--- tags: - devops - l0 - flashcard-deck - python-concepts --- [Portal](../../../../library/portal/index.md) | **Level:** [L0: Entry](../../../../library/portal/levels.md) | **Topics:** [Python Automation](../../../../library/portal/topics.md) | **Domain:** DevOps & Tooling id category difficulty tags question answer source_path pyconcept/if-elif-else python-concepts easy python, control-flow, keyword if / elif / else x = 10\nif x > 20:\n print("big")\nelif x > 5:\n print("medium") # ← this runs\nelse:\n print("small")\n\nConditions are evaluated top-to-bottom; first True branch wins. elif and else are optional. concept-reference pyconcept/for-loop python-concepts easy python, control-flow, keyword for loop for name in ["alice", "bob", "carol"]:\n print(name.upper())\n\nIterates over any iterable: lists, strings, dicts, files, generators, range(). concept-reference pyconcept/while-loop python-concepts easy python, control-flow, keyword while loop n = 5\nwhile n > 0:\n print(n)\n n -= 1\n\nRuns as long as the condition is True. Use break to exit early. concept-reference pyconcept/break python-concepts easy python, control-flow, keyword break for n in range(100):\n if n > 4:\n break\n print(n) # prints 0 1 2 3 4\n\nExits the innermost loop immediately. concept-reference pyconcept/continue python-concepts easy python, control-flow, keyword continue for n in range(5):\n if n == 2:\n continue\n print(n) # prints 0 1 3 4\n\nSkips the rest of the current iteration, jumps to the next one. concept-reference pyconcept/pass python-concepts easy python, control-flow, keyword pass class NotYet:\n pass # placeholder — class body can't be empty\n\ndef todo():\n pass # same for functions\n\nDoes nothing. Used as a syntactic placeholder. concept-reference pyconcept/def python-concepts easy python, functions, keyword def (function definition) def greet(name):\n return f"hello, {name}"\n\nprint(greet("alice")) # hello, alice\n\nFunctions are first-class objects — you can pass them, return them, store them. concept-reference pyconcept/lambda python-concepts easy python, functions, keyword lambda (anonymous function) square = lambda x: x ** 2\nprint(square(5)) # 25\n\nsorted(["bob", "alice"], key=lambda s: len(s))\n# ['bob', 'alice']\n\nSingle-expression functions. No statements, no multi-line. concept-reference pyconcept/return python-concepts easy python, functions, keyword return def add(a, b):\n return a + b\n\nresult = add(3, 4) # 7\n\nSends a value back to the caller and exits the function. Without return, function returns None. concept-reference pyconcept/yield python-concepts medium python, functions, keyword yield (generators) def countdown(n):\n while n > 0:\n yield n\n n -= 1\n\nfor x in countdown(3):\n print(x) # 3 2 1\n\nPauses the function and produces a value. Resumes on next iteration. Memory-efficient for large sequences. concept-reference pyconcept/class python-concepts easy python, oop, keyword class class Dog:\n def __init__(self, name):\n self.name = name\n def speak(self):\n return f"{self.name} says woof"\n\nd = Dog("Rex")\nprint(d.speak()) # Rex says woof concept-reference pyconcept/self python-concepts easy python, oop, keyword self class Counter:\n def __init__(self):\n self.count = 0\n def inc(self):\n self.count += 1\n\nself is the instance — it's how methods access the object's data. Not a keyword, but always the first parameter. concept-reference pyconcept/try-except python-concepts easy python, error-handling, keyword try / except try:\n x = int("not a number")\nexcept ValueError as e:\n print(f"bad input: {e}")\n\nCatch specific exceptions. Avoid bare except: — always name the exception type. concept-reference pyconcept/try-finally python-concepts medium python, error-handling, keyword try / except / else / finally try:\n f = open("data.txt")\nexcept FileNotFoundError:\n print("missing")\nelse:\n print(f.read()) # runs only if no exception\nfinally:\n print("always runs") # cleanup goes here concept-reference pyconcept/raise python-concepts easy python, error-handling, keyword raise def divide(a, b):\n if b == 0:\n raise ValueError("cannot divide by zero")\n return a / b\n\nRaise an exception explicitly. raise without args re-raises the current exception in an except block. concept-reference pyconcept/assert python-concepts easy