Short answers

How do I… in Python?

26 questions people type into a search box, each answered on one page: the code first, then the one or two things that trip people up. Press Run and the example works, here in your browser. Every output on these pages is checked when the site is built.

Files and data

CSV, JSON, text files, folders and SQLite.

How do I read a CSV file in Python?Open the file with newline='' and hand it to csv.DictReader: each row arrives as a dictionary keyed by the header. Every value is a string until you convert it.How do I write a CSV file in Python?Open the file with newline='' and use csv.DictWriter: writeheader() once, then writerows(). The csv module quotes commas and line breaks inside values for you.How do I read a JSON file in Python?json.load(f) reads a file, json.loads(text) parses a string. Objects become dicts, arrays become lists, and bad JSON raises JSONDecodeError with the line.How do I write a dictionary to a JSON file in Python?json.dump(data, f, indent=2) writes a readable file. ensure_ascii=False keeps accented names readable; default=str handles dates and other types JSON can't.How do I read a file line by line in Python?Loop over the open file: for line in f. It reads one line at a time, so a 10 GB log costs the same memory as a 10-line one. Strip the newline with rstrip().How do I check if a file exists in Python?Path('notes.txt').exists() answers it. About to open the file anyway? Skip the check and catch FileNotFoundError: the file can vanish in between.How do I list all files in a folder in Python?Path('folder').glob('*.csv') lists what matches, iterdir() lists everything, rglob() goes into subfolders. Sort the result: the order isn't guaranteed.How do I use SQLite in Python?sqlite3 ships with Python: connect to a file, run SQL, and always pass values with ? placeholders, never f-strings. A with block commits your writes.

Lists and dictionaries

Sorting, merging, de-duplicating, flattening.

How do I sort a dictionary by value in Python?sorted(d.items(), key=lambda kv: kv[1], reverse=True) gives (key, value) pairs, highest first. Wrap it in dict() to get a dictionary back, still in order.How do I merge two dictionaries in Python?defaults | overrides gives a new merged dict (Python 3.9+), and the right side wins on shared keys. update() changes one in place. Nested dicts are replaced.How do I remove duplicates from a list in Python?list(dict.fromkeys(items)) removes duplicates and keeps the order. list(set(items)) is shorter but scrambles it. Neither works on a list of dicts.How do I flatten a list of lists in Python?[x for row in nested for x in row] flattens one level, and itertools.chain does it lazily. Avoid sum(nested, []): it slows down fast as the list grows.How do I find an item in a list in Python?'x in items' answers yes or no. items.index(x) gives the position but raises ValueError if missing. next() with a default finds the first match by condition.

Strings and numbers

Converting, searching, splitting, reversing.

How do I convert a string to a number in Python?int('42') and float('3.5') convert text to numbers. int('3.5') fails, so go through float() first. Wrap input from users in try/except ValueError and ask again.How do I check if a string contains a substring in Python?'error' in line gives True or False. Lower-case both sides to ignore case, find() for the position, and a regex word boundary to match whole words only.How do I split a string in Python?text.split() splits on any whitespace, text.split(',') on an exact separator, maxsplit=1 only once. For several different separators, use re.split().How do I reverse a string in Python?text[::-1] returns the string reversed: a slice with a step of -1, walking from the end. ''.join(reversed(text)) does the same and reads more plainly.How do I generate a random number in Python?random.randint(1, 6) includes both ends; choice picks one item, sample picks several without repeats. For passwords and tokens, use secrets instead.

Dates and time

Today's date, parsing dates, the gap between two.

How do I get the current date and time in Python?datetime.now() for local time, date.today() for just the date, datetime.now(timezone.utc) for anything you store or send. Then format it with strftime.How do I convert a string to a date in Python?date.fromisoformat('2026-10-07') for ISO dates; datetime.strptime(text, '%d/%m/%Y') for others. The format must match the text exactly, or it's ValueError.How do I find the number of days between two dates in Python?Subtract the dates and read .days: (end - start).days. Add a timedelta to move a date forward or back, and numpy.busday_count for weekdays only.How do I measure how long my Python code takes to run?time.perf_counter() before and after, then subtract: it's the most precise clock Python has. To compare two small snippets fairly, use timeit.

The web and your computer

APIs, downloads, shell commands, environment variables.

How do I call an API in Python?requests.get(url, timeout=10), then raise_for_status() and .json(). Always set a timeout: without one, a server that stops answering hangs your program.How do I download a file from a URL in Python?For small files, requests.get(url).content written in 'wb' mode. For big ones, stream=True and iter_content() so a 2 GB download doesn't need 2 GB of memory.How do I run a shell command from Python?subprocess.run(['git', 'status'], capture_output=True, text=True, check=True). Pass the command as a list, and never use shell=True with anything a user typed.How do I read an environment variable in Python?os.environ.get('NAME', 'default') for optional settings; os.environ['NAME'] for required ones, so a missing value fails at start-up. Values are strings.

want the long version?The How-to track

These pages answer one question each. The How-to track covers the same ground properly — scraping, APIs, databases, CSV and JSON, testing, logging — when you have twenty minutes instead of two.