from fasthtml.common import *
from collections import namedtuple
from typing import TypedDict
from datetime import datetime
import json,timeHandling handlers
How handlers work in FastHTML
app = FastHTML()The FastHTML class is the main application class for FastHTML apps.
rt = app.routeapp.route is used to register route handlers. It is a decorator, which means we place it before a function that is used as a handler. Because it’s used frequently in most FastHTML applications, we often alias it as rt, as we do here.
Basic Route Handling
@rt("/hi")
def get(): return 'Hi there'Handler functions can return strings directly. These strings are sent as the response body to the client.
cli = Client(app)Client is a test client for FastHTML applications. It allows you to simulate requests to your app without running a server.
cli.get('/hi').text'Hi there'The get method on a Client instance simulates GET requests to the app. It returns a response object that has a .text attribute, which you can use to access the body of the response. It calls httpx.get internally – all httpx HTTP verbs are supported.
@rt("/hi")
def post(): return 'Postal'
cli.post('/hi').text'Postal'Handler functions can be defined for different HTTP methods on the same route. Here, we define a post handler for the /hi route. The Client instance can simulate different HTTP methods, including POST requests.
Request and Response Objects
@app.get("/hostie")
def show_host(req): return req.headers['host']
cli.get('/hostie').text'testserver'Handler functions can accept a req (or request) parameter, which represents the incoming request. This object contains information about the request, including headers. In this example, we return the host header from the request. The test client uses ‘testserver’ as the default host.
In this example, we use @app.get("/hostie") instead of @rt("/hostie"). The @app.get() decorator explicitly specifies the HTTP method (GET) for the route, while @rt() by default handles both GET and POST requests.
@rt
def yoyo(): return 'a yoyo'
cli.post('/yoyo').text'a yoyo'If the @rt decorator is used without arguments, it uses the function name as the route path. Here, the yoyo function becomes the handler for the /yoyo route. This handler responds to GET and POST methods, since a specific method wasn’t provided.
@rt
def ft1(): return Html(Div('Text.'))
print(cli.get('/ft1').text) <html>
<div>Text.</div>
</html>
Handler functions can return FT objects, which are automatically converted to HTML strings. The FT class can take other FT components as arguments, such as Div. This allows for easy composition of HTML elements in your responses.
@app.get
def autopost(): return Html(Div('Text.', hx_post=yoyo.rt()))
print(cli.get('/autopost').text) <html>
<div hx-post="/yoyo">Text.</div>
</html>
The rt decorator modifies the yoyo function by adding an rt() method. This method returns the route path associated with the handler. It’s a convenient way to reference the route of a handler function dynamically.
In the example, yoyo.rt() is used as the value for hx_post. This means when the div is clicked, it will trigger an HTMX POST request to the route of the yoyo handler. This approach allows for flexible, DRY code by avoiding hardcoded route strings and automatically updating if the route changes.
This pattern is particularly useful in larger applications where routes might change, or when building reusable components that need to reference their own routes dynamically.
@app.get
def autoget(): return Html(Body(Div('Text.', cls='px-2', hx_post=show_host.rt(a='b'))))
print(cli.get('/autoget').text) <html>
<body>
<div hx-post="/hostie?a=b" class="px-2">Text.</div>
</body>
</html>
The rt() method of handler functions can also accept parameters. When called with parameters, it returns the route path with a query string appended. In this example, show_host.rt(a='b') generates the path /hostie?a=b.
The Body component is used here to demonstrate nesting of FT components. Div is nested inside Body, showcasing how you can create more complex HTML structures.
The cls parameter is used to add a CSS class to the Div. This translates to the class attribute in the rendered HTML. (class can’t be used as a parameter name directly in Python since it’s a reserved word.)
