumerate), sort a list, split a string, join strings. Environment and tooling: virtual environment, install with pip, environment variables. For data science workflows see CSV with Pandas, Matplotlib, and related guides.
Use the sidebar or the list below to jump to a guide. If you hit an error, check the Errors & Exceptions section (e.g. FileNotFoundError, KeyError) or the linked Basics and Concepts pages.
How to read and write JSON
Use json.load, json.dump, and avoid common pitfalls.
How to loop over a list with index
Use enumerate() when you need both the item and its index.
How to sort a list
Use sorted() for a new list or list.sort() in place.
How to reverse a string
Use slicing s[::-1] or reversed() with ''.join().
How to check if a key exists in a dict
Use key in d or d.get() for safe access.
How to merge two dictionaries
Use ** unpacking, dict.update(), or | (Python 3.9+).
How to read a text file
Open with open(path, 'r') and use read(), readline(), or loop.
How to write to a text file
Open with open(path, 'w') and use write() or writelines().
How to parse dates and times
Use datetime.strptime() with a format string.
How to format dates and times
Use datetime.strftime() with format codes.
How to make an HTTP GET request
Use requests.get(url) and check response.status_code and response.text.
How to make an HTTP POST request
Use requests.post(url, json=...) or data=... for form data.
How to parse HTML with Beautiful Soup
Use BeautifulSoup(html, 'html.parser') and .find() / .select().
How to connect to SQLite
Use sqlite3.connect('file.db') and a cursor to run queries.
How to query SQLite safely
Use parameterized queries with ? placeholders to avoid SQL injection.
How to create a virtual environment
Run python -m venv .venv, then activate with source .venv/bin/activate or .venv\Scripts\activate.
How to install a package with pip
Run pip install <package>. Use pip install -r requirements.txt for a list.
How to read environment variables
Use os.environ['KEY'] or os.getenv('KEY', default).
How to run a subprocess
Use subprocess.run() for simple cases. Capture output with capture_output=True.
How to flatten a nested list
Use a list comprehension, itertools.chain.from_iterable, or a recursive helper.
How to remove duplicates from a list
Use list(dict.fromkeys(lst)) to preserve order, or list(set(lst)) if order does not matter.
How to count occurrences in a list
Use list.count(), collections.Counter, or a loop.
How to split a string
Use str.split() or str.splitlines(). Regex: re.split().
How to join strings
Use str.join(iterable). The separator is the string you call join on.
How to match text with regex
Use re.search() or re.match(). Compile with re.compile() for reuse.
How to replace text with regex
Use re.sub(pattern, repl, string). Use a callable as repl for dynamic replacement.
How to create a directory
Use os.makedirs(path) or Path.mkdir(). Use exist_ok=True to avoid errors if it exists.
How to list files in a directory
Use os.listdir(), pathlib.Path.iterdir(), or glob.glob().
How to delete a file
Use os.remove(path) or Path.unlink(). Check existence first if needed.
How to copy a file
Use shutil.copy(src, dst) or shutil.copy2 to preserve metadata.
How to move or rename a file
Use os.rename() or shutil.move(). Move works across filesystems.
How to read YAML files
Use pyyaml: yaml.safe_load(stream). Prefer safe_load over load.
How to write YAML files
Use yaml.dump(data, stream). Use default_flow_style=False for block style.
How to parse a URL
Use urllib.parse.urlparse() for components, or urlsplit / parse_qs for query strings.
How to encode and decode Base64
Use base64.b64encode() and b64decode(). Work with bytes.
How to hash a string
Use hashlib: hashlib.sha256(s.encode()).hexdigest(). Use SHA-256 or better for security.
How to generate a random string
Use secrets for security-sensitive randomness: secrets.token_hex(16) or token_urlsafe.
How to shuffle a list
Use random.shuffle(lst) to shuffle in place. Use random.sample(lst, k) for a random sample.
How to round a number
Use round(x, n) for rounding to n decimals. Use math.floor / ceil for down/up.
How to format numbers
Use f-strings f'{x:.2f}', format(), or locale for localized formatting.
How to convert a string to int
Use int(s). Use int(s, 16) for hex. Handle ValueError for invalid strings.
How to handle a missing dict key
Use d.get(key, default), try/except KeyError, or collections.defaultdict.
How to deep-copy a list
Use copy.deepcopy(x) for nested structures. list.copy() or lst[:] only copies the top level.
How to chunk a list into smaller lists
Use a list comprehension with slicing, or more_itertools.chunked. Example: [lst[i:i+n] for i in range(0,len(lst),n)].
How to zip two lists together
Use zip(a, b) to get pairs. Use list(zip(...)) if you need a list.
How to invert a dictionary
Swap keys and values. Use {v: k for k, v in d.items()}. Watch out for duplicate values.
How to sort a dict by value
Use sorted(d.items(), key=lambda x: x[1]). Use dict() to get a dict back (Python 3.7+ order preserved).
How to filter a list
Use list comprehension [x for x in lst if cond] or filter(func, lst).
How to map over a list
Use list comprehension [f(x) for x in lst] or map(f, lst). list(map(...)) if you need a list.
How to find the index of an item in a list
Use list.index(x). Use enumerate or a loop if you need to handle duplicates or a predicate.