python, error-handling, keyword assert def avg(nums):\n assert len(nums) > 0, "empty list"\n return sum(nums) / len(nums)\n\nRaises AssertionError if condition is False. Disabled with python -O. Use for sanity checks, not input validation. concept-reference pyconcept/with python-concepts easy python, keyword, context-manager with (context manager) with open("file.txt") as f:\n data = f.read()\n# f is automatically closed here\n\nGuarantees cleanup (close, unlock, etc.) even if an exception occurs. Calls __enter__ and __exit__ under the hood. concept-reference pyconcept/as python-concepts easy python, keyword as (aliasing) import numpy as np # module alias\nwith open("f") as fh: # bind context manager result\nexcept KeyError as e: # bind exception to name\n\nas binds a name in import, with, and except statements. concept-reference pyconcept/in python-concepts easy python, keyword, operator in / not in (membership) print(3 in [1, 2, 3]) # True\nprint("x" not in "hello") # True\nprint("key" in {"key": 1}) # True (checks keys)\n\nWorks on lists, tuples, sets, dicts (keys), strings (substring). concept-reference pyconcept/is python-concepts easy python, keyword, operator is / is not (identity) x = None\nif x is None:\n print("nothing here")\n\na = [1, 2]\nb = a\nprint(a is b) # True (same object)\nprint(a is [1,2]) # False (different object, same value)\n\nis checks identity (same object in memory). Use == for value equality. concept-reference pyconcept/del python-concepts easy python, keyword del (delete) x = 42\ndel x # unbinds the name\n\nlst = [1, 2, 3]\ndel lst[1] # [1, 3]\n\nd = {"a": 1}\ndel d["a"] # {}\n\nRemoves a name binding, list element, dict entry, or attribute. concept-reference pyconcept/global-nonlocal python-concepts medium python, keyword, scope global / nonlocal count = 0\ndef increment():\n global count\n count += 1\n\ndef outer():\n x = 10\n def inner():\n nonlocal x\n x += 1\n inner()\n print(x) # 11\n\nglobal refers to module scope; nonlocal refers to enclosing function scope. concept-reference pyconcept/none python-concepts easy python, types None def find(lst, target):\n for item in lst:\n if item == target:\n return item\n return None # explicit "not found"\n\nresult = find([1,2,3], 99)\nif result is None:\n print("not found")\n\nPython's null. There is exactly one None object. Always test with is None, not ==. concept-reference pyconcept/fstring python-concepts easy python, strings f-string (formatted string literal) name = "alice"\nage = 30\nprint(f"{name} is {age} years old")\n\nprint(f"{age:>10}") # right-align, width 10\nprint(f"{3.14159:.2f}") # 3.14\nprint(f"{1000:,}") # 1,000\n\nExpressions inside {} are evaluated at runtime. Python 3.6+. concept-reference pyconcept/str-methods python-concepts easy python, strings, methods Common string methods s = " Hello, World "\ns.strip() # "Hello, World"\ns.lower() # " hello, world "\ns.split(", ") # [' Hello', 'World ']\n"_".join(["a","b"]) # "a_b"\ns.replace("World", "Python")\ns.startswith(" He") # True\ns.find("World") # 9 (-1 if not found)\n"42".isdigit() # True concept-reference pyconcept/str-slicing python-concepts easy python, strings String slicing s = "python"\ns[0] # 'p'\ns[-1] # 'n'\ns[2:4] # 'th'\ns[:3] # 'pyt'\ns[3:] # 'hon'\ns[::-1] # 'nohtyp' (reverse)\ns[::2] # 'pto' (every other)\n\nSame [start:stop:step] syntax works on lists and tuples. concept-reference pyconcept/list-ops python-concepts easy python, data-structures, list List operations a = [3, 1, 2]\na.append(4) # [3, 1, 2, 4]\na.extend([5, 6]) # [3, 1, 2, 4, 5, 6]\na.insert(0, 99) # [99, 3, 1, 2, 4, 5, 6]\na.pop() # returns 6, list shrinks\na.remove(99) # removes first occurrence\na.sort() # in-place: [1, 2, 3, 4, 5]\na.reverse() # in-place: [5, 4, 3, 2, 1]\nlen(a) # 5 concept-reference pyconcept/list-comp python-concepts easy python, data-structures, comprehension List comprehension squares = [x**2 for x in range(5)] # [0, 1, 4, 9, 16]\nevens = [x for x in range(10) if x % 2 == 0] # [0, 2, 4, 6, 8]\n\nflat = [c for word in ["hi","yo"] for c in word] # ['h','i','y','o']\n\nMore readable than map/filter for simple transforms. concept-reference pyconcept/dict-ops python-concepts easy python, data-structures, dict Dict operations d = {"a": 1, "b": 2}\nd["c"] = 3 # add/update\nd.get("z", 0) # 0 (no