@rt('/ft2')
def get(): return Title('Foo'),H1('bar')
print(cli.get('/ft2').text) <!doctype html>
<html>
<head>
<title>Foo</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
<script src="https://unpkg.com/htmx.org@next/dist/htmx.min.js"></script>
<script src="https://cdn.jsdelivr.net/gh/answerdotai/fasthtml-js@main/fasthtml.js"></script>
<script src="https://cdn.jsdelivr.net/gh/answerdotai/surreal@main/surreal.js"></script>
<script src="https://cdn.jsdelivr.net/gh/gnat/css-scope-inline@main/script.js"></script>
</head>
<body>
<h1>bar</h1>
</body>
</html>
Handler functions can return multiple FT objects as a tuple. The first item is treated as the Title, and the rest are added to the Body. When the request is not an HTMX request, FastHTML automatically adds necessary HTML boilerplate, including default head content with required scripts.
When using app.route (or rt), if the function name matches an HTTP verb (e.g., get, post, put, delete), that HTTP method is automatically used for the route. In this case, a path must be explicitly provided as an argument to the decorator.
hxhdr = {'headers':{'hx-request':"1"}}
print(cli.get('/ft2', **hxhdr).text) <title>Foo</title>
<h1>bar</h1>
For HTMX requests (indicated by the hx-request header), FastHTML returns only the specified components without the full HTML structure. This allows for efficient partial page updates in HTMX applications.
@rt('/ft3')
def get(): return H1('bar')
print(cli.get('/ft3', **hxhdr).text) <h1>bar</h1>
When a handler function returns a single FT object for an HTMX request, it’s rendered as a single HTML partial.
@rt('/ft4')
def get(): return Html(Head(Title('hi')), Body(P('there')))
print(cli.get('/ft4').text) <html>
<head>
<title>hi</title>
</head>
<body>
<p>there</p>
</body>
</html>
Handler functions can return a complete Html structure, including Head and Body components. When a full HTML structure is returned, FastHTML doesn’t add any additional boilerplate. This gives you full control over the HTML output when needed.
@rt
def index(): return "welcome!"
print(cli.get('/').text)welcome!The index function is a special handler in FastHTML. When defined without arguments to the @rt decorator, it automatically becomes the handler for the root path ('/'). This is a convenient way to define the main page or entry point of your application.
Path and Query Parameters
@rt('/user/{nm}', name='gday')
def get(nm:str=''): return f"Good day to you, {nm}!"
cli.get('/user/Alexis').text'Good day to you, Alexis!'Handler functions can use path parameters, defined using curly braces in the route – this is implemented by Starlette directly, so all Starlette path parameters can be used. These parameters are passed as arguments to the function.
The name parameter in the decorator allows you to give the route a name, which can be used for URL generation.
In this example, {nm} in the route becomes the nm parameter in the function. The function uses this parameter to create a personalized greeting.
@app.get
def autolink(): return Html(Div('Text.', link=uri('gday', nm='Alexis')))
print(cli.get('/autolink').text) <html>
<div href="/user/Alexis">Text.</div>
</html>
The uri function is used to generate URLs for named routes. It takes the route name as its first argument, followed by any path or query parameters needed for that route.
In this example, uri('gday', nm='Alexis') generates the URL for the route named ‘gday’ (which we defined earlier as ‘/user/{nm}’), with ‘Alexis’ as the value for the ‘nm’ parameter.
The link parameter in FT components sets the href attribute of the rendered HTML element. By using uri(), we can dynamically generate correct URLs even if the underlying route structure changes.
This approach promotes maintainable code by centralizing route definitions and avoiding hardcoded URLs throughout the application.
@rt('/link')
def get(req): return f"{req.url_for('gday', nm='Alexis')}; {req.url_for('show_host')}"
cli.get('/link').text'http://testserver/user/Alexis; http://testserver/hostie'The url_for method of the request object can be used to generate URLs for named routes. It takes the route name as its first argument, followed by any path parameters needed for that route.
In this example, req.url_for('gday', nm='Alexis') generates the full URL for the route named ‘gday’, including the scheme and host. Similarly, req.url_for('show_host') generates the URL for the ‘show_host’ route.
