Collections and Data Structures in the Sifr Stdlib

Sifr’s sifr.collections module provides familiar collection types such as Counter, deque, defaultdict, and frozenset. Built-in set[T] is a first-class language type and needs no import. Indexing operations that could fail return Option instead of raising KeyError or IndexError, so every missing-value case is visible in the type system.

Importing

from sifr.collections import Counter, from_list, deque, defaultdict, frozenset

Do not use bare import collections — that triggers SIFR-IMPORT-0008. Always use from sifr.collections import ....

Lists

Standard list[T] is a first-class language type in Sifr. You do not need to import it. Lists are mutable, ordered, and support all comprehension forms:

numbers: list[int] = [1, 2, 3, 4, 5]
squares: list[int] = [x * x for x in numbers]
evens: list[int] = [x for x in numbers if x % 2 == 0]

Appending to a list requires the mut qualifier on the binding that owns it:

def append_all(mut target: list[bool], values: list[bool]):
    for value in values:
        target.append(value)

For the core language model behind lists, dictionaries, optional values, and iteration, see Values and Collections and Iteration.

Dicts and Tuples

dict[K, V] and tuple[T, ...] are built-in types. Dict literals and comprehensions work exactly as in Python:

scores: dict[str, int] = {"alice": 42, "bob": 17}
doubled: dict[str, int] = {k: v * 2 for k, v in scores.items()}

Safe access via .get(key) returns Option[V] (either the value or None). Direct indexing uses the same panic-free contract: scores["missing"] returns None instead of raising KeyError.

maybe_score: int | None = scores["charlie"]
if maybe_score is not None:
    print(maybe_score)

Sets

set[T] is a built-in generic mutable collection. Construct it directly and use its methods:

left: set[int] = set([1, 2, 3])
right: set[int] = set([3, 4, 5])

union_set: set[int] = left.union(right)
inter_set: set[int] = left.intersection(right)

print(len(union_set))        # 5
print(len(inter_set))        # 1

Counter

Counter[T] counts occurrences of hashable values. Build one from a list with from_list, then query counts or pull the most-common elements:

from sifr.collections import Counter, from_list

counts: Counter[str] = from_list(["x", "y", "x", "z", "x", "y"])

print(counts.get("x"))            # 3
print(counts.most_common(2))      # [("x", 3), ("y", 2)]

counts.get(key) returns the integer count for that key (0 if unseen). most_common(n) returns the top-n (value, count) pairs in descending order, exactly as Python’s Counter.most_common does.

deque

deque[T] is a double-ended queue. Pass maxlen to create a bounded deque that automatically drops the oldest element when capacity is exceeded — useful for sliding windows and ring buffers:

from sifr.collections import deque

d: deque[int] = deque(maxlen=2)
d.append(10)
d.append(20)
d.append(30)   # drops 10 because maxlen=2

print(d.len())       # 2
print(d.popleft())   # 20

pop() on an empty deque returns None rather than raising IndexError. Check the return value before using it:

value = d.pop()
if value is not None:
    print(value)

Full Demo

The following is the complete collections demo from the Sifr repository, showing sets, Counter, and deque working together:

from sifr.collections import Counter, from_list, deque

def collect_set_and_counter_checks() -> list[bool]:
    checks: list[bool] = []
    left: set[int] = set([1, 2, 3])
    right: set[int] = set([3, 4, 5])
    checks.append(len(left.union(right)) == 5)
    checks.append(len(left.intersection(right)) == 1)

    counts: Counter[str] = from_list(["x", "y", "x", "z", "x", "y"])
    checks.append(counts.get("x") == 3)
    checks.append(str(counts.most_common(2)) == "[(\"x\", 3), (\"y\", 2)]")
    return checks

def collect_deque_checks() -> list[bool]:
    checks: list[bool] = []
    d: deque[int] = deque(maxlen=2)
    d.append(10)
    d.append(20)
    d.append(30)
    checks.append(d.len() == 2 and d.popleft() == 20)

    _ = d.pop()
    checks.append(d.pop() is None)
    return checks

def append_all(mut target: list[bool], values: list[bool]):
    for value in values:
        target.append(value)

def main():
    checks: list[bool] = []
    append_all(checks, collect_set_and_counter_checks())
    append_all(checks, collect_deque_checks())

    all_passed: bool = True
    for check in checks:
        if not check:
            all_passed = False
    print("collections demo: " + ("pass" if all_passed else "FAIL"))

JSON and Serialization

For working with JSON data structures, use sifr.json alongside your collections:

from sifr.json import JsonValue, loads, dumps

try:
    parsed_obj: JsonValue = loads("{\"name\":\"sifr\"}")
    parsed_arr: JsonValue = loads("[1,2,3]")
    roundtrip: JsonValue = loads(dumps(7))

    print(str(parsed_obj))  # {"name":"sifr"}
    print(str(parsed_arr))  # [1,2,3]
    print(str(roundtrip))   # 7
except JSONDecodeError as e:
    print(e.message)

loads raises JSONDecodeError on malformed input. Use a try/except block whenever parsing untrusted data.

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Collections and Data Structures in the Sifr Stdlib

Importing

Lists

Dicts and Tuples

Sets

Counter

deque

Full Demo

JSON and Serialization

​Importing

​Lists

​Dicts and Tuples

​Sets

​Counter

​deque

​Full Demo

​JSON and Serialization

Importing

Lists

Dicts and Tuples

Sets

Counter

deque

Full Demo

JSON and Serialization