Mental Model — Thinking in Python

The biggest shift: the compiler is gone

In TypeScript, the compiler (tsc) is your first line of defence. It catches type mismatches before any code runs. When you move to Python, that safety net disappears at runtime — the interpreter does not check types. Optional type hints exist (and you should use them), but enforcement requires a separate tool like mypy or pyright.

Mental model: TypeScript says “prove it is safe before running.” Python says “run it, and if you pass the wrong type, you will get a runtime error.”

Dynamic typing

Python variables are labels that can point to any object. The type of the value lives on the object itself, not on the variable name. This is fundamentally different from TypeScript where the variable has a type at compile time.

TypeScript

// TypeScript: variable has a compile-time type
let count: number = 0;
count = "five"; // Error: Type 'string' is not assignable to type 'number'

function double(n: number): number {
  return n * 2;
}

Python

# Python: variable is just a label
count = 0
count = "five"   # perfectly legal at runtime

def double(n: int) -> int:   # hints are documentation only
    return n * 2

print(double(3))    # 6
print(double("x"))  # TypeError at RUNTIME, not before

The interpreter and the REPL

TypeScript compiles to JavaScript before running. Python is interpreted — you run source files directly with python3. There is no build step for development.

TypeScript

# TypeScript workflow
# tsc index.ts          → compiles to index.js
# node index.js         → runs the compiled output
# ts-node index.ts      → shortcut for development

Python

# Python workflow
# python3 script.py     → runs directly, no compile step
# python3               → opens the interactive REPL
# python3 -c "print(1)" → run a one-liner inline

Significant indentation — no curly braces

This is the change TypeScript developers notice first. Python uses indentation (4 spaces by convention) to delimit blocks. There are no { and }. A colon (:) always opens a new block.

TypeScript

// TypeScript
function classify(n: number): string {
  if (n > 0) {
    return "positive";
  } else if (n < 0) {
    return "negative";
  } else {
    return "zero";
  }
}

Python

# Python — indentation IS the block
def classify(n: int) -> str:
    if n > 0:
        return "positive"
    elif n < 0:         # note: elif, not else if
        return "negative"
    else:
        return "zero"

print(classify(5))   # positive
print(classify(-3))  # negative
print(classify(0))   # zero

A misindented line is a syntax error. Most Python editors enforce 4-space indentation automatically.

One obvious way — “Pythonic” code

TypeScript is flexible: you can use classes, functional composition, prototype chains, or plain objects interchangeably. Python encourages a single idiomatic style. The community has a name for it: Pythonic. Pythonic code is concise, readable, and uses language features as intended.

TypeScript

// TypeScript: multiple ways to filter
const evens1 = nums.filter(n => n % 2 === 0);
const evens2 = nums.reduce((acc, n) => n % 2 === 0 ? [...acc, n] : acc, []);
// Both work; TS community accepts both

Python

# Python: list comprehension is the Pythonic way
nums = [1, 2, 3, 4, 5, 6]
evens = [n for n in nums if n % 2 == 0]
print(evens)   # [2, 4, 6]

# Works but less Pythonic:
# evens = list(filter(lambda n: n % 2 == 0, nums))

Explicit self in methods

TypeScript class methods implicitly receive this. Python class methods require an explicit self parameter as the first argument — the interpreter fills it in when you call the method, but you must declare it.

TypeScript

// TypeScript: implicit this
class Counter {
  private count: number = 0;

  increment(): void {
    this.count++;          // 'this' is implicit
  }

  value(): number {
    return this.count;
  }
}

Python

# Python: explicit self
class Counter:
    def __init__(self):     # called on Counter()
        self.count = 0      # instance attribute

    def increment(self):    # self is the instance
        self.count += 1

    def value(self) -> int:
        return self.count

c = Counter()
c.increment()
c.increment()
print(c.value())   # 2

🟦 __init__ is not a constructor — it is an initialiser

TypeScript’s constructor creates the object and initialises it in one step. In Python:

1. __new__ creates the object (you rarely override this).
2. __init__ initialises the newly created object — this is what you override.

When you write Counter(), Python calls Counter.__new__(Counter) to allocate memory, then passes the result to Counter.__init__(self) so you can set attributes. The self parameter in __init__ is that freshly allocated instance.

In practice, you only ever write __init__. Think of it as “the constructor body” without the object-creation step.

Run ▸ Open in an online REPL

# Mental model: types live on objects, not variables
x = 42
print(type(x).__name__)   # int

x = "hello"
print(type(x).__name__)   # str

# Significant indentation
def classify(n: int) -> str:
    if n > 0:
        return "positive"
    elif n < 0:
        return "negative"
    else:
        return "zero"

for num in [5, -3, 0]:
    print(f"{num} is {classify(num)}")

# Pythonic list comprehension
evens = [n for n in range(1, 11) if n % 2 == 0]
print("Evens:", evens)

# Explicit self
class Counter:
    def __init__(self):
        self.count = 0

    def increment(self):
        self.count += 1

    def value(self) -> int:
        return self.count

c = Counter()
c.increment()
c.increment()
print("Counter:", c.value())

In Python, where is the type of a value stored?

In the variable declaration In a separate type table maintained by the interpreter On the object itself, not on the variable In the function signature where the variable is created

What opens a new indented block in Python?

An opening curly brace `{` The `begin` keyword A colon `:` at the end of the statement The `do` keyword

Why must Python class methods declare `self` as their first parameter?

It is a syntax requirement — Python does not pass the instance implicitly It is optional but recommended by convention It is needed only in `__init__`, not in other methods `self` is a keyword that initialises instance memory

What is the Pythonic way to filter even numbers from a list?

`list(filter(lambda n: n % 2 == 0, nums))` `[n for n in nums if n % 2 == 0]` `nums.filter(n => n % 2 === 0)` `nums.where(lambda n: n % 2 == 0)`

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