This method is particularly useful when you need to generate absolute URLs, such as for email links or API responses. It ensures that the correct host and scheme are included, even if the application is accessed through different domains or protocols.
app.url_path_for('gday', nm='Jeremy')'/user/Jeremy'The url_path_for method of the application can be used to generate URL paths for named routes. Unlike url_for, it returns only the path component of the URL, without the scheme or host.
In this example, app.url_path_for('gday', nm='Jeremy') generates the path ‘/user/Jeremy’ for the route named ‘gday’.
This method is useful when you need relative URLs or just the path component, such as for internal links or when constructing URLs in a host-agnostic manner.
@rt('/oops')
def get(nope): return nope
r = cli.get('/oops?nope=1')
print(r)
r.text<Response [200 OK]>/Users/jhoward/Documents/GitHub/fasthtml/fasthtml/core.py:175: UserWarning: `nope has no type annotation and is not a recognised special name, so is ignored.
if arg!='resp': warn(f"`{arg} has no type annotation and is not a recognised special name, so is ignored.")''Handler functions can include parameters, but they must be type-annotated or have special names (like req) to be recognized. In this example, the nope parameter is not annotated, so it’s ignored, resulting in a warning.
When a parameter is ignored, it doesn’t receive the value from the query string. This can lead to unexpected behavior, as the function attempts to return nope, which is undefined.
The cli.get('/oops?nope=1') call succeeds with a 200 OK status because the handler doesn’t raise an exception, but it returns an empty response, rather than the intended value.
To fix this, you should either add a type annotation to the parameter (e.g., def get(nope: str):) or use a recognized special name like req.
@rt('/html/{idx}')
def get(idx:int): return Body(H4(f'Next is {idx+1}.'))
print(cli.get('/html/1', **hxhdr).text) <body>
<h4>Next is 2.</h4>
</body>
Path parameters can be type-annotated, and FastHTML will automatically convert them to the specified type if possible. In this example, idx is annotated as int, so it’s converted from the string in the URL to an integer.
reg_re_param("imgext", "ico|gif|jpg|jpeg|webm")
@rt(r'/static/{path:path}{fn}.{ext:imgext}')
def get(fn:str, path:str, ext:str): return f"Getting {fn}.{ext} from /{path}"
print(cli.get('/static/foo/jph.ico').text)Getting jph.ico from /foo/The reg_re_param function is used to register custom path parameter types using regular expressions. Here, we define a new path parameter type called “imgext” that matches common image file extensions.
Handler functions can use complex path patterns with multiple parameters and custom types. In this example, the route pattern r'/static/{path:path}{fn}.{ext:imgext}' uses three path parameters:
path: A Starlette built-in type that matches any path segmentsfn: The filename without extensionext: Our custom “imgext” type that matches specific image extensions
ModelName = str_enum('ModelName', "alexnet", "resnet", "lenet")
@rt("/models/{nm}")
def get(nm:ModelName): return nm
print(cli.get('/models/alexnet').text)alexnetWe define ModelName as an enum with three possible values: “alexnet”, “resnet”, and “lenet”. Handler functions can use these enum types as parameter annotations. In this example, the nm parameter is annotated with ModelName, which ensures that only valid model names are accepted.
When a request is made with a valid model name, the handler function returns that name. This pattern is useful for creating type-safe APIs with a predefined set of valid values.
@rt("/files/{path}")
async def get(path: Path): return path.with_suffix('.txt')
print(cli.get('/files/foo').text)foo.txtHandler functions can use Path objects as parameter types. The Path type is from Python’s standard library pathlib module, which provides an object-oriented interface for working with file paths. In this example, the path parameter is annotated with Path, so FastHTML automatically converts the string from the URL to a Path object.
This approach is particularly useful when working with file-related routes, as it provides a convenient and platform-independent way to handle file paths.
fake_db = [{"name": "Foo"}, {"name": "Bar"}]
@rt("/items/")
def get(idx:int|None = 0): return fake_db[idx]
print(cli.get('/items/?idx=1').text){"name":"Bar"}Handler functions can use query parameters, which are automatically parsed from the URL. In this example, idx is a query parameter with a default value of 0. It’s annotated as int|None, allowing it to be either an integer or None.
