Response Types

Response Types

Flask is flexible about what your view functions return. The simplest case is a string — Flask automatically wraps it in a proper HTTP response with a 200 status code and HTML content type:

@app.route("/")
def home():
    return "Hello, World!"

Return a dictionary and Flask converts it to JSON:

@app.route("/api/data")
def api_data():
    return {"message": "Success", "count": 42}

This is incredibly handy for building APIs. Flask sets the Content-Type to application/json automatically.

For more control, use make_response(). It lets you customize the status code, headers, and content:

from flask import make_response

@app.route("/custom")
def custom_response():
    response = make_response("Custom response", 201)
    response.headers["X-Custom-Header"] = "hello"
    return response

When you need to send users somewhere else, use redirect():

from flask import redirect, url_for

@app.route("/old-page")
def old_page():
    return redirect(url_for("new_page"))

@app.route("/new-page")
def new_page():
    return "This is the new page!"

And when something goes wrong, abort() stops processing and returns an error page:

from flask import abort

@app.route("/secret")
def secret():
    if not user_is_authenticated():
        abort(401)
    return "Top secret content"

You can also return a tuple with a response body and a status code, or a body, status code, and headers — all in one line:

@app.route("/created")
def created():
    return "Resource created", 201

@app.route("/with-headers")
def with_headers():
    return "OK", 200, {"X-Request-Id": "abc123"}

The Content-Type header tells the browser how to interpret your response. Flask defaults to text/html for strings and application/json for dicts. You can override it when needed:

@app.route("/plain")
def plain_text():
    return "Just plain text", 200, {"Content-Type": "text/plain"}