Flask Cheatsheet

Blueprints

Use this Flask reference while you build software engineering projects, review code for technical interview prep, or polish examples for a software engineer resume.

What Blueprints Are

A Blueprint is a collection of routes, templates, static files, and error handlers that can be registered on a Flask app. Use them to split a large application into modular components.

Creating a Blueprint

# myapp/blueprints/auth.py
from flask import Blueprint, render_template, redirect, url_for

auth_bp = Blueprint(
    "auth",                   # blueprint name (used in url_for: "auth.login")
    __name__,                 # import name (for template/static lookup)
    url_prefix="/auth",       # prepend to all routes in this blueprint
    template_folder="templates",  # blueprint-local templates/ dir
    static_folder="static",       # blueprint-local static/ dir
    static_url_path="/auth/static",
)

@auth_bp.route("/login", methods=["GET", "POST"])
def login():
    return render_template("auth/login.html")

@auth_bp.route("/logout")
def logout():
    return redirect(url_for("auth.login"))

Registering a Blueprint

# myapp/__init__.py
from flask import Flask
from .blueprints.auth import auth_bp
from .blueprints.api import api_bp
from .blueprints.admin import admin_bp

def create_app():
    app = Flask(__name__)

    app.register_blueprint(auth_bp)
    app.register_blueprint(api_bp, url_prefix="/api/v1")   # override prefix at registration
    app.register_blueprint(admin_bp, url_prefix="/admin", name="admin_v2")  # rename

    return app

register_blueprint options:

Option	Description
`url_prefix`	Override the blueprint's own `url_prefix`
`name`	Override the blueprint name (endpoint namespace)
`subdomain`	Mount blueprint on a subdomain
`url_defaults`	Dict of default URL values for all routes
`cli_group`	CLI command group name

Blueprint `url_for`

Always namespace with blueprint_name.view_function:

url_for("auth.login")
url_for("api.get_user", user_id=1)
url_for("admin.dashboard")

# From within the same blueprint, use ".view_name" shorthand:
url_for(".login")       # resolves to current blueprint's endpoint

Blueprint Hooks

Hooks on a blueprint apply only to that blueprint's requests:

@auth_bp.before_request
def require_login():
    if not current_user.is_authenticated:
        return redirect(url_for("auth.login"))

@auth_bp.after_request
def add_auth_header(response):
    response.headers["X-Auth-By"] = "MyApp"
    return response

@auth_bp.teardown_request
def teardown(exception):
    pass

App-wide hooks registered on the blueprint (rare — use app hooks instead):

@auth_bp.before_app_request
def before_all_requests():
    ...

@auth_bp.after_app_request
def after_all_requests(response):
    return response

Error Handlers on Blueprints

Blueprint error handlers only catch errors raised within that blueprint's views (Flask 2.3+ — earlier versions require app-level handlers for HTTP errors):

@auth_bp.errorhandler(403)
def forbidden(e):
    return render_template("auth/403.html"), 403

@auth_bp.app_errorhandler(404)
def not_found(e):
    return render_template("errors/404.html"), 404   # app-wide 404

Template Lookup with Blueprints

Flask searches the app's templates/ before blueprint templates. To avoid name collisions, namespace blueprint templates in a subfolder:

myapp/
  templates/
    base.html              # app templates
  blueprints/
    auth/
      templates/
        auth/
          login.html       # render as "auth/login.html"
      static/
        auth.css

auth_bp = Blueprint("auth", __name__, template_folder="templates")

@auth_bp.route("/login")
def login():
    return render_template("auth/login.html")   # finds blueprint template

Static Files with Blueprints

auth_bp = Blueprint(
    "auth", __name__,
    static_folder="static",
    static_url_path="/auth-static",
)

# In templates:
# url_for("auth.static", filename="auth.css")
# → /auth-static/auth.css

Context Processors per Blueprint

@auth_bp.context_processor
def inject_user():
    return {"current_user": get_current_user()}

CLI Commands per Blueprint

import click

@auth_bp.cli.command("create-admin")
@click.argument("email")
def create_admin(email):
    """Create an admin user."""
    user = User(email=email, role="admin")
    db.session.add(user)
    db.session.commit()
    click.echo(f"Admin {email} created.")

flask auth create-admin admin@example.com

Nesting Blueprints (Flask 2.0+)

parent_bp = Blueprint("parent", __name__, url_prefix="/parent")
child_bp  = Blueprint("child",  __name__, url_prefix="/child")

@child_bp.route("/page")
def page():
    return "child page"    # url: /parent/child/page

parent_bp.register_blueprint(child_bp)
app.register_blueprint(parent_bp)

Typical Blueprint File Structure

myapp/
  blueprints/
    auth/
      __init__.py          # Blueprint definition
      routes.py            # view functions (optional split)
      forms.py
      models.py
      templates/
        auth/
          login.html
      static/
        auth.css
    api/
      __init__.py
      v1.py
      v2.py

# blueprints/auth/__init__.py
from flask import Blueprint

auth_bp = Blueprint("auth", __name__,
                    template_folder="templates",
                    static_folder="static")

from . import routes   # import views to register them  # noqa: E402,F401

Registering the Same Blueprint Multiple Times

# Mount the same blueprint at two prefixes with different names
app.register_blueprint(api_bp, url_prefix="/api/v1", name="api_v1")
app.register_blueprint(api_bp, url_prefix="/api/v2", name="api_v2")

url_for("api_v1.get_user", id=1)  # /api/v1/users/1
url_for("api_v2.get_user", id=1)  # /api/v2/users/1

Common Gotchas

Forgetting blueprint_name. in url_for raises BuildError.
Blueprint templates shadow app templates if names collide — use subdirectories.
Blueprint before_request hooks only fire for that blueprint's routes, not the whole app.
static_url_path defaults to static_folder name if not set; set it explicitly to avoid collisions with app.static_url_path.
Running flask routes shows all endpoints including blueprint namespace.