How to replace an item in a list
Use lst[i] = new_value. Use [new if x==old else x for x in lst] to replace all matches.
How to use defaultdict
Use collections.defaultdict(factory). Missing keys get the default from factory().
How to use Counter
Use collections.Counter(iterable) for frequency counts. most_common(n) returns top n.
How to parse a JSON string
Use json.loads(s) for a string. Use json.load(f) for a file.
How to pretty-print JSON
Use json.dumps(obj, indent=2). Use json.dump(..., indent=2) when writing to a file.
How to retry HTTP requests
Use a loop with try/except, or a library like tenacity or requests-retry.
How to set a timeout for requests
Pass timeout=10 to requests.get() or post(). Use a tuple (connect, read) for finer control.
How to download a file with requests
Use requests.get(url) and write response.content to a file. Use streaming for large files.
How to parse CSV with Pandas
Use pd.read_csv('file.csv'). Use encoding, sep, and header as needed.
How to filter a DataFrame
Use boolean indexing: df[df['col'] > 0]. Use query() for string expressions.
How to use groupby in Pandas
Use df.groupby('col') then .sum(), .mean(), .agg(), etc.
How to read Excel files
Use pd.read_excel('file.xlsx'). Install openpyxl for .xlsx. Use sheet_name to pick sheets.
How to write Excel files
Use df.to_excel('out.xlsx'). Use ExcelWriter for multiple sheets.
How to create a plot with Matplotlib
Use plt.plot(x, y), then plt.show(). Use plt.figure() and ax for more control.
How to save a Matplotlib figure
Use plt.savefig('out.png'). Set dpi, bbox_inches='tight' as needed.
How to use argparse
Create ArgumentParser(), add arguments with add_argument(), then parse_args().
How to read from stdin
Use sys.stdin (line by line: for line in sys.stdin) or input() for interactive prompts.
How to log to a file
Use logging.basicConfig(filename='app.log') or add a FileHandler to a logger.
How to use dataclasses
Use @dataclass and define attributes. Get __init__, __repr__ automatically.
How to use enums
Use class Color(Enum): RED = 1; GREEN = 2. Access with Color.RED.
How to run tests with pytest
Install pytest, name files test_*.py, define def test_*(). Run pytest.
How to mock and patch in tests
Use unittest.mock.patch or pytest fixtures. Patch where the object is used, not where it is defined.
How to use a context manager
Use with expr as x:. Create one with @contextmanager or a class with __enter__ / __exit__.
How to use a generator
Use yield instead of return in a function. Call it to get an iterator; iterate with for or next().
How to run async code with asyncio
Define async def functions, use await. Run with asyncio.run(main()).
How to use a thread pool
Use concurrent.futures.ThreadPoolExecutor. Submit tasks with executor.submit(fn, *args) or map.
How to use a decorator
Define a function that takes a callable and returns a callable. Use @decorator above the target.
How to cache a function
Use functools.lru_cache for memoization. Use @lru_cache(maxsize=128) above the function.
How to validate user input
Check type and value, use try/except for conversions. Use a library like pydantic for schemas.
How to sanitize a filename
Remove or replace unsafe characters. Use re.sub(r'[\/:*?"<>|]', '_', s) or a whitelist of allowed chars.
How to load .env files
Use python-dotenv: load_dotenv() then os.getenv(). Call load_dotenv() early in the app.
How to use ConfigParser
Use configparser.ConfigParser(), read('config.ini'), then get(section, key).
How to create a ZIP file
Use zipfile.ZipFile('out.zip', 'w') and write(path) or writestr.
How to extract a ZIP file
Use zipfile.ZipFile('file.zip', 'r') and extractall() or extract(member).
How to send email
Use smtplib.SMTP or SMTP_SSL. Create a message with email.mime, then sendmail.
How to validate email with regex
Use a regex pattern for basic checks. For robust validation, use a library like email-validator.
How to use a requests session
Use requests.Session() to reuse TCP connections and cookies across requests.
How to handle cookies in requests
Use requests.Session() for automatic cookie handling. Access response.cookies or set cookies=....
How to use pytest fixtures
Define def fixture_name() and pass it as an argument to tests. Use @pytest.fixture.
How to parametrize tests in pytest
Use @pytest.mark.parametrize('arg', values) to run the same test with different inputs.
How to parse XML
Use xml.etree.ElementTree: ET.parse('file.xml') or ET.fromstring(s), then find / findall.
How to use type hints
Add annotations like def f(x: int) -> str:. Use list[int], dict[str, int], Optional, etc.
How to convert a list to a dict
Use dict(zip(keys, values)) or {k: v for k, v in pairs}. Use dict.fromkeys(keys, default) for same value.
How to transpose a matrix
Use list(zip(*matrix)) for a list of tuples, or numpy.array(m).T for NumPy.
How to iterate pairwise
Use zip(lst, lst[1:]) or itertools.pairwise (Python 3.10+).
How to use deque
Use collections.deque for O(1) append/pop at both ends. appendleft, popleft, maxlen for bounded queues.
How to parse command-line arguments
Use argparse for full control, or sys.argv for minimal parsing.
How to redirect stdout
Assign sys.stdout to a file-like object, or use contextlib.redirect_stdout.
How to set headers in requests
Pass headers={'User-Agent': '...', 'Authorization': 'Bearer ...'} to get() or post().