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A Comprehensive Beginner's Guide to API Development with FastAPI

The FAST API is a modern, fast (hence the name), and asynchronous web framework for building APIs. In this article, I’ll be sharing a guide to developing an API using the FastAPI micro-framework.First, we need to install PostgreSQL for setting up our database..

The Web technology industry is profound as it is continuously evolving; thousands of web application development tools are available to build any application of your choice. Considering the numerous options available, its difficult for anyone to make prudent choices. However, with clear requirements in mind, one can accomplish the tedious task of choosing a framework.

If your application’s requirements are not huge and want to develop a greatly performant application with built-in validation support, great documentation as well as remarkable concurrency, you should definitely choose FAST API.


The FAST API is a modern, fast (hence the name), and asynchronous web framework for building APIs. FastAPI is based on Pydantic and type hints to validate, serialize, and deserialize data. It is designed to be easy to use and to provide high performance, making it a popular choice for building APIs.

A notable characterstic of FastAPI is that it can handle multiple requests concurrently without blocking the execution of other requests through the use of asynchronous programming and can leverage multiple threads or processes for handling requests concurrently. Another asynchronous web framework, Node.js, on the other hand, achieves concurrency through it’s single-threaded event loop, utilizing non-blocking I/O operations and callbacks to handle concurrent requests.

Many existing Python web frameworks are built on top of WSGI (Web Server Gateway Interface) example: Flask, Django, Pyramid, etc, whereas FastAPI uses ASGI (Asynchronous Server Gateway Interface) and its implementation is growing in new frameworks such as Starlette, Quart, etc but not all frameworks, have full ASGI compatibility.

ASGI and WSGI are both specifications that define how web servers and web applications communicate with each other in Python. WSGI is a synchronous protocol, meaning that the server waits for the response from the application before moving on to the next request. On the other hand, ASGI supports both synchronous and asynchronous frameworks, allowing developers to choose the best approach for their application’s requirements.

In this article, I’ll be sharing a guide to developing an API using the FastAPI micro-framework. The starter project is available in my GitHub repository. Although, It is highly recommended to follow this guide for novices while creating your own version.

Setting up development environment

First of all, make sure Python3.6+ is installed on the system. Then, we will create a new folder, inside it a new virtual environment to install all our dependencies in isolation and activate our virtual environment before installing the required packages. For this, we need to run the following command in the root directory of our project folder.

python3 -m venv env
source env/bin/activate

We’ll need to install our main dependency i.e fastapi by running the following command:

pip install fastapi

Once we have installed the FAST API, we can start building our API. We’ll create a new Python file main.py and import the FastAPI class from the fastapi module as shown below.

from fastapi import FastAPI
app = FastAPI()

This creates a new instance of the FastAPI class app instance, which we can use to define our API endpoints. To create a new endpoint, we’ll use the @app.route() decorator and specify the HTTP method and URL path:

async def hello(name: str):
    return {"message": f"Hello, {name}!"}

This creates a new endpoint that responds to GET requests to the /hello/{name} URL path. The endpoint returns a JSON response with a message key that contains a personalized greeting.

Now to run our simple API server, we can use the uvicorn command-line tool. First we will install the uvicorn python package, run:

pip install uvicorn

Then run the following command to start the server:

uvicorn main:app --reload

This starts the server on http://localhost:8000 and automatically reloads the server when you make changes to the code.

That’s it! We’ve now created a simple API endpoint using the FAST API in Python.

Database Setup

First, we need to install PostgreSQL for setting up our database. Refer to the PostgreSQL-installation document for installing PostgreSQL on other systems of choice. We can use the following command on MacOS.

brew install postgresql

Start PostgreSQL server

brew services start postgresql

Check Postgres version

postgres - V;

We need to create a new PostgreSQL user and Database. To create a new PostgreSQL user Role ‘subash’, we can use the following commands.



createuser subash
createuser subash —createdb

Now, we can log in with the new user to the PostgreSQL console and create a database with access to the new user ‘subash’.

psql postgres -U subash
CREATE DATABASE mypostgresdb;


Configure an ORM (Object Relational Mapping)

To dive into Relational database integration with an ORM, we will be using the PostgreSQL database with SQLalchemy ORM. To integrate Postgres with our FastAPI application, we can use an ORM library such as SQLAlchemy.

First, we need to install the sqlalchemy python package:

pip install sqlalchemy

🛠 Apple Silicon M1 chip Users may encounter an error during “psycopg2” package installation which is an adapter for Postgres in MacOS. To resolve the issue, separately install the binary using the following command.

pip uninstall psycopg2
pip install psycopg2-binary

Then, create an SQLAlchemy engine object to connect to the Postgres database:

from sqlalchemy import create_engine

DATABASE_URL = "postgresql://user:password@localhost/dbname"
engine = create_engine(DATABASE_URL)

Replace user, password, and dbname with the PostgreSQL credentials.

