What are coupling and cohesion?

Question

Accepted Answer

**Coupling** measures how much one module depends on another; **cohesion** measures how strongly the parts *inside* a module belong together. The goal: **low coupling, high cohesion**.

## The two axes

```text
HIGH cohesion  → a class does ONE well-defined job, parts relate strongly  ✅
LOW  coupling  → modules interact through small, stable interfaces         ✅
```

## Low cohesion (bad) vs. high cohesion (good)

```python
# LOW cohesion: unrelated responsibilities jammed together
class Utils:
    def parse_csv(self, s): ...
    def send_email(self, to): ...
    def hash_password(self, p): ...   # nothing in common

# HIGH cohesion: each class has a single, focused purpose
class CsvParser:     ...
class Mailer:        ...
class PasswordHasher: ...
```

## Tight vs. loose coupling

```python
# TIGHT: OrderService is welded to a concrete class
class OrderService:
    def __init__(self):
        self.db = MySqlDatabase()        # can't swap or test in isolation

# LOOSE: depends on an abstraction, injected in
class OrderService:
    def __init__(self, db: Database):    # any Database works
        self.db = db
```

Loose coupling lets you replace `MySqlDatabase` with an in-memory fake for tests, with no edit to `OrderService`.

## Why it matters

Tight coupling is what makes a change "ripple" — touch one class and ten others break; loose coupling contains changes to their source.

High cohesion makes code findable and testable: when a class does one thing, you know where logic lives and what a test should cover.

These two metrics underlie nearly every other design principle.