The function uses this parameter to index into a fake database (fake_db). When a request is made with a valid idx query parameter, the handler returns the corresponding item from the database.
print(cli.get('/items/').text){"name":"Foo"}When no idx query parameter is provided, the handler function uses the default value of 0. This results in returning the first item from the fake_db list, which is {"name":"Foo"}.
This behavior demonstrates how default values for query parameters work in FastHTML. They allow the API to have a sensible default behavior when optional parameters are not provided.
print(cli.get('/items/?idx=g'))<Response [404 Not Found]>When an invalid value is provided for a typed query parameter, FastHTML returns a 404 Not Found response. In this example, ‘g’ is not a valid integer for the idx parameter, so the request fails with a 404 status.
This behavior ensures type safety and prevents invalid inputs from reaching the handler function.
@app.get("/booly/")
def _(coming:bool=True): return 'Coming' if coming else 'Not coming'
print(cli.get('/booly/?coming=true').text)
print(cli.get('/booly/?coming=no').text)Coming
Not comingHandler functions can use boolean query parameters. In this example, coming is a boolean parameter with a default value of True. FastHTML automatically converts string values like ‘true’, ‘false’, ‘1’, ‘0’, ‘on’, ‘off’, ‘yes’, and ‘no’ to their corresponding boolean values.
The underscore _ is used as the function name in this example to indicate that the function’s name is not important or won’t be referenced elsewhere. This is a common Python convention for throwaway or unused variables, and it works here because FastHTML uses the route decorator parameter, when provided, to determine the URL path, not the function name. By default, both get and post methods can be used in routes that don’t specify an http method (by either using app.get, def get, or the methods parameter to app.route).
@app.get("/datie/")
def _(d:parsed_date): return d
date_str = "17th of May, 2024, 2p"
print(cli.get(f'/datie/?d={date_str}').text)2024-05-17 14:00:00Handler functions can use date objects as parameter types. FastHTML uses dateutil.parser library to automatically parse a wide variety of date string formats into date objects.
@app.get("/ua")
async def _(user_agent:str): return user_agent
print(cli.get('/ua', headers={'User-Agent':'FastHTML'}).text)FastHTMLHandler functions can access HTTP headers by using parameter names that match the header names. In this example, user_agent is used as a parameter name, which automatically captures the value of the ‘User-Agent’ header from the request.
The Client instance allows setting custom headers for test requests. Here, we set the ‘User-Agent’ header to ‘FastHTML’ in the test request.
@app.get("/hxtest")
def _(htmx): return htmx.request
print(cli.get('/hxtest', headers={'HX-Request':'1'}).text)
@app.get("/hxtest2")
def _(foo:HtmxHeaders, req): return foo.request
print(cli.get('/hxtest2', headers={'HX-Request':'1'}).text)1
1Handler functions can access HTMX-specific headers using either the special htmx parameter name, or a parameter annotated with HtmxHeaders. Both approaches provide access to HTMX-related information.
In these examples, the htmx.request attribute returns the value of the ‘HX-Request’ header.
app.chk = 'foo'
@app.get("/app")
def _(app): return app.chk
print(cli.get('/app').text)fooHandler functions can access the FastHTML application instance using the special app parameter name. This allows handlers to access application-level attributes and methods.
In this example, we set a custom attribute chk on the application instance. The handler function then uses the app parameter to access this attribute and return its value.
@app.get("/app2")
def _(foo:FastHTML): return foo.chk,HttpHeader("mykey", "myval")
r = cli.get('/app2', **hxhdr)
print(r.text)
print(r.headers)foo
Headers({'mykey': 'myval', 'content-length': '4', 'content-type': 'text/html; charset=utf-8'})Handler functions can access the FastHTML application instance using a parameter annotated with FastHTML. This allows handlers to access application-level attributes and methods, just like using the special app parameter name.
