Rust for Python Programmers

Variables and Mutability

What you’ll learn: Immutable-by-default variables, explicit mut, primitive numeric types vs Python’s arbitrary-precision int, String vs &str (the hardest early concept), string formatting, and Rust’s required type annotations.

Difficulty: 🟢 Beginner

Python Variable Declaration

# Python — everything is mutable, dynamically typed
count = 0          # Mutable, type inferred as int
count = 5          # ✅ Works
count = "hello"    # ✅ Works — type can change! (dynamic typing)

# "Constants" are just convention:
MAX_SIZE = 1024    # Nothing prevents MAX_SIZE = 999 later

Rust Variable Declaration

#![allow(unused)]
fn main() {
// Rust — immutable by default, statically typed
let count = 0;           // Immutable, type inferred as i32
// count = 5;            // ❌ Compile error: cannot assign twice to immutable variable
// count = "hello";      // ❌ Compile error: expected integer, found &str

let mut count = 0;       // Explicitly mutable
count = 5;               // ✅ Works
// count = "hello";      // ❌ Still can't change type

const MAX_SIZE: usize = 1024; // True constant — enforced by compiler
}

Key Mental Shift for Python Developers

#![allow(unused)]
fn main() {
// Python: variables are labels that point to objects
// Rust: variables are named storage locations that OWN their values

// Variable shadowing — unique to Rust, very useful
let input = "42";              // &str
let input = input.parse::<i32>().unwrap();  // Now it's i32 — new variable, same name
let input = input * 2;         // Now it's 84 — another new variable

// In Python, you'd just reassign and lose the old type:
input = "42"
input = int(input)   # Same name, different type — Python allows this too
But in Rust, each `let` creates a genuinely new binding. The old one is gone.
}

Practical Example: Counter

# Python version
class Counter:
    def __init__(self):
        self.value = 0
    
    def increment(self):
        self.value += 1
    
    def get_value(self):
        return self.value

c = Counter()
c.increment()
print(c.get_value())  # 1

// Rust version
struct Counter {
    value: i64,
}

impl Counter {
    fn new() -> Self {
        Counter { value: 0 }
    }

    fn increment(&mut self) {     // &mut self = I will modify this
        self.value += 1;
    }

    fn get_value(&self) -> i64 {  // &self = I only read this
        self.value
    }
}

fn main() {
    let mut c = Counter::new();   // Must be `mut` to call increment()
    c.increment();
    println!("{}", c.get_value()); // 1
}

Key difference: In Rust, &mut self in the method signature tells you (and the compiler) that increment modifies the counter. In Python, any method can mutate anything — you have to read the code to know.

Primitive Types Comparison

flowchart LR
    subgraph Python ["Python Types"]
        PI["int\n(arbitrary precision)"] 
        PF["float\n(64-bit only)"]
        PB["bool"]
        PS["str\n(Unicode)"]
    end
    subgraph Rust ["Rust Types"]
        RI["i8 / i16 / i32 / i64 / i128\nu8 / u16 / u32 / u64 / u128"]
        RF["f32 / f64"]
        RB["bool"]
        RS["String / &str"]
    end
    PI -->|"fixed-size"| RI
    PF -->|"choose precision"| RF
    PB -->|"same"| RB
    PS -->|"owned vs borrowed"| RS
    style Python fill:#ffeeba
    style Rust fill:#d4edda

Numeric Types

# Python — one integer type, arbitrary precision
x = 42                     # int — can grow to any size
big = 2 ** 1000            # Still works — thousands of digits
y = 3.14                   # float — always 64-bit

#![allow(unused)]
fn main() {
// Rust — explicit sizes, overflow is a compile/runtime error
let x: i32 = 42;           // 32-bit signed integer
let y: f64 = 3.14;         // 64-bit float (Python's float equivalent)
let big: i128 = 2_i128.pow(100); // 128-bit max — no arbitrary precision
// For arbitrary precision: use the `num-bigint` crate

// Underscores for readability (like Python's 1_000_000):
let million = 1_000_000;   // Same syntax as Python!