Next, we need to create a SQLAlchemy SessionLocal class to manage database sessions:

from sqlalchemy.orm import sessionmaker

SessionLocal = sessionmaker(bind=engine, autocommit=False, autoflush=False)

We can use this class to create a new database session whenever we need to interact with the database:

def get_db():
    db = SessionLocal()
        yield db

Finally, we can use the get_db function to inject a database session into our FastAPI endpoints using the Depends function.

Database Schema

Before starting, CRUD operations we need to define our database schema. We will be creating a simple Application that stores users’ information. The database will have two tables the user table for storing user info and the phone_number table for storing multiple phone numbers of a user. These two tables will have a ONE-TO-MANY relationship respectively. Let’s create both User and Phone ORM.

from typing import List
from typing import Optional
from sqlalchemy import ForeignKey
from sqlalchemy import String
from sqlalchemy.orm import Mapped
from sqlalchemy.orm import mapped_column
from sqlalchemy.orm import relationship
from .base import Base

class User(Base):
    __tablename__ = "user_account"

    id: Mapped[int] = mapped_column(primary_key=True)
    name: Mapped[str] = mapped_column(String(30))
    fullname: Mapped[Optional[str]]

    phone_numbers: Mapped[List["Phone"]] = relationship(
        back_populates="user", cascade="all, delete-orphan"

    def __repr__(self) -> str:
        return f"User(id={self.id!r}, name={self.name!r}, fullname={self.fullname!r})"

class Phone(Base):
    __tablename__ = "phone_number"

    id: Mapped[int] = mapped_column(primary_key=True)
    phone_number: Mapped[str]
    user_id: Mapped[int] = mapped_column(ForeignKey("user_account.id"))

    user: Mapped["User"] = relationship(back_populates="phone_numbers")

    def __repr__(self) -> str:
        return f"Phone(id={self.id!r}, phone_number={self.phone_number!r})"

CRUD Operations

Create User

We will define a Pydantic model UserCreate with three fields: name, fullname, and phone_numbers. The name, fullname and phone_numbers fields are required. This model is used to validate incoming user data in the create_user endpoint.

from fastapi import FastAPI, Depends
from typing import Union
from pydantic import BaseModel
from sqlalchemy.orm import Session
from app.orm.user import User
from app.database.dbengine import get_db

app = FastAPI()

# define a pydantic type definition
class UserCreate(BaseModel):
    name: str
    fullname: str
    phone_numbers: Optional[List[str]] = None

async def create_user(user: UserCreate, db: Session = Depends(get_db)):
    db_user = User(**user.dict())
    return {"id": db_user.id, "name": db_user.name}

Here, the UserCreate class and User model must be defined first.

Read User

We can read a user from the database using a GET request with a unique user ID in the request parameters as shown below. In this example, the read_user endpoint uses the get_db function to inject a database session into the db argument. The User model is defined using SQLAlchemy’s declarative syntax.

async def read_user(user_id: int, db: Session = Depends(get_db)):
        user = db.query(User).filter(User.id == user_id).first()
        return {"id": user.id, "name": user.name, "fullname": user.fullname, "phone_numbers": user.phone_numbers}
    except BaseException as e:
        return {"message": "Not found"}

List Users

To create an endpoint to list all users from the database, you can define a new endpoint using the @app.get() decorator. This endpoint queries all User objects from the database and returns them as a JSON response with a users key containing a list of objects with id and name keys.

async def list_users(db: Session = Depends(get_db)):
    users = db.query(User).all()
    return {"users": [{"id": user.id, "name": user.name} for user in users]}

Update User

To update an existing user document in the database we can create an update endpoint as follows. The following function accepts

async def update_user(user_id: int, user: dict, db: Session = Depends(get_db)):
    # Retrieve the user from the database
    db_user = db.query(User).filter(User.id == user_id).first()
    if not db_user:
        raise HTTPException(
            detail=f"User with ID {user_id} not found.",

   # Update the user's properties
    for key in user.keys():
        if key == 'phone_numbers':
            for phone_number in user.get("phone_numbers"):
                new_phone = Phone(phone_number=phone_number)
        db_user.__setattr__(key, user[key])

    # Commit the changes to the database
    return {"id": db_user.id, "name": db_user.name, "fullname": db_user.fullname, "phone_numbers": db_user.phone_numbers}

With this PUT request we can now update existing entries in DB for an user.

Delete User

Let’s create a delete API request to delete a user entry with associated phone number entries.

async def delete_user(user_id: int, db: Session = Depends(get_db)):
    db_user = db.query(User).filter(User.id == user_id).first()

    if db_user is None:
        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="User not found")


    return {"message": "User deleted successfully"}

That’s it! We’ve now integrated PostgreSQL with our FastAPI application using SQLAlchemy. From here, we can continue to build more complex database-driven APIs to suit our needs.

Setup Database Migration

Database Migration is crucial for large-scale products that require constant updates and adding a proper mechanism to insert data and update data in the database in advance would be beneficial.