Handlers can return tuples containing both content and HttpHeader objects. HttpHeader allows setting custom HTTP headers in the response.
In this example:
- We define a handler that returns both the
chkattribute from the application and a custom header. - The
HttpHeader("mykey", "myval")sets a custom header in the response. - We use the test client to make a request and examine both the response text and headers.
- The response includes the custom header “mykey” along with standard headers like content-length and content-type.
@app.get("/app3")
def _(foo:FastHTML): return HtmxResponseHeaders(location="http://example.org")
r = cli.get('/app3')
print(r.headers)Headers({'hx-location': 'http://example.org', 'content-length': '0', 'content-type': 'text/html; charset=utf-8'})Handler functions can return HtmxResponseHeaders objects to set HTMX-specific response headers. This is useful for HTMX-specific behaviors like client-side redirects.
In this example we define a handler that returns an HtmxResponseHeaders object with a location parameter, which sets the HX-Location header in the response. HTMX uses this for client-side redirects.
@app.get("/app4")
def _(foo:FastHTML): return Redirect("http://example.org")
cli.get('/app4', follow_redirects=False)<Response [303 See Other]>Handler functions can return Redirect objects to perform HTTP redirects. This is useful for redirecting users to different pages or external URLs.
In this example:
- We define a handler that returns a
Redirectobject with the URL “http://example.org”. - The
cli.get('/app4', follow_redirects=False)call simulates a GET request to the ‘/app4’ route without following redirects. - The response has a 303 See Other status code, indicating a redirect.
The follow_redirects=False parameter is used to prevent the test client from automatically following the redirect, allowing us to inspect the redirect response itself.
Redirect.__response__<function fasthtml.core.Redirect.__response__(self, req)>The Redirect class in FastHTML implements a __response__ method, which is a special method recognized by the framework. When a handler returns a Redirect object, FastHTML internally calls this __response__ method to replace the original response.
The __response__ method takes a req parameter, which represents the incoming request. This allows the method to access request information if needed when constructing the redirect response.
@rt
def meta():
return ((Title('hi'),H1('hi')),
(Meta(property='image'), Meta(property='site_name')))
print(cli.post('/meta').text) <!doctype html>
<html>
<head>
<title>hi</title>
<meta property="image">
<meta property="site_name">
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
<script src="https://unpkg.com/htmx.org@next/dist/htmx.min.js"></script>
<script src="https://cdn.jsdelivr.net/gh/answerdotai/fasthtml-js@main/fasthtml.js"></script>
<script src="https://cdn.jsdelivr.net/gh/answerdotai/surreal@main/surreal.js"></script>
<script src="https://cdn.jsdelivr.net/gh/gnat/css-scope-inline@main/script.js"></script>
</head>
<body>
<h1>hi</h1>
</body>
</html>
FastHTML automatically identifies elements typically placed in the <head> (like Title and Meta) and positions them accordingly, while other elements go in the <body>.
In this example: - (Title('hi'), H1('hi')) defines the title and main heading. The title is placed in the head, and the H1 in the body. - (Meta(property='image'), Meta(property='site_name')) defines two meta tags, which are both placed in the head.
Form Data and JSON Handling
@app.post('/profile/me')
def profile_update(username: str): return username
print(cli.post('/profile/me', data={'username' : 'Alexis'}).text)
r = cli.post('/profile/me', data={})
print(r.text)
rAlexis
Missing required field: username<Response [400 Bad Request]>Handler functions can accept form data parameters, without needing to manually extract it from the request. In this example, username is expected to be sent as form data.
If required form data is missing, FastHTML automatically returns a 400 Bad Request response with an error message.
The data parameter in the cli.post() method simulates sending form data in the request.
@app.post('/pet/dog')
def pet_dog(dogname: str = None): return dogname or 'unknown name'
print(cli.post('/pet/dog', data={}).text)unknown nameHandlers can have optional form data parameters with default values. In this example, dogname is an optional parameter with a default value of None.
