# How to fix ModuleNotFoundError: No module named 'psycopg2'

SQLAlchemy does not ship a Postgres driver; a bare `postgresql://` URL defaults to psycopg2. Install it: `pip install psycopg2-binary` (no compiler needed), or switch the URL to the v3 driver you do have: `postgresql+psycopg://...` with `pip install "psycopg[binary]"`. The driver name in the URL must match the installed package.

## The exact error

```text
ModuleNotFoundError: No module named 'psycopg2'
```

## Fix it

1. Check which driver you have: `python -m pip show psycopg2-binary psycopg`.
   Expected: you see which (if any) is installed

2. Easiest: `python -m pip install psycopg2-binary` and keep `postgresql://`.
   Expected: create_engine connects

3. Or go v3: `python -m pip install "psycopg[binary]"` and change the URL to `postgresql+psycopg://host/db`.
   Expected: connects via psycopg3

## When this applies

`create_engine('postgresql://...')` raises No module named 'psycopg2'

## When this does NOT apply

`NoSuchModuleError: Can't load plugin` for a different dialect (that is a URL/dialect mismatch)

## Versions

SQLAlchemy with postgresql:// URLs

## Why it happens

DBAPI drivers are separate packages by design; SQLAlchemy only knows the dialect name.

## Edge cases

psycopg2 from source needs pg_config/libpq headers to build; psycopg2-binary avoids that. Keep the password in env vars, not code.