WebOb File-Serving Example

This document shows how you can make a static-file-serving application using WebOb. We’ll quickly build this up from minimal functionality to a high-quality file serving application.

Note

Starting from 1.2b4, WebOb ships with a webob.static module which implements a webob.static.FileApp WSGI application similar to the one described below.

This document stays as a didactic example how to serve files with WebOb, but you should consider using applications from webob.static in production.

First we’ll setup a really simple shim around our application, which we can use as we improve our application:

>>> from webob import Request, Response
>>> import os
>>> class FileApp(object):
...     def __init__(self, filename):
...         self.filename = filename
...     def __call__(self, environ, start_response):
...         res = make_response(self.filename)
...         return res(environ, start_response)
>>> import mimetypes
>>> def get_mimetype(filename):
...     type, encoding = mimetypes.guess_type(filename)
...     # We'll ignore encoding, even though we shouldn't really
...     return type or 'application/octet-stream'

Now we can make different definitions of make_response. The simplest version:

>>> def make_response(filename):
...     res = Response(content_type=get_mimetype(filename))
...     res.body = open(filename, 'rb').read()
...     return res

Let’s give it a go. We’ll test it out with a file test-file.txt in the WebOb doc directory:

>>> fn = os.path.join(doc_dir, 'test-file.txt')
>>> open(fn).read()
'This is a test.  Hello test people!'
>>> app = FileApp(fn)
>>> req = Request.blank('/')
>>> print req.get_response(app)
200 OK
Content-Type: text/plain; charset=UTF-8
Content-Length: 35

This is a test.  Hello test people!

Well, that worked. But it’s not a very fancy object. First, it reads everything into memory, and that’s bad. We’ll create an iterator instead:

>>> class FileIterable(object):
...     def __init__(self, filename):
...         self.filename = filename
...     def __iter__(self):
...         return FileIterator(self.filename)
>>> class FileIterator(object):
...     chunk_size = 4096
...     def __init__(self, filename):
...         self.filename = filename
...         self.fileobj = open(self.filename, 'rb')
...     def __iter__(self):
...         return self
...     def next(self):
...         chunk = self.fileobj.read(self.chunk_size)
...         if not chunk:
...             raise StopIteration
...         return chunk
...     __next__ = next # py3 compat
>>> def make_response(filename):
...     res = Response(content_type=get_mimetype(filename))
...     res.app_iter = FileIterable(filename)
...     res.content_length = os.path.getsize(filename)
...     return res

And testing:

>>> req = Request.blank('/')
>>> print req.get_response(app)
200 OK
Content-Type: text/plain; charset=UTF-8
Content-Length: 35

This is a test.  Hello test people!

Well, that doesn’t look different, but lets imagine that it’s different because we know we changed some code. Now to add some basic metadata to the response:

>>> def make_response(filename):
...     res = Response(content_type=get_mimetype(filename),
...                    conditional_response=True)
...     res.app_iter = FileIterable(filename)
...     res.content_length = os.path.getsize(filename)
...     res.last_modified = os.path.getmtime(filename)
...     res.etag = '%s-%s-%s' % (os.path.getmtime(filename),
...                              os.path.getsize(filename), hash(filename))
...     return res

Now, with conditional_response on, and with last_modified and etag set, we can do conditional requests:

>>> req = Request.blank('/')
>>> res = req.get_response(app)
>>> print res
200 OK
Content-Type: text/plain; charset=UTF-8
Content-Length: 35
Last-Modified: ... GMT
ETag: ...-...

This is a test.  Hello test people!
>>> req2 = Request.blank('/')
>>> req2.if_none_match = res.etag
>>> req2.get_response(app)
<Response ... 304 Not Modified>
>>> req3 = Request.blank('/')
>>> req3.if_modified_since = res.last_modified
>>> req3.get_response(app)
<Response ... 304 Not Modified>

We can even do Range requests, but it will currently involve iterating through the file unnecessarily. When there’s a range request (and you set conditional_response=True) the application will satisfy that request. But with an arbitrary iterator the only way to do that is to run through the beginning of the iterator until you get to the chunk that the client asked for. We can do better because we can use fileobj.seek(pos) to move around the file much more efficiently.

So we’ll add an extra method, app_iter_range, that Response looks for:

>>> class FileIterable(object):
...     def __init__(self, filename, start=None, stop=None):
...         self.filename = filename
...         self.start = start
...         self.stop = stop
...     def __iter__(self):
...         return FileIterator(self.filename, self.start, self.stop)
...     def app_iter_range(self, start, stop):
...         return self.__class__(self.filename, start, stop)
>>> class FileIterator(object):
...     chunk_size = 4096
...     def __init__(self, filename, start, stop):
...         self.filename = filename
...         self.fileobj = open(self.filename, 'rb')
...         if start:
...             self.fileobj.seek(start)
...         if stop is not None:
...             self.length = stop - start
...         else:
...             self.length = None
...     def __iter__(self):
...         return self
...     def next(self):
...         if self.length is not None and self.length <= 0:
...             raise StopIteration
...         chunk = self.fileobj.read(self.chunk_size)
...         if not chunk:
...             raise StopIteration
...         if self.length is not None:
...             self.length -= len(chunk)
...             if self.length < 0:
...                 # Chop off the extra:
...                 chunk = chunk[:self.length]
...         return chunk
...     __next__ = next # py3 compat

Now we’ll test it out:

>>> req = Request.blank('/')
>>> res = req.get_response(app)
>>> req2 = Request.blank('/')
>>> # Re-fetch the first 5 bytes:
>>> req2.range = (0, 5)
>>> res2 = req2.get_response(app)
>>> res2
<Response ... 206 Partial Content>
>>> # Let's check it's our custom class:
>>> res2.app_iter
<FileIterable object at ...>
>>> res2.body
'This '
>>> # Now, conditional range support:
>>> req3 = Request.blank('/')
>>> req3.if_range = res.etag
>>> req3.range = (0, 5)
>>> req3.get_response(app)
<Response ... 206 Partial Content>
>>> req3.if_range = 'invalid-etag'
>>> req3.get_response(app)
<Response ... 200 OK>