Here, if the form data doesn’t include the dogname field, the function uses the default value. The function returns either the provided dogname or ‘unknown name’ if dogname is None.
@dataclass
class Bodie: a:int;b:str
@rt("/bodie/{nm}")
def post(nm:str, data:Bodie):
res = asdict(data)
res['nm'] = nm
return res
print(cli.post('/bodie/me', data=dict(a=1, b='foo', nm='me')).text){"a":1,"b":"foo","nm":"me"}You can use dataclasses to define structured form data. In this example, Bodie is a dataclass with a (int) and b (str) fields.
FastHTML automatically converts the incoming form data to a Bodie instance where attribute names match parameter names. Other form data elements are matched with parameters with the same names (in this case, nm).
Handler functions can return dictionaries, which FastHTML automatically JSON-encodes.
@app.post("/bodied/")
def bodied(data:dict): return data
d = dict(a=1, b='foo')
print(cli.post('/bodied/', data=d).text){"a":"1","b":"foo"}dict parameters capture all form data as a dictionary. In this example, the data parameter is annotated with dict, so FastHTML automatically converts all incoming form data into a dictionary.
Note that when form data is converted to a dictionary, all values become strings, even if they were originally numbers. This is why the ‘a’ key in the response has a string value “1” instead of the integer 1.
nt = namedtuple('Bodient', ['a','b'])
@app.post("/bodient/")
def bodient(data:nt): return asdict(data)
print(cli.post('/bodient/', data=d).text){"a":"1","b":"foo"}Handler functions can use named tuples to define structured form data. In this example, Bodient is a named tuple with a and b fields.
FastHTML automatically converts the incoming form data to a Bodient instance where field names match parameter names. As with the previous example, all form data values are converted to strings in the process.
class BodieTD(TypedDict): a:int;b:str='foo'
@app.post("/bodietd/")
def bodient(data:BodieTD): return data
print(cli.post('/bodietd/', data=d).text){"a":1,"b":"foo"}You can use TypedDict to define structured form data with type hints. In this example, BodieTD is a TypedDict with a (int) and b (str) fields, where b has a default value of ‘foo’.
FastHTML automatically converts the incoming form data to a BodieTD instance where keys match the defined fields. Unlike with regular dictionaries or named tuples, FastHTML respects the type hints in TypedDict, converting values to the specified types when possible (e.g., converting ‘1’ to the integer 1 for the ‘a’ field).
class Bodie2:
a:int|None; b:str
def __init__(self, a, b='foo'): store_attr()
@app.post("/bodie2/")
def bodie(d:Bodie2): return f"a: {d.a}; b: {d.b}"
print(cli.post('/bodie2/', data={'a':1}).text)a: 1; b: fooCustom classes can be used to define structured form data. Here, Bodie2 is a custom class with a (int|None) and b (str) attributes, where b has a default value of ‘foo’. The store_attr() function (from fastcore) automatically assigns constructor parameters to instance attributes.
FastHTML automatically converts the incoming form data to a Bodie2 instance, matching form fields to constructor parameters. It respects type hints and default values.
@app.post("/b")
def index(it: Bodie): return Titled("It worked!", P(f"{it.a}, {it.b}"))
s = json.dumps({"b": "Lorem", "a": 15})
print(cli.post('/b', headers={"Content-Type": "application/json", 'hx-request':"1"}, data=s).text) <title>It worked!</title>
<main class="container">
<h1>It worked!</h1>
<p>15, Lorem</p>
</main>
Handler functions can accept JSON data as input, which is automatically parsed into the specified type. In this example, it is of type Bodie, and FastHTML converts the incoming JSON data to a Bodie instance.
The Titled component is used to create a page with a title and main content. It automatically generates an <h1> with the provided title, wraps the content in a <main> tag with a “container” class, and adds a title to the head.
When making a request with JSON data: - Set the “Content-Type” header to “application/json” - Provide the JSON data as a string in the data parameter of the request