KeyError)\nd.pop("b") # returns 2, removes key\nd.update({"a": 10}) # merge\nd.setdefault("d", 4) # insert only if missing\n\nfor k, v in d.items():\n print(k, v) concept-reference pyconcept/dict-comp python-concepts easy python, data-structures, comprehension Dict comprehension squares = {x: x**2 for x in range(5)}\n# {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}\n\ninverse = {v: k for k, v in squares.items()}\n# {0: 0, 1: 1, 4: 2, 9: 3, 16: 4} concept-reference pyconcept/set-ops python-concepts easy python, data-structures, set Set operations a = {1, 2, 3}\nb = {2, 3, 4}\n\na | b # {1, 2, 3, 4} union\na & b # {2, 3} intersection\na - b # {1} difference\na ^ b # {1, 4} symmetric difference\n\n3 in a # True (O(1) lookup)\na.add(5)\na.discard(99) # no error if missing concept-reference pyconcept/set-comp python-concepts easy python, data-structures, comprehension Set comprehension words = ["hello", "world", "hello"]\nunique_lengths = {len(w) for w in words} # {5} concept-reference pyconcept/tuple-ops python-concepts easy python, data-structures, tuple Tuple operations t = (1, "two", 3.0)\nt[0] # 1\nt[-1] # 3.0\nlen(t) # 3\na, b, c = t # unpacking\n\n# single-element tuple needs trailing comma\none = (42,)\n\n# tuples are immutable — no append, no assignment\n# t[0] = 99 → TypeError concept-reference pyconcept/unpacking python-concepts medium python, syntax Unpacking (* and **) # Iterable unpacking\nfirst, *rest = [1, 2, 3, 4] # first=1, rest=[2,3,4]\na, _, b = (1, 2, 3) # ignore middle value\n\n# Function argument unpacking\ndef f(a, b, c): return a + b + c\nargs = [1, 2, 3]\nf(*args) # 6\n\nkw = {"a": 1, "b": 2, "c": 3}\nf(**kw) # 6\n\n# Merge dicts\nmerged = {**d1, **d2} concept-reference pyconcept/enumerate python-concepts easy python, builtins enumerate() for i, name in enumerate(["alice", "bob", "carol"]):\n print(f"{i}: {name}")\n# 0: alice\n# 1: bob\n# 2: carol\n\nfor i, ch in enumerate("abc", start=1):\n print(i, ch) # 1 a, 2 b, 3 c\n\nReplaces the manual counter pattern. Always prefer over range(len(x)). concept-reference pyconcept/zip python-concepts easy python, builtins zip() names = ["alice", "bob"]\nages = [30, 25]\n\nfor name, age in zip(names, ages):\n print(f"{name} is {age}")\n\ndict(zip(names, ages)) # {"alice": 30, "bob": 25}\n\nStops at the shortest iterable. Use itertools.zip_longest to pad. concept-reference pyconcept/map-filter python-concepts easy python, builtins map() and filter() nums = [1, 2, 3, 4, 5]\n\nlist(map(str, nums)) # ['1','2','3','4','5']\nlist(filter(lambda x: x>3, nums)) # [4, 5]\n\nBoth return lazy iterators. Usually prefer comprehensions:\n[str(n) for n in nums]\n[n for n in nums if n > 3] concept-reference pyconcept/any-all python-concepts easy python, builtins any() and all() nums = [2, 4, 6, 7, 8]\nany(x > 5 for x in nums) # True (at least one)\nall(x > 0 for x in nums) # True (every one)\nall(x % 2 == 0 for x in nums) # False (7 is odd)\n\nShort-circuit: any() stops at first True, all() stops at first False. concept-reference pyconcept/sorted python-concepts easy python, builtins sorted() and .sort() nums = [3, 1, 2]\nsorted(nums) # [1, 2, 3] — returns new list\nnums.sort() # in-place, returns None\n\nwords = ["banana", "apple", "cherry"]\nsorted(words, key=len) # ['apple', 'banana', 'cherry']\nsorted(words, key=len, reverse=True) # ['cherry', 'banana', 'apple']\n\nsorted() works on any iterable. .sort() is list-only. concept-reference pyconcept/isinstance python-concepts easy python, builtins, types isinstance() and type() x = 42\nisinstance(x, int) # True\nisinstance(x, (int, float)) # True (either type)\ntype(x) # \ntype(x) is int # True\n\nPrefer isinstance() — it handles subclasses. type() is for exact type checks. concept-reference pyconcept/ternary python-concepts easy python, syntax Conditional expression (ternary) age = 20\nstatus = "adult" if age >= 18 else "minor"\n\n# In a list\nlabels = ["even" if x % 2 == 0 else "odd" for x in range(5)]\n# ['even', 'odd', 'even', 'odd', 'even'] concept-reference pyconcept/walrus python-concepts medium python, syntax, operator Walrus operator (:=) # Assign and test in one expression\nif (n := len("hello")) > 3:\n print(f"long: {n} chars") # long: 5 chars\n\n# In a while loop\nwhile (line := input("> ")) != "quit":\n print(f"you said: {line}")\n\n# In