Python
on Mac

Essential Terminal Commands

The simplest program

Python's print() function outputs text to the terminal. That's it — no curly braces, no semicolons, no main() required for small scripts.

Variables

No var, let, or int keyword. Just write name = value. Python infers the type dynamically.

f-strings (Python 3.6+)

Prefix a string with f and embed expressions directly: f"Hello, {name}!". The {...} is evaluated at runtime. This is the preferred way to format strings in modern Python.

Running on your Mac

Save the file as hello.py, then in Terminal:

The interactive REPL

Type python in Terminal to open the REPL (Read-Eval-Print Loop). Try expressions line by line. Exit with ⌃D or type exit().

Indentation is syntax

Python uses 4 spaces per level. Tabs vs spaces is a hard error if mixed. VS Code handles this automatically — check the bottom-right corner shows "Spaces: 4".

Comments

# single line — everything after # is ignored. There are no multi-line block comments; use consecutive # lines. Triple-quoted strings """...""" are used as docstrings (function/class documentation), not general comments.

Operators — differences from other languages

• //Floor (integer) division: 7 // 2 → 3. Plain / always returns a float.
• **Exponentiation: 2 ** 8 → 256. Not ^ (which is XOR in Python).
• and/orLogical operators are words, not && and ||.
• notLogical negation — a word, not !.
• isIdentity check (same object). Use only for None, True, False.
• inMembership: "a" in ["a","b"] → True.

String literals

• '...'Single quotes — identical to double quotes.
• "..."Double quotes — use whichever avoids escaping apostrophes.
• """..."""Triple quotes — spans multiple lines. Used for docstrings.
• r"..."Raw string — backslashes are literal. Use for file paths and regex.
• f"..."F-string — embeds expressions. f"Pi is {3.14:.2f}"

Numeric types

Type inspection

Use type(x) to check type. Prefer isinstance(x, int) for conditional checks — it handles subclasses.

Type hints (Python 3.5+)

Hints are optional documentation. The interpreter does not enforce them. Tools like mypy and VS Code use them for static analysis and autocomplete.

def greet(name: str) -> str:

age: int = 30

Truthiness — what is falsy?

In Python, every value has a boolean interpretation. These are falsy (everything else is truthy):

• NonePython's null value
• FalseBoolean false
• 0, 0.0Zero numbers of any numeric type
• "", [], {}Empty string, list, dict, set, tuple

for loops — iterate directly

Python's for loop iterates over any iterable — lists, strings, dicts, files, generators. You almost never need a manual index.

• range(n)Generates integers 0 to n-1. range(2, 10, 2) → 2,4,6,8.
• enumeratefor i, val in enumerate(lst): — gives index AND value. Never write range(len(lst)).
• zip()for a, b in zip(lst1, lst2): — iterate two lists together.
• for…elseThe else block runs if the loop completed without hitting a break.

Ternary expression

value_if_true if condition else value_if_false

Example: label = "even" if n % 2 == 0 else "odd"

match / case (Python 3.10+)

Structural pattern matching. More powerful than a switch statement — it can match values, types, sequences, and guard conditions.

Argument types

• positionalStandard args. greet("Alice")
• keywordNamed at call site. greet(name="Alice") — order doesn't matter.
• defaultdef f(x, n=10): — n is optional. Defaults evaluated once at definition time.
• *argsVariadic positional. Collected into a tuple inside the function.
• **kwargsVariadic keyword. Collected into a dict inside the function.

Generators

A function with yield becomes a generator. It produces values lazily — pausing at each yield and resuming when the caller asks for the next item. Memory-efficient for large sequences.

Decorators

@decorator above a function is shorthand for f = decorator(f). Decorators wrap a function to add behavior (logging, timing, caching) without modifying its code.

self is not a keyword

self is just the conventional name for the first parameter of an instance method — Python passes the instance automatically. You could name it anything, but always use self.

Privacy conventions

@dataclass — modern Python

The @dataclass decorator auto-generates __init__, __repr__, and __eq__ from field annotations. Recommended for data-holding classes.

Key dunder methods

Choosing the right collection

* dicts preserve insertion order since Python 3.7

Slicing — works on any sequence

lst[start:stop:step]

• lst[2:5]Items at index 2, 3, 4 (stop is exclusive)
• lst[:3]First 3 items
• lst[-3:]Last 3 items (negative counts from end)
• lst[::-1]Reverse a sequence
• lst[::2]Every second item

Tuple unpacking

x, y = (3, 4) — assigns both at once. Works with any iterable. Use *rest to collect remaining items: first, *rest = [1, 2, 3, 4]

The pattern

[expression for item in iterable if condition]

All three parts map directly to what you'd write in a for loop:

• expressionWhat to do with each item (transform)
• for item inSource iterable to loop over
• if conditionOptional filter — only include items where this is truthy

Four types

try/except/else/finally

• tryThe code that might raise an exception.
• exceptRuns if an exception matches. Can have multiple except clauses.
• elseRuns only if no exception was raised. Good for code that shouldn't be in try.
• finallyAlways runs — exception or not. Use for cleanup (closing files, releasing resources).

Common exceptions

File modes

pathlib on Mac

Path.home() returns /Users/username on Mac. Use / operator to build paths: Path.home() / "Documents" / "data.json"

Always specify encoding

macOS defaults to UTF-8, but always specify it explicitly for portability: open("f.txt", encoding="utf-8"). This prevents issues when files are shared with Windows users.

Virtual environments — always use them

A virtual environment creates an isolated Python installation for each project. Packages installed in one environment don't affect others.

The script guard

When Python runs a file directly: __name__ == "__main__". When it's imported as a module: __name__ == "modulename". Always guard your entry point:

if __name__ == "__main__":
    main()

VS Code on Mac

After activating your venv, open the project folder: code . in Terminal. Press ⌘ShiftP → "Python: Select Interpreter" → choose .venv/bin/python.

Context managers — with statement

Any object with __enter__ and __exit__ methods can be used with with. The __exit__ is always called — even on exceptions — making cleanup reliable.

Walrus operator := (Python 3.8+)

Assigns and returns a value in a single expression. Useful in while loops and if statements to avoid redundant calls:

while chunk := file.read(8192):

async / await

Cooperative multitasking for I/O-bound code (network requests, file reading). async def defines a coroutine. await suspends it until the awaited operation completes. Run with asyncio.run(main()).

functools.lru_cache

Memoizes a function — caches return values by arguments. @lru_cache(maxsize=128) above a recursive or expensive function can give dramatic speedups.