author | Lennard de Rijk <ljvderijk@gmail.com> |
Sat, 04 Jul 2009 16:23:00 +0200 | |
changeset 2535 | d987dc40ea5d |
parent 109 | 620f9b141567 |
permissions | -rw-r--r-- |
109
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parents:
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1 |
Comment Example |
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2 |
=============== |
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parents:
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3 |
|
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4 |
.. contents:: |
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5 |
|
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6 |
Introduction |
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7 |
------------ |
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8 |
|
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9 |
This is an example of how to write WSGI middleware with WebOb. The |
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10 |
specific example adds a simple comment form to HTML web pages; any |
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11 |
page served through the middleware that is HTML gets a comment form |
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12 |
added to it, and shows any existing comments. |
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13 |
|
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14 |
Code |
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15 |
---- |
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16 |
|
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17 |
The finished code for this is available in |
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18 |
`docs/comment-example-code/example.py |
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19 |
<http://svn.pythonpaste.org/Paste/WebOb/trunk/docs/comment-example-code/example.py>`_ |
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-- you can run that file as a script to try it out. |
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21 |
|
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22 |
Instantiating Middleware |
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23 |
------------------------ |
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24 |
|
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25 |
Middleware of any complexity at all is usually best created as a |
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26 |
class with its configuration as arguments to that class. |
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27 |
|
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28 |
Every middleware needs an application (``app``) that it wraps. This |
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29 |
middleware also needs a location to store the comments; we'll put them |
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30 |
all in a single directory. |
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31 |
|
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32 |
.. code-block:: |
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33 |
|
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34 |
import os |
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35 |
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36 |
class Commenter(object): |
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37 |
def __init__(self, app, storage_dir): |
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self.app = app |
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39 |
self.storage_dir = storage_dir |
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40 |
if not os.path.exists(storage_dir): |
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41 |
os.makedirs(storage_dir) |
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42 |
|
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43 |
When you use this middleware, you'll use it like: |
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44 |
|
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45 |
.. code-block:: |
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46 |
|
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47 |
app = ... make the application ... |
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48 |
app = Commenter(app, storage_dir='./comments') |
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49 |
|
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50 |
For our application we'll use a simple static file server that is |
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51 |
included with `Paste <http://pythonpaste.org>`_ (use ``easy_install |
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52 |
Paste`` to install this). The setup is all at the bottom of |
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53 |
``example.py``, and looks like this: |
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54 |
|
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55 |
.. code-block:: |
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56 |
|
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57 |
if __name__ == '__main__': |
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58 |
import optparse |
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59 |
parser = optparse.OptionParser( |
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60 |
usage='%prog --port=PORT BASE_DIRECTORY' |
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61 |
) |
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62 |
parser.add_option( |
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63 |
'-p', '--port', |
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64 |
default='8080', |
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65 |
dest='port', |
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66 |
type='int', |
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67 |
help='Port to serve on (default 8080)') |
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68 |
parser.add_option( |
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69 |
'--comment-data', |
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70 |
default='./comments', |
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71 |
dest='comment_data', |
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72 |
help='Place to put comment data into (default ./comments/)') |
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73 |
options, args = parser.parse_args() |
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74 |
if not args: |
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75 |
parser.error('You must give a BASE_DIRECTORY') |
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76 |
base_dir = args[0] |
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77 |
from paste.urlparser import StaticURLParser |
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78 |
app = StaticURLParser(base_dir) |
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79 |
app = Commenter(app, options.comment_data) |
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80 |
from wsgiref.simple_server import make_server |
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81 |
httpd = make_server('localhost', options.port, app) |
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82 |
print 'Serving on http://localhost:%s' % options.port |
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83 |
try: |
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84 |
httpd.serve_forever() |
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85 |
except KeyboardInterrupt: |
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86 |
print '^C' |
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87 |
|
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88 |
I won't explain it here, but basically it takes some options, creates |
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89 |
an application that serves static files |
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90 |
(``StaticURLParser(base_dir)``), wraps it with ``Commenter(app, |
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91 |
options.comment_data)`` then serves that. |
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92 |
|
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93 |
The Middleware |
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94 |
-------------- |
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95 |
|
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96 |
While we've created the class structure for the middleware, it doesn't |
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97 |
actually do anything. Here's a kind of minimal version of the |
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98 |
middleware (using WebOb): |
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99 |
|
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100 |
.. code-block:: |
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101 |
|
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102 |
from webob import Request |
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103 |
|
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104 |
class Commenter(object): |
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105 |
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106 |
def __init__(self, app, storage_dir): |
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107 |
self.app = app |
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108 |
self.storage_dir = storage_dir |
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109 |
if not os.path.exists(storage_dir): |
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110 |
os.makedirs(storage_dir) |
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111 |
|
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112 |
def __call__(self, environ, start_response): |
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113 |
req = Request(environ) |