a comprehension\nresults = [y for x in data if (y := expensive(x)) > threshold]\n\nPython 3.8+. Use sparingly — readability first. concept-reference pyconcept/match-case python-concepts medium python, control-flow, keyword match / case (structural pattern matching) def handle(command):\n match command.split():\n case ["quit"]:\n return "bye"\n case ["go", direction]:\n return f"going {direction}"\n case ["pick", "up", item]:\n return f"picked up {item}"\n case _:\n return "unknown"\n\nhandle("go north") # "going north"\nhandle("pick up key") # "picked up key"\n\nPython 3.10+. Matches structure, not just values. concept-reference pyconcept/decorator python-concepts medium python, functions Decorator def log_calls(fn):\n def wrapper(*args, **kwargs):\n print(f"calling {fn.__name__}")\n return fn(*args, **kwargs)\n return wrapper\n\n@log_calls\ndef add(a, b):\n return a + b\n\nadd(1, 2) # prints "calling add", returns 3\n\n@decorator is syntactic sugar for: add = log_calls(add) concept-reference pyconcept/closure python-concepts medium python, functions Closure def make_counter(start=0):\n count = start\n def increment():\n nonlocal count\n count += 1\n return count\n return increment\n\nc = make_counter()\nc() # 1\nc() # 2\nc() # 3\n\nThe inner function captures variables from the enclosing scope. They persist after the outer function returns. concept-reference pyconcept/args-kwargs python-concepts medium python, functions *args and **kwargs def f(*args, **kwargs):\n print(args) # tuple of positional args\n print(kwargs) # dict of keyword args\n\nf(1, 2, x=3, y=4)\n# (1, 2)\n# {'x': 3, 'y': 4}\n\n# Keyword-only args (after *):\ndef g(a, b, *, verbose=False):\n pass # verbose must be passed by name concept-reference pyconcept/type-hints python-concepts medium python, types Type hints def greet(name: str, times: int = 1) -> str:\n return (f"hello {name}\\n") * times\n\nfrom typing import Optional\ndef find(lst: list[int], target: int) -> Optional[int]:\n return target if target in lst else None\n\nHints are not enforced at runtime. Use mypy for static checking. Python 3.9+ allows list[int] instead of List[int]. concept-reference pyconcept/dataclass python-concepts medium python, oop dataclass from dataclasses import dataclass\n\n@dataclass\nclass Point:\n x: float\n y: float\n\np = Point(1.0, 2.0)\nprint(p) # Point(x=1.0, y=2.0)\np == Point(1.0, 2.0) # True (auto __eq__)\n\nAuto-generates __init__, __repr__, __eq__. Use frozen=True for immutable. concept-reference pyconcept/namedtuple python-concepts medium python, data-structures NamedTuple from typing import NamedTuple\n\nclass Point(NamedTuple):\n x: float\n y: float\n\np = Point(1.0, 2.0)\np.x # 1.0 (attribute access)\nx, y = p # unpacking\np[0] # 1.0 (index access)\n\nImmutable, lightweight, hashable. Great for simple records. concept-reference pyconcept/genexpr python-concepts medium python, functions, comprehension Generator expression # Parentheses instead of brackets → lazy evaluation\ntotal = sum(x**2 for x in range(1000000)) # no list in memory\n\nlines = (line.strip() for line in open("big.log"))\nfor line in lines:\n if "ERROR" in line:\n print(line) concept-reference pyconcept/context-mgr-custom python-concepts medium python, context-manager Custom context manager from contextlib import contextmanager\n\n@contextmanager\ndef timer():\n import time\n start = time.time()\n yield # code inside 'with' runs here\n print(f"elapsed: {time.time() - start:.2f}s")\n\nwith timer():\n sum(range(10**7))\n# elapsed: 0.23s concept-reference pyconcept/property python-concepts medium python, oop @property class Circle:\n def __init__(self, radius):\n self._radius = radius\n\n @property\n def area(self):\n return 3.14159 * self._radius ** 2\n\nc = Circle(5)\nprint(c.area) # 78.53975 — no parentheses\n\nMakes a method look like an attribute. Add @area.setter for write access. concept-reference pyconcept/staticmethod-classmethod python-concepts medium python, oop @staticmethod / @classmethod class Date:\n def __init__(self, y, m, d):\n self.y, self.m, self.d = y, m, d\n\n @classmethod\n def from_str(cls, s):\n return cls(*map(int, s.split("-")))\n\n @staticmethod\n def is_valid(s):\n return len(s.split("-")) == 3\n\nDate.from_str("2026-04-11") # creates Date\nDate.is_valid("2026-04-11") # True