148 commands · 9 cheat sheets · Python

Collections and Iteration master quick reference

Browse 148 commands from 9 focused cheat sheets on page 1 of 1. Each example opens its matching detailed section.

Collections and Iteration · 17 commands

Python Collections, Heaps, and Bisect Cheat Sheet

Use	Syntax	Examples
Count hashable values	`counts = Counter(values)`	View examples
Get common values	`top = counts.most_common(3)`	View examples
Sum counts	`size = counts.total()`	View examples
Group values	`groups = defaultdict(list)`	View examples
Initialize numeric counts	`counts = defaultdict(int)`	View examples
Add to the left	`queue.appendleft(item)`	View examples
Remove from the left	`item = queue.popleft()`	View examples
Keep a bounded history	`history = deque(maxlen=100)`	View examples
Layer mappings	`config = ChainMap(cli, environment, defaults)`	View examples
Add a child scope	`child = config.new_child(overrides)`	View examples
Declare a tuple record	`Point = namedtuple('Point', ['x', 'y'])`	View examples
Copy with field changes	`moved = point._replace(x=10)`	View examples
Push onto a min-heap	`heappush(heap, (priority, item))`	View examples
Pop the smallest entry	`priority, item = heappop(heap)`	View examples
Select a few smallest values	`lowest = nsmallest(3, values)`	View examples
Find a right insertion point	`index = bisect_right(values, target)`	View examples
Insert while sorted	`insort(values, item)`	View examples

Collections and Iteration · 12 commands

Python Comprehensions and Looping Patterns Cheat Sheet

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Use	Syntax	Examples
Transform into a list	`squares = [n * n for n in numbers]`	View examples
Filter and transform	`names = [user.name for user in users if user.active]`	View examples
Build a dictionary	`by_id = {user.id: user for user in users}`	View examples
Build unique results	`domains = {email.rsplit('@', 1)[-1] for email in emails}`	View examples
Stream transformed values	`total = sum(n * n for n in numbers)`	View examples
Pair with counters	`for line_number, line in enumerate(lines, start=1): handle(line_number, line)`	View examples
Pair equal-length inputs	`for name, score in zip(names, scores, strict=True): report(name, score)`	View examples
Sort by a key	`ordered = sorted(users, key=lambda user: user.name.casefold())`	View examples
Flatten one level	`flat = [item for group in groups for item in group]`	View examples
Group adjacent keys	`for key, group in groupby(sorted(rows, key=key_fn), key=key_fn): consume(key, group)`	View examples
Chain iterables lazily	`for item in chain(first, second): consume(item)`	View examples
Detect no break	`for ... else ...`	View examples

Collections and Iteration · 14 commands

Python Dictionaries Cheat Sheet

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Use	Syntax	Examples
Create a dictionary	`user = {"name": "Ada", "role": "engineer"}`	View examples
Create from keywords	`user = dict(name="Ada", role="engineer")`	View examples
Read a required key	`name = user["name"]`	View examples
Read an optional key	`timezone = user.get("timezone", "UTC")`	View examples
Check for a key	`if "role" in user: print(user["role"])`	View examples
Set one value	`user["active"] = True`	View examples
Update several values	`user.update({"role": "lead", "active": True})`	View examples
Create a merged dictionary	`combined = defaults \| overrides`	View examples
Remove and return a value	`role = user.pop("role", None)`	View examples
Delete a required key	`del user["active"]`	View examples
Iterate over pairs	`for key, value in user.items(): print(key, value)`	View examples
Iterate over keys	`for key in user: print(key)`	View examples
Build with a comprehension	`squares = {number: number ** 2 for number in range(4)}`	View examples
Initialize a missing key	`tags = groups.setdefault("python", [])`	View examples

Collections and Iteration · 18 commands

Python Iterators and Generators Cheat Sheet

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Use	Syntax	Examples
Create an iterator	`iterator = iter(values)`	View examples
Advance an iterator	`value = next(iterator)`	View examples
Use an exhaustion default	`value = next(iterator, None)`	View examples
Consume with a loop	`for value in iterable: print(value)`	View examples
Add indexes	`for index, value in enumerate(values, start=1): print(index, value)`	View examples
Pair iterables	`for name, score in zip(names, scores, strict=True): print(name, score)`	View examples
Create a lazy expression	`squares = (number * number for number in numbers)`	View examples
Filter lazily	`even = (number for number in numbers if number % 2 == 0)`	View examples
Yield a value	`yield value`	View examples
Delegate iteration	`yield from iterable`	View examples
End a generator	`return`	View examples
Compose lazy stages	`result = sum(value for value in source if value > 0)`	View examples
Materialize values	`cached = list(generator)`	View examples
Stop when any value matches	`found = any(value > 100 for value in values)`	View examples
Take a lazy slice	`first_five = islice(iterable, 5)`	View examples
Chain iterables	`combined = chain(first, second)`	View examples
Generate a counter	`identifiers = count(start=1000, step=10)`	View examples
Close a generator	`generator.close()`	View examples