Alembic is a migration tool for SQLAlchemy. Alembic complements SQLAlchemy by providing a comprehensive solution for managing database schema changes and version control. It simplifies the process of evolving your database schema.

Here, is a sample database migration script using Alembic to create the necessary tables and populate them with sample data on application startup.

First, install Alembic python package:

pip install alembic

Then, create a new directory called alembic in your project directory. Inside this directory, create a new file called alembic.ini with the following contents:

script_location = alembic

This tells Alembic where to find the migration scripts.

Next, create a new file called env.py in the alembic directory with the following contents:

from sqlalchemy import engine_from_config
from sqlalchemy import pool

from alembic import context
from fastapi import FastAPI

from app.models import Base
from app.database import engine, get_db
from app import models

# This is the Alembic Config object, which provides
# access to the values within the .ini file in use.
config = context.config

# Add your model's MetaData object here for 'autogenerate' support.
# This should be a string name, not the actual object.
# For example:
# target_metadata = None
target_metadata = Base.metadata

# other values from the ini file, defined by the needs of env.py,
# can be acquired:
# my_important_option = config.get_main_option("my_important_option")
# ... etc.

# Interpret the config file for Python logging.
# This line sets up loggers basically.
from logging.config import fileConfig


# add your model's MetaData object here
# for 'autogenerate' support
# target_metadata = mymodel.Base.metadata

def run_migrations_offline():
    """Run migrations in 'offline' mode.

    This configures the context with just a URL
    and not an Engine, though an Engine is acceptable
    here as well.  By skipping the Engine creation
    we don't even need a DBAPI to be available.
    Calls to context.execute() here emit the given string to the
    script output.
    url = config.get_main_option("sqlalchemy.url")

    with context.begin_transaction():

def run_migrations_online():
    """Run migrations in 'online' mode.

    In this scenario we need to create an Engine
    and associate a connection with the context.
    # this callback is used to prevent an error caused by SQLAlchemy 1.4
    # see <https://github.com/sqlalchemy/sqlalchemy/issues/6141>
    def process_revision_directives(context, revision, directives):
        if config.cmd_opts.autogenerate:
            script = directives[0]
            if script.upgrade_ops.is_empty():
                directives[:] = []

        for directive in directives:
            if directive.upgrade_ops.is_empty():

    connectable = engine

    # access to the app object from the context
    context.app = FastAPI()
    context.app.dependency_overrides[get_db] = lambda: SessionLocal()
    with connectable.connect() as connection:

        with context.begin_transaction():

if context.is_offline_mode():

This script imports the necessary modules, including the FastAPI application, SQLAlchemy models, and the get_db function. It also defines two functions, run_migrations_offline() and run_migrations_online(), that will be used to run the migrations.

Writing migration scripts

Next, create a new directory called alembic/versions and create a new migration script in this directory called 1_initial.py with the following contents:

"""create user table

Revision ID: 1
Create Date: 2023-11-20 19:17:01.434529


import sys
import os

SCRIPT_DIR = os.path.dirname(os.path.relpath('../'))

from sqlalchemy.orm import Session
from app.orm import Base
from app.database.dbengine import engine
from app.orm import User, Phone

# revision identifiers, used by Alembic.
revision = '1'
down_revision = None
branch_labels = None
depends_on = None

def upgrade():
    db = Session(bind=engine)

    # sample user with phone number
    user1 = User(id=1, name="Alice", fullname="Alice Sharma")

    phone1 = Phone(id=1, phone_number="9832412736", user_id=1)

    # sample use without phone number
    user2 = User(id=2, name="Bob", fullname="Bob Sharma")


def downgrade():

This script creates the necessary tables using SQLAlchemy’s Base.metadata.create_all() method and then creates some sample data to populate the database.

Finally, we can run the migrations using the following command:

alembic upgrade head

This will run all of the migrations in the alembic/versions directory, including the 1_initial.py script that we’ve just created.

That’s it! We’ve now created a database migration script to create the necessary tables and populate them with sample data on application startup.

Test Setup

It is always prudent to test your changes to ensure that they function as intended. We can write a simple FastAPI test case as follows for testing our API endpoint.

We will create a tests folder at the root directory of our project where we will be keeping all our tests.

from fastapi.testclient import TestClient
from ..app.main import app;

client = TestClient(app)

def test_read_main():
    response = client.get("/")
    assert response.status_code == 200
    assert response.json() == {"msg": "Hello World"}

To run unit tests locally:

  1. Install additional dependencies for your test environment listed on test_requirements.txt file
pip install -r test_requirements.txt
  1. Activate test environment
source ./testenv/bin/activate
  1. Run the following command
python3 -m pytest

This was a very basic guide to creating a starter project. If you have ideas to add additional configurations, feel free to contribute to the GitHub repository by following the contributing guide.

Please provide any suggestions or feedback on the article as well. Thank you 🙂 !!