// Type suffix syntax:
let a = 42u8;              // u8
let b = 3.14f32;           // f32
}

Size Types (Important!)

#![allow(unused)]
fn main() {
// usize and isize — pointer-sized integers, used for indexing
let length: usize = vec![1, 2, 3].len();  // .len() returns usize
let index: usize = 0;                     // Array indices are always usize

// In Python, len() returns int and indices are int — no distinction.
// In Rust, mixing i32 and usize requires explicit conversion:
let i: i32 = 5;
// let item = vec[i];    // ❌ Error: expected usize, found i32
let item = vec[i as usize]; // ✅ Explicit conversion
}

Type Inference

#![allow(unused)]
fn main() {
// Rust infers types but they're FIXED — not dynamic
let x = 42;          // Compiler infers i32 (default integer type)
let y = 3.14;        // Compiler infers f64 (default float type)
let s = "hello";     // Compiler infers &str (string slice)
let v = vec![1, 2];  // Compiler infers Vec<i32>

// You can always be explicit:
let x: i64 = 42;
let y: f32 = 3.14;

// Unlike Python, the type can NEVER change after inference:
let x = 42;
// x = "hello";      // ❌ Error: expected integer, found &str
}

String Types: String vs &str

This is one of the biggest surprises for Python developers. Rust has two main string types where Python has one.

Python String Handling

# Python — one string type, immutable, reference counted
name = "Alice"          # str — immutable, heap allocated
greeting = f"Hello, {name}!"  # f-string formatting
chars = list(name)      # Convert to list of characters
upper = name.upper()    # Returns new string (immutable)

Rust String Types

#![allow(unused)]
fn main() {
// Rust has TWO string types:

// 1. &str (string slice) — borrowed, immutable, like a "view" into string data
let name: &str = "Alice";           // Points to string data in the binary
                                     // Closest to Python's str, but it's a REFERENCE

// 2. String (owned string) — heap-allocated, growable, owned
let mut greeting = String::from("Hello, ");  // Owned, can be modified
greeting.push_str(name);
greeting.push('!');
// greeting is now "Hello, Alice!"
}

When to Use Which?

#![allow(unused)]
fn main() {
// Think of it like this:
// &str  = "I'm looking at a string someone else owns"  (read-only view)
// String = "I own this string and can modify it"        (owned data)

// Function parameters: prefer &str (accepts both types)
fn greet(name: &str) -> String {          // accepts &str AND &String
    format!("Hello, {}!", name)           // format! creates a new String
}

let s1 = "world";                         // &str literal
let s2 = String::from("Rust");            // String

greet(s1);      // ✅ &str works directly
greet(&s2);     // ✅ &String auto-converts to &str (Deref coercion)
}

Practical Examples

# Python string operations
name = "alice"
upper = name.upper()               # "ALICE"
contains = "lic" in name           # True
parts = "a,b,c".split(",")         # ["a", "b", "c"]
joined = "-".join(["a", "b", "c"]) # "a-b-c"
stripped = "  hello  ".strip()     # "hello"
replaced = name.replace("a", "A") # "Alice"

#![allow(unused)]
fn main() {
// Rust equivalents
let name = "alice";
let upper = name.to_uppercase();           // String — new allocation
let contains = name.contains("lic");       // bool
let parts: Vec<&str> = "a,b,c".split(',').collect();  // Vec<&str>
let joined = ["a", "b", "c"].join("-");    // String
let stripped = "  hello  ".trim();         // &str — no allocation!
let replaced = name.replace("a", "A");     // String

// Key insight: some operations return &str (no allocation), others return String.
// .trim() returns a slice of the original — efficient!
// .to_uppercase() must create a new String — allocation required.
}

Python Developers: Think of it This Way

Python str     ≈ Rust &str     (you usually read strings)
Python str     ≈ Rust String   (when you need to own/modify)

Rule of thumb:
- Function parameters → use &str (most flexible)
- Struct fields       → use String (struct owns its data)
- Return values       → use String (caller needs to own it)
- String literals     → automatically &str

