Python essentials: 50 core concepts

All 50 cards

1. 1
   Squares of 0–4 with a list comprehension
   python

   [x * x for x in range(5)]  # [0, 1, 4, 9, 16]

2. 2
   Dict comprehension
   python

   {word: len(word) for word in words}

3. 3
   Generator expression
   Like a list comprehension in parentheses, but lazy: values are produced on demand

   python

   total = sum(x * x for x in nums)

4. 4
   Format a float to 2 decimals with an f-string
   python

   f"{price:.2f}"  # "3.14"

5. 5
   What does *args collect?
   Extra positional arguments, as a tuple
6. 6
   What does **kwargs collect?
   Extra keyword arguments, as a dict
7. 7
   Why is def f(items=[]) a bug?
   The default list is created once and shared between calls; use None and create the list inside

   python

   def f(items=None):
       items = [] if items is None else items

8. 8
   Reverse a list or string with slicing
   python

   s[::-1]

9. 9
   What is seq[-1]?
   The last item
10. 10
    List vs tuple
    Lists are mutable; tuples are immutable (and hashable if their items are)
11. 11
    is vs ==
    is checks identity (same object); == checks equality
12. 12
    Falsy values in Python
    None, False, 0, 0.0, '', and empty containers like [], {}, set()
13. 13
    What does a with block guarantee?
    The context manager’s cleanup runs even if an exception is raised

    python

    with open("notes.txt") as f:
        text = f.read()  # file closed afterwards

14. 14
    The context manager protocol
    __enter__ and __exit__ methods (or contextlib.contextmanager)
15. 15
    What is a decorator?
    A function that takes a function and returns a wrapped version, applied with @

    python

    def logged(fn):
        def wrapper(*args, **kwargs):
            print("calling", fn.__name__)
            return fn(*args, **kwargs)
        return wrapper

16. 16
    Why use functools.wraps?
    It copies the wrapped function’s name, docstring and metadata onto the wrapper
17. 17
    What does yield do?
    Turns a function into a generator that produces values one at a time, pausing between them
18. 18
    yield from
    Delegates to another iterable or generator, yielding all its values
19. 19
    Loop with index and value
    python

    for i, item in enumerate(items, start=1):
        print(i, item)

20. 20
    What does zip do with uneven lengths?
    Stops at the shortest iterable

    Since Python 3.10, zip(a, b, strict=True) raises instead.
21. 21
    Read a dict key with a fallback
    python

    d.get("key", default)

22. 22
    collections.defaultdict
    A dict that creates a default value for missing keys

    python

    groups = defaultdict(list)
    groups[key].append(item)

23. 23
    collections.Counter
    Counts hashable items

    python

    Counter("banana").most_common(1)  # [('a', 3)]

24. 24
    sorted(x) vs x.sort()
    sorted returns a new list; .sort() sorts in place and returns None
25. 25
    Sort words by length
    python

    sorted(words, key=len)

26. 26
    What is a lambda?
    An anonymous function limited to a single expression

    python

    square = lambda x: x * x

27. 27
    Extended unpacking
    python

    first, *rest = [1, 2, 3]  # first = 1, rest = [2, 3]

28. 28
    The walrus operator :=
    Assigns a value inside an expression (Python 3.8+)

    python

    while (line := f.readline()):
        process(line)

29. 29
    When does the else of a try run?
    Only if no exception was raised in the try block

    finally always runs.
30. 30
    Raise an exception
    python

    raise ValueError("deck name is empty")

31. 31
    What is __init__?
    The method that initializes a new instance
32. 32
    What is self?
    The instance a method is called on, passed explicitly as the first parameter
33. 33
    @classmethod vs @staticmethod
    A classmethod receives the class (cls); a staticmethod receives neither the class nor the instance
34. 34
    What does @property do?
    Lets a method be read like an attribute

    python

    class Card:
        @property
        def is_new(self):
            return self.reps == 0

35. 35
    What does @dataclass generate?
    __init__, __repr__ and __eq__ from the class’s annotated fields
36. 36
    What is if __name__ == "__main__": for?
    Code under it runs when the file is executed directly, not when it is imported
37. 37
    Create a virtual environment
    bash

    python -m venv .venv
    source .venv/bin/activate

38. 38
    The GIL
    In standard CPython, the global interpreter lock lets only one thread run Python bytecode at a time

    Threads still help for I/O-bound work; use processes for CPU-bound work. Python 3.13 adds an optional free-threaded build.
39. 39
    Is the stop value of range(start, stop) included?
    No, stop is exclusive: range(1, 4) gives 1, 2, 3
40. 40
    7 // 2 and -7 // 2
    3 and -4: floor division rounds toward negative infinity
41. 41
    Shallow vs deep copy of a list
    items[:] or list(items) copy one level; copy.deepcopy copies nested objects too
42. 42
    Join a list of strings
    python

    ", ".join(words)

43. 43
    Split a string on commas
    python

    "a,b,c".split(",")  # ['a', 'b', 'c']

44. 44
    Type hints
    python

    def add(a: int, b: int) -> int:
        return a + b

    Hints are not enforced at runtime; tools like mypy check them.
45. 45
    any and all
    any is true if at least one item is truthy; all is true if every item is (and for an empty iterable)
46. 46
    What do map and filter return in Python 3?
    Lazy iterators, not lists
47. 47
    Are dicts ordered?
    Yes. Since Python 3.7, dicts keep insertion order as a language guarantee
48. 48
    Set operations
    python

    a | b   # union
    a & b   # intersection
    a - b   # difference

49. 49
    Check if a key is in a dict
    python

    if "key" in d:
        ...

50. 50
    Unpack a dict into keyword arguments
    python

    connect(**config)