Collections and Iteration · 19 commands

Python itertools, functools, and operator Cheat Sheet

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Use	Syntax	Examples
Concatenate iterables lazily	`items = chain(first, second)`	View examples
Flatten one iterable level	`items = chain.from_iterable(groups)`	View examples
Slice an iterator	`page = islice(source, start, stop, step)`	View examples
Create fixed-size batches	`chunks = batched(source, 3)`	View examples
Require complete batches	`chunks = batched(source, 3, strict=True)`	View examples
Iterate adjacent pairs	`edges = pairwise(values)`	View examples
Compute running totals	`totals = accumulate(values)`	View examples
Accumulate with a function	`products = accumulate(values, operator.mul, initial=1)`	View examples
Fork an iterator	`left, right = tee(source, 2)`	View examples
Group adjacent equal keys	`groups = groupby(records, key=operator.itemgetter('team'))`	View examples
Select with a Boolean mask	`selected = compress(data, selectors)`	View examples
Build a Cartesian product	`coordinates = product(rows, columns)`	View examples
Choose without ordering	`pairs = combinations(values, 2)`	View examples
Create an item lookup	`by_score = operator.itemgetter('score')`	View examples
Create an attribute lookup	`by_name = operator.attrgetter('profile.name')`	View examples
Create a method call	`normalize = operator.methodcaller('strip')`	View examples
Reduce to one value	`total = reduce(operator.add, values, 0)`	View examples
Pre-bind call arguments	`parse_hex = partial(int, base=16)`	View examples
Adapt a comparison function	`key = cmp_to_key(compare)`	View examples

Collections and Iteration · 19 commands

Python Lists Cheat Sheet

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Use	Syntax	Examples
Create a list	`items = ["alpha", "beta"]`	View examples
Create from an iterable	`items = list(range(1, 5))`	View examples
Read the first item	`first = items[0]`	View examples
Read the last item	`last = items[-1]`	View examples
Replace an item	`items[1] = "updated"`	View examples
Append one item	`items.append("beta")`	View examples
Append many items	`items.extend(["gamma", "delta"])`	View examples
Remove by value	`items.remove("beta")`	View examples
Remove the last item	`value = items.pop()`	View examples
Remove by index	`value = items.pop(0)`	View examples
Slice a range	`subset = items[1:4]`	View examples
Slice with a step	`subset = items[::2]`	View examples
Reverse with slicing	`reversed_items = items[::-1]`	View examples
Transform every item	`squares = [x ** 2 for x in items]`	View examples
Filter items	`evens = [x for x in items if x % 2 == 0]`	View examples
Sort the existing list	`items.sort()`	View examples
Create a sorted copy	`result = sorted(items)`	View examples
Sort with a key	`items.sort(key=str.lower)`	View examples
Sort descending	`items.sort(reverse=True)`	View examples

Collections and Iteration · 17 commands

Python Sets and Frozensets Cheat Sheet

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Use	Syntax	Examples
Create a populated set	`colors = {'red', 'green', 'blue'}`	View examples
Create an empty set	`values = set()`	View examples
Remove duplicates	`unique = set(values)`	View examples
Test membership	`if user_id in active_ids: allow()`	View examples
Add one member	`tags.add('python')`	View examples
Add several members	`tags.update(['css', 'html'])`	View examples
Remove a required member	`tags.remove('legacy')`	View examples
Discard if present	`tags.discard('legacy')`	View examples
Combine members	`all_tags = required \| optional`	View examples
Keep shared members	`shared = left & right`	View examples
Subtract members	`missing = required - supplied`	View examples
Keep non-shared members	`changed = before ^ after`	View examples
Test subset	`required <= supplied`	View examples
Test proper subset	`required < supplied`	View examples
Test for no overlap	`left.isdisjoint(right)`	View examples
Build with a comprehension	`lengths = {len(word) for word in words}`	View examples
Create an immutable set	`permissions = frozenset({'read', 'write'})`	View examples

Collections and Iteration · 20 commands

Python Strings Cheat Sheet

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Use	Syntax	Examples
Create a string	`message = "Hello, Python"`	View examples
Create multiline text	`message = "First line\\nSecond line"`	View examples
Count code points	`count = len(message)`	View examples
Read one character	`first = message[0]`	View examples
Slice a range	`prefix = message[:5]`	View examples
Reverse code points	`reversed_text = message[::-1]`	View examples
Trim surrounding whitespace	`clean = raw.strip()`	View examples
Lowercase text	`lowered = name.lower()`	View examples
Normalize for comparison	`matches = left.casefold() == right.casefold()`	View examples
Check for text	`contains = "error" in message`	View examples
Check a prefix	`is_secure = url.startswith("https://")`	View examples
Find a position	`position = message.find("Python")`	View examples
Replace text	`updated = message.replace("draft", "ready")`	View examples
Split fields	`fields = line.split(",")`	View examples
Split once	`key, value = line.split("=", maxsplit=1)`	View examples
Join strings	`path = "/".join(parts)`	View examples
Interpolate values	`message = f"{name} has {count} tasks"`	View examples
Format a number	`label = f"Total: ${total:,.2f}"`	View examples
Encode text	`payload = message.encode("utf-8")`	View examples
Decode bytes	`message = payload.decode("utf-8")`	View examples

Collections and Iteration · 12 commands

Python Tuples, Ranges, and Sequence Operations Cheat Sheet

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Use	Syntax	Examples
Create a tuple	`point = (3, 4)`	View examples
One-item tuple	`single = ('only',)`	View examples
Pack values	`record = name, score, active`	View examples
Unpack positions	`name, score = record`	View examples
Capture the middle	`first, *middle, last = values`	View examples
Count from zero	`range(5)`	View examples
Use start, stop, step	`range(10, 0, -2)`	View examples
Read by index	`last = values[-1]`	View examples
Slice a sequence	`copy = values[start:stop:step]`	View examples
Test membership	`if target in values: handle(target)`	View examples
Iterate with positions	`for index, value in enumerate(values, start=1): print(index, value)`	View examples
Pair sequences strictly	`pairs = zip(names, scores, strict=True)`	View examples

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