concept-reference pyconcept/dunder-init-repr python-concepts easy python, oop __init__ and __repr__ class Point:\n def __init__(self, x, y):\n self.x = x\n self.y = y\n def __repr__(self):\n return f"Point({self.x}, {self.y})"\n\np = Point(1, 2)\nprint(p) # Point(1, 2)\n\n__init__ sets up the instance. __repr__ controls how it prints. concept-reference pyconcept/dunder-len-getitem python-concepts medium python, oop __len__ and __getitem__ class Deck:\n def __init__(self):\n self.cards = list(range(52))\n def __len__(self):\n return len(self.cards)\n def __getitem__(self, i):\n return self.cards[i]\n\nd = Deck()\nlen(d) # 52\nd[0] # 0\nd[-1] # 51\nd[2:5] # [2, 3, 4] — slicing works too concept-reference pyconcept/dunder-enter-exit python-concepts medium python, oop, context-manager __enter__ and __exit__ class TempDir:\n def __enter__(self):\n import tempfile\n self.path = tempfile.mkdtemp()\n return self.path\n def __exit__(self, exc_type, exc_val, exc_tb):\n import shutil\n shutil.rmtree(self.path)\n return False # don't suppress exceptions\n\nwith TempDir() as d:\n print(d) # /tmp/xyz — auto-cleaned up after concept-reference pyconcept/dunder-add-eq python-concepts medium python, oop __add__, __eq__, __lt__ (operator overloading) class Vec:\n def __init__(self, x, y):\n self.x, self.y = x, y\n def __add__(self, other):\n return Vec(self.x + other.x, self.y + other.y)\n def __eq__(self, other):\n return self.x == other.x and self.y == other.y\n\nVec(1,2) + Vec(3,4) # Vec(4, 6)\nVec(1,2) == Vec(1,2) # True concept-reference pyconcept/inheritance python-concepts easy python, oop Inheritance class Animal:\n def speak(self):\n return "..."\n\nclass Dog(Animal):\n def speak(self):\n return "woof"\n\nclass Cat(Animal):\n def speak(self):\n return "meow"\n\nfor a in [Dog(), Cat()]:\n print(a.speak()) # woof, meow\n\nSubclasses override parent methods. Use super() to call the parent. concept-reference pyconcept/super python-concepts medium python, oop super() class Base:\n def __init__(self, x):\n self.x = x\n\nclass Child(Base):\n def __init__(self, x, y):\n super().__init__(x)\n self.y = y\n\nc = Child(1, 2)\nprint(c.x, c.y) # 1 2\n\nDelegates to the next class in the MRO. Always use super() in __init__ for cooperative inheritance. concept-reference pyconcept/async-await python-concepts medium python, async, keyword async / await import asyncio\n\nasync def fetch(url):\n await asyncio.sleep(1) # simulate I/O\n return f"data from {url}"\n\nasync def main():\n results = await asyncio.gather(\n fetch("a.com"),\n fetch("b.com"),\n )\n print(results) # both finish in ~1s, not 2s\n\nasyncio.run(main())\n\nFor I/O-bound concurrency. Not parallelism — use multiprocessing for CPU-bound work. concept-reference pyconcept/exception-hierarchy python-concepts medium python, error-handling Exception hierarchy # Common built-in exceptions:\n# BaseException\n# ├── KeyboardInterrupt\n# ├── SystemExit\n# └── Exception\n# ├── ValueError\n# ├── TypeError\n# ├── KeyError\n# ├── IndexError\n# ├── FileNotFoundError\n# ├── PermissionError\n# ├── AttributeError\n# └── RuntimeError\n\n# Custom exceptions:\nclass AppError(Exception):\n pass\n\nclass NotFoundError(AppError):\n pass concept-reference pyconcept/custom-exception python-concepts medium python, error-handling Custom exceptions class ValidationError(Exception):\n def __init__(self, field, message):\n self.field = field\n super().__init__(f"{field}: {message}")\n\ntry:\n raise ValidationError("email", "invalid format")\nexcept ValidationError as e:\n print(e) # email: invalid format\n print(e.field) # email concept-reference pyconcept/comprehension-nested python-concepts medium python, comprehension Nested comprehension matrix = [[1,2,3], [4,5,6], [7,8,9]]\n\n# Flatten\nflat = [x for row in matrix for x in row]\n# [1, 2, 3, 4, 5, 6, 7, 8, 9]\n\n# Transpose\ntranspose = [[row[i] for row in matrix] for i in range(3)]\n# [[1,4,7], [2,5,8], [3,6,9]] concept-reference pyconcept/defaultdict python-concepts easy python, data-structures, collections collections.defaultdict from collections import defaultdict\n\ncounts = defaultdict(int)\nfor word in "the cat sat on the mat".split():\n counts[word] += 1\n# {'the': 2, 'cat': 1, 'sat': 1, 'on': 1, 'mat': 1}\n\ngroups = defaultdict(list)\nfor name, dept in [("alice","eng"), ("bob","eng"), ("carol","hr")]:\n groups[dept].append(name)\n# {'eng': ['alice','bob'], 'hr': ['carol']} concept-reference pyconcept/counter python-concepts easy python, data-structures, collections collections.Counter from collections import Counter\n\nc = Counter("mississippi")\n# Counter({'s': 4, 'i': 4, 'p': 2, 'm': 1})\nc.most_common(2) # [('s', 4), ('i', 4)]\n\nwords = "the cat sat on the mat".split()\nCounter(words) # Counter({'the': 2, 'cat': 1, ...