LLM Best Practices

Python Collections Cheatsheet

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Overview

collections provides containers that replace common hand-written dict and list patterns. Typed record alternatives are in python-typing. Import what you need:

from collections import Counter, defaultdict, namedtuple, deque, ChainMap, OrderedDict

Counter

Use Counter for tallies and top-N queries.

PatternCodeResult
Count itemsc = Counter("aababc"){'a': 3, 'b': 2, 'c': 1}
Top Nc.most_common(2)[('a', 3), ('b', 2)]
Missing keyc["z"]0, not KeyError
Add / subtractc + d, c - dNew Counter; keeps positive counts only.
In-place subtractc.subtract("aab")Keeps zero and negative counts.
Expand / sumlist(c.elements()), c.total()Repeated elements; total count (3.10+).

defaultdict

Use defaultdict to group or count without existence checks.

PatternCode
Group into listsd = defaultdict(list); d[k].append(v)
Countd = defaultdict(int); d[k] += 1
Nestedd = defaultdict(lambda: defaultdict(int))
Custom defaultdefaultdict(lambda: "N/A"); the factory is any zero-argument callable.
Read without insertingd.get(k) or k in d

namedtuple and NamedTuple

Use a dataclass when you need mutability or methods.

from typing import NamedTuple
 
class Config(NamedTuple):
    host: str
    port: int = 5432
 
cfg = Config("localhost")
cfg._replace(port=6543)     # new instance
cfg._asdict()               # {'host': 'localhost', 'port': 5432}

namedtuple("P", ["x", "y"], defaults=[0, 0]) is the untyped form.

deque

Use deque for queues and sliding windows.

OperationCode
Bounded bufferdeque(maxlen=100) drops the oldest item when full.
Both ends in O(1)append, appendleft, pop, popleft
Rotated.rotate(1) right, negative left.
Extend leftd.extendleft([1, 2]) adds in reverse order.

ChainMap

Lookups search the maps in order; writes go to the first map only.

config = ChainMap(cli_args, env_vars, defaults)   # cli_args wins
config.new_child({"debug": True})                 # push a layer
config.parents                                     # drop the first layer
config.maps                                        # underlying dicts in priority order

OrderedDict

dict preserves insertion order since Python 3.7. Use OrderedDict only for move_to_end(), popitem(last=False), or equality that depends on order.

Gotchas

Avoid these; each loses data or inserts keys silently.

GotchaFix
c - d silently drops zero and negative counts.Use subtract() when you need them.
d[missing] on a defaultdict inserts the default.Use d.get(k) or k in d to probe.
A full deque(maxlen=n) discards data silently on append.Check len(d) == d.maxlen first if loss matters.
ChainMap writes never reach lower maps.Update the underlying dict directly.
Positional construction of a namedtuple breaks when fields are reordered.Construct with keywords.

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