How to write a web framework in Python
I want to write a small web app today.
Don’t you think using a web framework is an overkill for a hello world app? I think a simple wsgi app will be good enough.
def application(environ, start_response):
"""Simplest possible application object"""
status = '200 OK'
response_headers = [('Content-type', 'text/plain')]
start_response(status, response_headers)
return ['Hello world!\n']I may want to handle 2 URLs, won’t it be better if I have a class instead of a function.
class application:
def __init__(self, environ, start_response):
self.environ = environ
self.start = start_response
def __iter__(self):
status = '200 OK'
response_headers = [('Content-type', 'text/plain')]
self.start(status, response_headers)
yield "Hello world!\n"Now let me add a way to delegate stuff based on URLs.
def __iter__(self):
path = self.environ['PATH_INFO']
if path == "/":
return self.GET_index()
elif path == "/hello":
return self.GET_hello()
else:
return self.notfound()
def GET_index(self):
status = '200 OK'
response_headers = [('Content-type', 'text/plain')]
self.start(status, response_headers)
yield "Welcome!\n"
def GET_hello(self):
status = '200 OK'
response_headers = [('Content-type', 'text/plain')]
self.start(status, response_headers)
yield "Hello world!\n"
def notfound(self):
status = '404 Not Found'
response_headers = [('Content-type', 'text/plain')]
self.start(status, response_headers)
yield "Not Found\n"Neat, isn’t it?
Why should I hardcode URLs? Won’t it be better to abstract that out?
urls = [
("/", "index"),
("/hello", "hello")
]
def __iter__(self):
path_info = self.environ['PATH_INFO']
method = self.environ['REQUEST_METHOD']
for path, name in self.urls:
if path == path_info:
funcname = method.upper() + "_" + name
func = getattr(self, funcname)
return func()
return self.notfound()Looks good, but something is missing. How about adding ability to parameterize
the URLs? All it takes is a regex match. Then I can do urls like /hello/foo!
urls = [
("/", "index"),
("/hello/(.*)", "hello")
]
def __iter__(self):
path = self.environ['PATH_INFO']
method = self.environ['REQUEST_METHOD']
for pattern, name in self.urls:
m = re.match('^' + pattern + '$', path)
if m:
# pass the matched groups as arguments to the function
args = m.groups()
funcname = method.upper() + "_" + name
func = getattr(self, funcname)
return func(*args)
return self.notfound()
def GET_hello(self, name):
status = '200 OK'
response_headers = [('Content-type', 'text/plain')]
self.start(status, response_headers)
yield "Hello %s!\n" % nameThe __iter__ method is really doing delegation. Don’t you think we should move it into a separate method?
def __iter__(self):
return self.delegate()
def delegate(self):
path = self.environ['PATH_INFO']
method = self.environ['REQUEST_METHOD']
for pattern, name in self.urls:
m = re.match('^' + pattern + '$', path)
if m:
# pass the matched groups as arguments to the function
args = m.groups()
funcname = method.upper() + "_" + name
func = getattr(self, funcname)
return func(*args)
return self.notfound()You know what? I think now there is lot of code that has nothing to do with my app at all. May be it is a good idea to move it into a base class.
class wsgiapp:
"""Base class for my wsgi application."""
def __init__(self, environ, start_response):
self.environ = environ
self.start = start_response
def __iter__(self):
return self.delegate()
def delegate(self):
path = self.environ['PATH_INFO']
method = self.environ['REQUEST_METHOD']
for pattern, name in self.urls:
m = re.match('^' + pattern + '$', path)
if m:
# pass the matched groups as arguments to the function
args = m.groups()
funcname = method.upper() + "_" + name
func = getattr(self, funcname)
return func(*args)
return self.notfound()
class application(wsgiapp):
urls = [
("/", "index"),
("/hello/(.*)", "index")
]
def GET_index(self):
status = '200 OK'
response_headers = [('Content-type', 'text/plain')]
self.start(status, response_headers)
yield "Welcome!\n"
def GET_hello(self, name):
status = '200 OK'
response_headers = [('Content-type', 'text/plain')]
self.start(status, response_headers)
yield "Hello %s!\n" % nameMuch better now. Don’t you think?
Hey, wait a minute! Do you notice the duplication? The self.start method
is called almost in the same way in both GET_ methods. How about keeping the
status and headers in the wsgiapp and provide a header function to add any
new headers, if required?
class wsgiapp:
"""Base class for my wsgi application."""
def __init__(self, environ, start_response):
self.environ = environ
self.start = start_response
self.status = "200 OK"
self._headers = []
def header(self, name, value):
self._headers.append((name, value))
def __iter__(self):
x = self.delegate()
self.start(self.status, self._headers)
# return value can be a string or a list. we should be able to
# return an iter in both the cases.
if isinstance(x, str):
return iter([x])
else:
return iter(x)
...
class application(wsgiapp):
urls = [
("/", "index"),
("/hello/(.*)", "index")
]
def GET_index(self):
self.header("content-type": "text/plain")
return "Welcome!\n"
def GET_hello(self, name):
self.header("content-type": "text/plain")
return "Hello %s!\n" % nameLooking good. Isn’t it?
I introduced an error by mistake and the server started giving a blank response. Believe me, it was too hard to spot the mistake. Let me add a check for it.
def __iter__(self):
try:
x = self.delegate()
self.start(self.status, self._headers)
except:
headers = [("Content-Type": "text/plain")]
self.start("500 Internal Error", headers)
x = "Internal Error:\n\n" + traceback.format_exc()
# return value can be a string or a list. we should be able to
# return an iter in both the cases.
if isinstance(x, str):
return iter([x])
else:
return iter(x)Oh my god! It looks like I’ve invented a new web framework.
Here is the complete code:
import re
import traceback
class wsgiapp:
"""The most beatiful micro web framwork.
How to use:
class application(wsgiapp):
urls = [
("/(.*)", "index"),
]
def GET_hello(self, name):
self.header("Content-Type", "text/plain")
return "Hello, %s!" % name
"""
def __init__(self, environ, start_response):
self.environ = environ
self.start = start_response
self.status = "200 OK"
self._headers = []
def header(self, name, value):
self._headers.append((name, value))
def __iter__(self):
try:
x = self.delegate()
self.start(self.status, self._headers)
except:
headers = [("Content-Type", "text/plain")]
self.start("500 Internal Error", headers)
x = "Internal Error:\n\n" + traceback.format_exc()
# return value can be a string or a list. we should be able to
# return an iter in both the cases.
if isinstance(x, str):
return iter([x])
else:
return iter(x)
def delegate(self):
path = self.environ['PATH_INFO']
method = self.environ['REQUEST_METHOD']
for pattern, name in self.urls:
m = re.match('^' + pattern + '$', path)
if m:
# pass the matched groups as arguments to the function
args = m.groups()
funcname = method.upper() + "_" + name
func = getattr(self, funcname)
return func(*args)
return self.notfound()