}) concept-reference pyconcept/deque python-concepts medium python, data-structures, collections collections.deque from collections import deque\n\nq = deque([1, 2, 3])\nq.append(4) # right: [1, 2, 3, 4]\nq.appendleft(0) # left: [0, 1, 2, 3, 4]\nq.pop() # 4\nq.popleft() # 0\n\n# Fixed-size buffer (keeps last N)\nbuf = deque(maxlen=3)\nfor i in range(5):\n buf.append(i)\nprint(buf) # deque([2, 3, 4]) concept-reference pyconcept/itertools python-concepts medium python, stdlib, itertools itertools essentials import itertools\n\n# chain — flatten iterables\nlist(itertools.chain([1,2], [3,4])) # [1, 2, 3, 4]\n\n# islice — lazy head/tail\nlist(itertools.islice(range(100), 5)) # [0, 1, 2, 3, 4]\n\n# groupby (must be sorted first)\nfrom itertools import groupby\ndata = sorted(["aa", "ab", "ba", "bb"], key=lambda x: x[0])\nfor k, g in groupby(data, key=lambda x: x[0]):\n print(k, list(g)) # a ['aa','ab'], b ['ba','bb']\n\n# product — nested loops\nlist(itertools.product("AB", "12")) # [('A','1'),('A','2'),('B','1'),('B','2')] concept-reference pyconcept/functools python-concepts medium python, stdlib, functools functools essentials from functools import lru_cache, partial, reduce\n\n# lru_cache — memoization\n@lru_cache(maxsize=128)\ndef fib(n):\n return n if n < 2 else fib(n-1) + fib(n-2)\nfib(100) # instant\n\n# partial — fix some arguments\nfrom functools import partial\nint_from_hex = partial(int, base=16)\nint_from_hex("ff") # 255\n\n# reduce — fold a sequence\nreduce(lambda a, b: a * b, [1,2,3,4]) # 24 concept-reference pyconcept/pathlib python-concepts easy python, stdlib, filesystem pathlib basics from pathlib import Path\n\np = Path("/var/log/syslog")\np.name # "syslog"\np.parent # Path("/var/log")\np.suffix # "" (no extension)\np.exists() # True/False\np.read_text() # file contents as string\n\n# Build paths with /\nconf = Path.home() / ".config" / "myapp"\nconf.mkdir(parents=True, exist_ok=True)\n\n# Glob\nfor f in Path("/var/log").glob("*.log"):\n print(f) concept-reference pyconcept/subprocess-basic python-concepts easy python, stdlib, subprocess subprocess basics import subprocess\n\n# Run a command, capture output\nresult = subprocess.run(\n ["ls", "-la"],\n capture_output=True, text=True, check=True,\n)\nprint(result.stdout)\n\n# NEVER use shell=True with user input\n# Always pass args as a list concept-reference pyconcept/re-basics python-concepts medium python, stdlib, regex re (regular expressions) import re\n\n# Search\nm = re.search(r"(\\d+)", "error on line 42")\nm.group(1) # "42"\n\n# Find all\nre.findall(r"\\b\\w+@\\w+\\.\\w+\\b", text)\n\n# Sub (replace)\nre.sub(r"\\s+", " ", "too many spaces") # "too many spaces"\n\n# Split\nre.split(r"[;,]\\s*", "a, b; c") # ['a', 'b', 'c']\n\n# Compile for reuse\npattern = re.compile(r"ERROR: (.+)") concept-reference pyconcept/json-yaml python-concepts easy python, stdlib, serialization JSON and YAML import json\n\n# JSON\ndata = {"name": "alice", "age": 30}\njson.dumps(data) # string\njson.dumps(data, indent=2) # pretty\njson.loads('{"x": 1}') # dict\n\nwith open("f.json") as f:\n data = json.load(f) # from file\n\n# YAML (pip install pyyaml)\nimport yaml\nyaml.safe_load(open("f.yaml"))\nyaml.dump(data, default_flow_style=False) concept-reference pyconcept/datetime python-concepts easy python, stdlib datetime from datetime import datetime, timedelta\n\nnow = datetime.now()\nprint(now.strftime("%Y-%m-%d %H:%M:%S"))\n\ndt = datetime.strptime("2026-04-11", "%Y-%m-%d")\n\ntomorrow = now + timedelta(days=1)\ndelta = datetime(2026, 12, 31) - now\nprint(f"{delta.days} days left") concept-reference pyconcept/logging python-concepts medium python, stdlib logging import logging\n\nlogging.basicConfig(\n level=logging.INFO,\n format="%(asctime)s %(levelname)s %(message)s",\n)\nlog = logging.getLogger(__name__)\n\nlog.debug("details") # hidden (below