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Python code examples · runnable

20 runnable Python code examples.

Open any Python example to explore the code and run it online. No setup, no signup.Explore practical Python code examples with clear explanations. Open any example to edit and run it online — no setup, no signup.

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Python

How to reverse a string in PythonThree ways to reverse a string — slicing, reversed() + join(), and a loop — plus how to reverse the words in a sentence.How to remove duplicates from a list in PythonRemove duplicate items from a list — dict.fromkeys() (keeps order), set() (fastest), and a loop — plus how to dedupe unhashable items like dicts.How to merge two dictionaries in PythonMerge two dictionaries — the | operator (3.9+), {**a, **b} unpacking, and .update() — plus how to deep-merge nested dicts. See which mutates and which builds a new dict.How to flatten a list of lists in PythonMake a flat list out of a list of lists — a nested comprehension, itertools.chain.from_iterable(), and sum(nested, []) — plus how to flatten arbitrarily nested lists.How to find the index of a list item in PythonFind the index of an item in a list — list.index() for the first match, an in-guard or try/except for missing items, and an enumerate() comprehension for every index — plus finding the first item that matches a condition.How to get the current date and time in PythonGet the current date and time with datetime.now(), format it with strftime(), make it timezone-aware with timezone.utc or ZoneInfo, and read the clock as a Unix timestamp.How to convert bytes to a string in PythonDecode bytes to str with .decode(), why str(b) gives you b'...' instead, how to survive a UnicodeDecodeError, and where bytes come from in the first place.How to list all files in a directory in PythonList a folder with os.listdir() and Path.iterdir(), keep only the files, match patterns with glob, walk subfolders recursively, and read sizes with os.scandir().How to concatenate two lists in PythonFour ways to concatenate lists — the + operator, extend() / += in place, unpacking with [*a, *b], and itertools.chain — plus the aliasing gotcha += hides.How to check if a list is empty in PythonCheck whether a list is empty — the truthiness test if not items:, len(items) == 0, and why items == [] is discouraged — plus how to tell an empty list from None.How to slice a list in PythonSlice a list with items[start:stop] — the half-open rule, omitted ends, negative indices, the step argument, and slice assignment to edit a list in place.How to count occurrences in a list in PythonCount how often items appear in a list — list.count() for a single value, collections.Counter for all of them in one pass, sum() with a condition, and a plain dict tally.How to remove a key from a dictionary in PythonRemove a key from a dict — del for keys that must exist, pop() when you want the removed value, pop(key, None) to never raise — plus a comprehension that drops several keys at once.How to check if a key exists in a dictionary in PythonFour ways to check for a dictionary key — the in operator, dict.get() with a default, try/except KeyError, and setdefault() — plus the falsy-value trap that breaks if d.get(key).How to sort a dictionary by value in PythonSort a dictionary by value — sorted() on .items() with a lambda, operator.itemgetter(1), reverse=True for descending, a tuple key for ties, and Counter.most_common() for counts.How to convert two lists into a dictionary in PythonTurn a list of keys and a list of values into a dict — dict(zip(...)), zip(strict=True), a dict comprehension, and itertools.zip_longest — plus what happens on uneven lengths and duplicate keys.How to check if a string contains a substring in PythonCheck whether a string contains a substring with the in operator, find() and index() when you need the position, casefold() for case-insensitive matching, and re.search() for patterns.How to join a list into a string in PythonJoin a list into a string with ", ".join(items) — plus map(str, ...) for lists of numbers, "\n".join() for line-by-line output, and why += in a loop is the wrong tool.How to convert a string to an int in PythonTurn a string into an int with int() — plus try/except ValueError for untrusted input, int(float()) for decimal strings, int(s, base) for binary and hex, and how to convert a whole list.How to loop with an index in PythonLoop with an index in Python — enumerate(), enumerate(items, start=1) for human-facing numbering, the range(len()) habit to avoid, and zip() for walking two lists together.

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