Printing and String Formatting

Basic Output

# Python
print("Hello, World!")
print("Name:", name, "Age:", age)    # Space-separated
print(f"Name: {name}, Age: {age}")   # f-string

#![allow(unused)]
fn main() {
// Rust
println!("Hello, World!");
println!("Name: {} Age: {}", name, age);    // Positional {}
println!("Name: {name}, Age: {age}");       // Inline variables (Rust 1.58+, like f-strings!)
}

Format Specifiers

# Python formatting
print(f"{3.14159:.2f}")          # "3.14" — 2 decimal places
print(f"{42:05d}")               # "00042" — zero-padded
print(f"{255:#x}")               # "0xff" — hex
print(f"{42:>10}")               # "        42" — right-aligned
print(f"{'left':<10}|")          # "left      |" — left-aligned

#![allow(unused)]
fn main() {
// Rust formatting (very similar to Python!)
println!("{:.2}", 3.14159);         // "3.14" — 2 decimal places
println!("{:05}", 42);              // "00042" — zero-padded
println!("{:#x}", 255);             // "0xff" — hex
println!("{:>10}", 42);             // "        42" — right-aligned
println!("{:<10}|", "left");        // "left      |" — left-aligned
}

Debug Printing

# Python — repr() and pprint
print(repr([1, 2, 3]))             # "[1, 2, 3]"
from pprint import pprint
pprint({"key": [1, 2, 3]})         # Pretty-printed

#![allow(unused)]
fn main() {
// Rust — {:?} and {:#?}
println!("{:?}", vec![1, 2, 3]);       // "[1, 2, 3]" — Debug format
println!("{:#?}", vec![1, 2, 3]);      // Pretty-printed Debug format

// To make your types printable, derive Debug:
#[derive(Debug)]
struct Point { x: f64, y: f64 }

let p = Point { x: 1.0, y: 2.0 };
println!("{:?}", p);                   // "Point { x: 1.0, y: 2.0 }"
println!("{p:?}");                     // Same, with inline syntax
}

Quick Reference

Type Annotations: Optional vs Required

Python Type Hints (Optional, Not Enforced)

# Python — type hints are documentation, not enforcement
def add(a: int, b: int) -> int:
    return a + b

add(1, 2)         # ✅
add("a", "b")     # ✅ Python doesn't care — returns "ab"
add(1, "2")       # ✅ Until it crashes at runtime: TypeError

# Union types, Optional
def find(key: str) -> int | None:
    ...

# Generic types
def first(items: list[int]) -> int | None:
    return items[0] if items else None

# Type aliases
UserId = int
Mapping = dict[str, list[int]]

Rust Type Declarations (Required, Compiler-Enforced)

#![allow(unused)]
fn main() {
// Rust — types are enforced. Always. No exceptions.
fn add(a: i32, b: i32) -> i32 {
    a + b
}

add(1, 2);         // ✅
// add("a", "b");  // ❌ Compile error: expected i32, found &str

// Optional values use Option<T>
fn find(key: &str) -> Option<i32> {
    // Returns Some(value) or None
    Some(42)
}

// Generic types
fn first(items: &[i32]) -> Option<i32> {
    items.first().copied()
}

// Type aliases
type UserId = i64;
type Mapping = HashMap<String, Vec<i32>>;
}

Key insight: In Python, type hints help your IDE and mypy but don’t affect runtime. In Rust, types ARE the program — the compiler uses them to guarantee memory safety, prevent data races, and eliminate null pointer errors.

📌 See also: Ch. 6 — Enums and Pattern Matching shows how Rust’s type system replaces Python’s Union types and isinstance() checks.

Exercises

fn celsius_to_fahrenheit(c: f64) -> f64 {
    c * 9.0 / 5.0 + 32.0
}

fn classify(temp_f: f64) -> &'static str {
    if temp_f < 50.0 { "cold" }
    else if temp_f < 77.0 { "mild" }
    else { "hot" }
}

fn main() {
    for c in [0.0, 20.0, 35.0] {
        let f = celsius_to_fahrenheit(c);
        println!("{c:.1}°C = {f:.1}°F — {}", classify(f));
    }
}