INFO)\nlog.info("started") # shown\nlog.warning("slow") # shown\nlog.error("failed") # shown\n\nUse logging, not print(), in production code. concept-reference pyconcept/argparse python-concepts medium python, stdlib, cli argparse import argparse\n\np = argparse.ArgumentParser(description="greet user")\np.add_argument("name")\np.add_argument("-n", "--count", type=int, default=1)\np.add_argument("-v", "--verbose", action="store_true")\nargs = p.parse_args()\n\nfor _ in range(args.count):\n print(f"hello {args.name}")\n\n# Usage: python greet.py alice -n 3 -v concept-reference pyconcept/threading python-concepts medium python, concurrency threading from concurrent.futures import ThreadPoolExecutor\nimport urllib.request\n\ndef fetch(url):\n return urllib.request.urlopen(url).read()[:100]\n\nurls = ["https://example.com"] * 5\nwith ThreadPoolExecutor(max_workers=5) as pool:\n results = list(pool.map(fetch, urls))\n\nGood for I/O-bound tasks. GIL prevents true CPU parallelism — use ProcessPoolExecutor for that. concept-reference pyconcept/multiprocessing python-concepts medium python, concurrency multiprocessing from concurrent.futures import ProcessPoolExecutor\n\ndef crunch(n):\n return sum(i*i for i in range(n))\n\nwith ProcessPoolExecutor(max_workers=4) as pool:\n results = list(pool.map(crunch, [10**6]*4))\n\nBypasses the GIL — true parallelism for CPU-bound work. Each worker is a separate process. concept-reference pyconcept/slots python-concepts hard python, oop, performance __slots__ class Point:\n __slots__ = ("x", "y")\n def __init__(self, x, y):\n self.x = x\n self.y = y\n\np = Point(1, 2)\n# p.z = 3 → AttributeError (no dynamic attrs)\n\n~40% less memory per instance. Use when creating millions of objects. concept-reference pyconcept/abc python-concepts medium python, oop Abstract base class (ABC) from abc import ABC, abstractmethod\n\nclass Shape(ABC):\n @abstractmethod\n def area(self) -> float:\n ...\n\nclass Circle(Shape):\n def __init__(self, r):\n self.r = r\n def area(self):\n return 3.14 * self.r ** 2\n\n# Shape() → TypeError (can't instantiate)\nCircle(5).area() # 78.5 concept-reference pyconcept/protocol python-concepts hard python, types Protocol (structural typing) from typing import Protocol\n\nclass Closable(Protocol):\n def close(self) -> None: ...\n\ndef shutdown(obj: Closable) -> None:\n obj.close()\n\n# Any object with a .close() method works\n# No inheritance needed — duck typing + type checking\nshutdown(open("/dev/null")) # file has .close() concept-reference pyconcept/string-bytes python-concepts medium python, types str vs bytes s = "hello" # str (unicode text)\nb = b"hello" # bytes (raw bytes)\n\n# Encode: str → bytes\nb = s.encode("utf-8") # b'hello'\n\n# Decode: bytes → str\ns = b.decode("utf-8") # 'hello'\n\n# You WILL hit this at network/file boundaries\n# Always decode bytes to str as early as possible concept-reference pyconcept/os-environ python-concepts easy python, stdlib, os os.environ import os\n\nhome = os.environ["HOME"] # raises KeyError if missing\ndb = os.environ.get("DB_HOST", "localhost") # default if missing\nos.environ["MY_VAR"] = "value" # set for this process\n\n# List all env vars starting with AWS_\naws = {k: v for k, v in os.environ.items() if k.startswith("AWS_")} concept-reference pyconcept/shutil python-concepts easy python, stdlib, filesystem shutil import shutil\n\nshutil.copy2("a.txt", "b.txt") # cp -p (preserves metadata)\nshutil.copytree("src/", "dst/") # cp -r\nshutil.move("old.txt", "new.txt") # mv\nshutil.rmtree("/tmp/build") # rm -rf\nshutil.which("python3") # /usr/bin/python3\n\nusage = shutil.disk_usage("/")\nprint(f"{usage.free // 1e9:.0f} GB free") concept-reference pyconcept/tempfile python-concepts easy python, stdlib, filesystem tempfile import tempfile\n\n# Temp file (auto-deleted)\nwith tempfile.NamedTemporaryFile(mode="w", suffix=".txt") as f:\n f.write("data")\n print(f.name) # /tmp/tmpXXXX.txt\n\n# Temp directory (auto-deleted)\nwith tempfile.TemporaryDirectory() as d:\n print(d) # /tmp/tmpXXXXXX concept-reference pyconcept/secrets python-concepts easy python, stdlib, security secrets (secure random) import secrets\n\n# Secure token\ntok = secrets.token_hex(16) # "a3f2...d1b7" (32 hex chars)\ntok = secrets.token_urlsafe(16) # URL-safe base64\n\n# Secure random integer\nn = secrets.randbelow(100) # 0-99\n\n# Secure password\nimport string\nalpha = string.ascii_letters + string.digits\npwd = "".join(secrets.choice(alpha) for _ in range(16))\n\nUse secrets, not random, for anything security-related. concept-reference pyconcept/hashlib python-concepts easy python, stdlib, security hashlib import hashlib\n\nh = hashlib.sha256(b"hello world").hexdigest()\n# "b94d27b9934d3e08a52e52d7da7dabfa..."