Standard Library Essentials¶

from collections import Counter

c = Counter('mississippi')  # Counter({'i': 4, 's': 4, 'p': 2, 'm': 1})
c.most_common(2)           # [('i', 4), ('s', 4)]
c['nonexistent']           # 0 (no KeyError!)
c.update(['a', 'b', 'a']) # adds counts (unlike dict.update which replaces)

# Operators
c1 + c2    # add counts (excludes zero/negative)
c1 - c2    # subtract counts (excludes zero/negative)
c1 & c2    # intersection (min)
c1 | c2    # union (max)

from collections import defaultdict

word_count = defaultdict(int)     # missing key -> 0
for word in text.split():
    word_count[word] += 1

groups = defaultdict(list)        # missing key -> []
for name, dept in employees:
    groups[dept].append(name)

from collections import ChainMap

d1 = {'a': 1, 'b': 2}
d2 = {'b': 3, 'c': 4}
cm = ChainMap(d1, d2)
cm['b']          # 1 (first match wins)
cm.new_child()   # new ChainMap with empty dict prepended

from collections import OrderedDict

od = OrderedDict()
od['a'] = 1
od['b'] = 2
od.move_to_end('a')  # move 'a' to end

from datetime import date, time, datetime, timedelta, timezone

# Date
today = date.today()
d = date(2024, 12, 25)
d.strftime('%d.%m.%Y')     # '25.12.2024'
d.weekday()                # 0=Monday, 6=Sunday

# DateTime
now = datetime.now()
utc = datetime.now(timezone.utc)
dt = datetime.strptime('15/03/2024 14:30', '%d/%m/%Y %H:%M')  # parse string
dt.strftime('%Y-%m-%d %H:%M:%S')  # format to string
dt.isoformat()             # '2024-03-15T14:30:00'

from datetime import timedelta

tomorrow = date.today() + timedelta(days=1)
week_ago = datetime.now() - timedelta(weeks=1)
diff = date(2024, 12, 31) - date(2024, 1, 1)
diff.days       # 365
diff.total_seconds()  # everything as seconds (float)

from zoneinfo import ZoneInfo  # Python 3.9+

eastern = ZoneInfo("America/New_York")
moscow = ZoneInfo("Europe/Moscow")
dt = datetime.now(eastern)
dt_moscow = dt.astimezone(moscow)

# Date range generator
def date_range(start, end):
    current = start
    while current <= end:
        yield current
        current += timedelta(days=1)

# Parse multiple formats
formats = ['%d.%m.%Y', '%Y-%m-%d', '%d/%m/%Y']
def parse_date(s):
    for fmt in formats:
        try:
            return datetime.strptime(s, fmt)
        except ValueError:
            continue
    raise ValueError(f"Cannot parse: {s}")

# Age calculation
def calculate_age(birth_date):
    today = date.today()
    age = today.year - birth_date.year
    if (today.month, today.day) < (birth_date.month, birth_date.day):
        age -= 1
    return age

import sys
sys.argv       # command-line arguments
sys.path       # module search paths
sys.getrecursionlimit()  # default 1000
sys.getsizeof(obj)       # memory size in bytes

import random
random.randint(1, 10)           # random int in [1, 10]
random.choice(['a', 'b', 'c']) # random element
random.shuffle(my_list)         # shuffle in-place
random.seed(42)                 # reproducible results

from functools import lru_cache, partial, reduce

@lru_cache(maxsize=128)  # memoization with LRU eviction
def fib(n): ...

add_five = partial(add, 5)  # partial application
total = reduce(lambda a, b: a + b, [1, 2, 3, 4])  # 10