\n\n# Hash a file\ndef file_hash(path):\n h = hashlib.sha256()\n with open(path, "rb") as f:\n for chunk in iter(lambda: f.read(8192), b""):\n h.update(chunk)\n return h.hexdigest() concept-reference pyconcept/for-else python-concepts medium python, control-flow for/else and while/else for n in [2, 3, 5, 7]:\n if n == 4:\n print("found 4")\n break\nelse:\n print("4 not found") # ← runs because no break\n\nThe else block runs only if the loop completes without break. Useful for search patterns. concept-reference pyconcept/assignment-expr python-concepts easy python, syntax Augmented assignment x = 10\nx += 5 # 15\nx -= 3 # 12\nx *= 2 # 24\nx //= 5 # 4\nx **= 3 # 64\nx %= 10 # 4\n\nAlso works with strings: s = "hi"; s += "!" → "hi!" concept-reference pyconcept/truthiness python-concepts easy python, types Truthy and falsy values # Falsy values:\nbool(0) # False\nbool(0.0) # False\nbool("") # False\nbool([]) # False\nbool({}) # False\nbool(None) # False\nbool(False) # False\n\n# Everything else is truthy:\nbool(1) # True\nbool("hi") # True\nbool([0]) # True (non-empty list)\n\n# So you can write:\nif my_list: # instead of if len(my_list) > 0 concept-reference pyconcept/scope-legb python-concepts medium python, scope LEGB scope rule x = "global"\n\ndef outer():\n x = "enclosing"\n def inner():\n x = "local"\n print(x) # local\n inner()\n print(x) # enclosing\n\nouter()\nprint(x) # global\n\n# Python searches: Local → Enclosing → Global → Built-in\n# import builtins to see the B layer concept-reference pyconcept/string-multiline python-concepts easy python, strings Multi-line strings and docstrings # Triple quotes for multi-line\nquery = """\nSELECT *\nFROM users\nWHERE active = true\n"""\n\n# Docstrings\ndef add(a, b):\n """Return the sum of a and b."""\n return a + b\n\nprint(add.__doc__) # "Return the sum of a and b." concept-reference pyconcept/frozen-set python-concepts easy python, data-structures frozenset a = frozenset([1, 2, 3])\n# a.add(4) → AttributeError (immutable)\n\n# Can use as dict key or set element\ncache = {frozenset([1,2]): "result"}\n\n# Same set operations work\na | frozenset([3, 4]) # frozenset({1, 2, 3, 4}) concept-reference pyconcept/zip-dict python-concepts easy python, builtins zip tricks # Transpose a matrix\nmatrix = [(1,2,3), (4,5,6)]\nlist(zip(*matrix)) # [(1,4), (2,5), (3,6)]\n\n# Unzip\npairs = [("a",1), ("b",2), ("c",3)]\nkeys, vals = zip(*pairs)\n# keys = ('a','b','c'), vals = (1,2,3) concept-reference pyconcept/classvar-vs-instancevar python-concepts medium python, oop Class variable vs instance variable class Dog:\n species = "canine" # class variable (shared)\n\n def __init__(self, name):\n self.name = name # instance variable (per-object)\n\na = Dog("rex")\nb = Dog("fido")\nDog.species # "canine"\na.name # "rex"\n\n# Gotcha: mutable class vars are shared!\nclass Bad:\n items = [] # all instances share this list concept-reference --- ## Wiki Navigation ### Related Content - Perl Flashcards *(CLI)* (flashcard_deck, L1) — Python Automation - Python Algorithms Flashcards *(CLI)* (flashcard_deck, L2) — Python Automation - [Python Async & Concurrency](../../../../library/topics/python-async-concurrency/index.md) (Topic Pack, L2) — Python Automation - [Python Debugging](../../../../library/topics/python-debugging/index.md) (Topic Pack, L1) — Python Automation - [Python Drills](../../../../library/drills/python_drills.md) (Drill, L0) — Python Automation - Python Exercises (Quest Ladder) *(CLI)* (Exercise Set, L0) — Python Automation - Python Flashcards *(CLI)* (flashcard_deck, L1) — Python Automation - Python OOP Flashcards *(CLI)* (flashcard_deck, L2) — Python Automation - [Python Packaging](../../../../library/topics/python-packaging/index.md) (Topic Pack, L2) — Python Automation - [Python for Infrastructure](../../../../library/topics/python-infra/index.md) (Topic Pack, L1) — Python Automation