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114 |
resp = req.get_response(self.app) |
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115 |
return resp(environ, start_response) |
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116 |
|
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117 |
This doesn't modify the response it any way. You could write it like |
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118 |
this without WebOb: |
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119 |
|
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120 |
.. code-block:: |
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121 |
|
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122 |
class Commenter(object): |
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123 |
... |
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124 |
def __call__(self, environ, start_response): |
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125 |
return self.app(environ, start_response) |
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126 |
|
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127 |
But it won't be as convenient later. First, lets create a little bit |
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of infrastructure for our middleware. We need to save and load |
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129 |
per-url data (the comments themselves). We'll keep them in pickles, |
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130 |
where each url has a pickle named after the url (but double-quoted, so |
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131 |
``http://localhost:8080/index.html`` becomes |
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132 |
``http%3A%2F%2Flocalhost%3A8080%2Findex.html``). |
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133 |
|
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134 |
.. code-block:: |
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135 |
|
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136 |
from cPickle import load, dump |
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137 |
|
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138 |
class Commenter(object): |
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139 |
... |
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140 |
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141 |
def get_data(self, url): |
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filename = self.url_filename(url) |
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143 |
if not os.path.exists(filename): |
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144 |
return [] |
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145 |
else: |
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146 |
f = open(filename, 'rb') |
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data = load(f) |
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148 |
f.close() |
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149 |
return data |
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150 |
|
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151 |
def save_data(self, url, data): |
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filename = self.url_filename(url) |
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f = open(filename, 'wb') |
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154 |
dump(data, f) |
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155 |
f.close() |
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156 |
|
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157 |
def url_filename(self, url): |
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158 |
# Double-quoting makes the filename safe |
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159 |
return os.path.join(self.storage_dir, urllib.quote(url, '')) |
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160 |
|
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161 |
You can get the full request URL with ``req.url``, so to get the |
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162 |
comment data with these methods you do ``data = |
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163 |
self.get_data(req.url)``. |
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164 |
|
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165 |
Now we'll update the ``__call__`` method to filter *some* responses, |
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166 |
and get the comment data for those. We don't want to change responses |
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167 |
that were error responses (anything but ``200``), nor do we want to |
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168 |
filter responses that aren't HTML. So we get: |
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169 |
|
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170 |
.. code-block:: |
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171 |
|
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172 |
class Commenter(object): |
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173 |
... |
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174 |
|
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175 |
def __call__(self, environ, start_response): |
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176 |
req = Request(environ) |
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177 |
resp = req.get_response(self.app) |
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178 |
if resp.content_type != 'text/html' or resp.status_int != 200: |
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179 |
return resp(environ, start_response) |
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180 |
data = self.get_data(req.url) |
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181 |
... do stuff with data, update resp ... |
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182 |
return resp(environ, start_response) |
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183 |
|
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184 |
So far we're punting on actually adding the comments to the page. We |
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185 |
also haven't defined what ``data`` will hold. Let's say it's a list |
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186 |
of dictionaries, where each dictionary looks like ``{'name': 'John |
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187 |
Doe', 'homepage': 'http://blog.johndoe.com', 'comments': 'Great |
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188 |
site!'}``. |
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189 |
|
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190 |
We'll also need a simple method to add stuff to the page. We'll use a |
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191 |
regular expression to find the end of the page and put text in: |
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192 |
|
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193 |
.. code-block:: |
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194 |
|
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195 |
import re |
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196 |
|
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197 |
class Commenter(object): |
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198 |
... |
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199 |
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200 |
_end_body_re = re.compile(r'</body.*?>', re.I|re.S) |
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201 |
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202 |
def add_to_end(self, html, extra_html): |
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203 |
""" |
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204 |
Adds extra_html to the end of the html page (before </body>) |
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205 |
""" |
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206 |
match = self._end_body_re.search(html) |
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207 |
if not match: |
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208 |
return html + extra_html |
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209 |
else: |
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210 |
return html[:match.start()] + extra_html + html[match.start():] |
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211 |
|
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212 |
And then we'll use it like: |
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213 |
|
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214 |
.. code-block:: |
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215 |
|
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216 |
data = self.get_data(req.url) |
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217 |
body = resp.body |
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218 |
body = self.add_to_end(body, self.format_comments(data)) |
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219 |
resp.body = body |
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220 |
return resp(environ, start_response) |
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221 |
|
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222 |
We get the body, update it, and put it back in the response. This |
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223 |
also updates ``Content-Length``. Then we define: |
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224 |
|
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225 |
.. code-block:: |
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226 |
|
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227 |
from webob import html_escape |
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228 |
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class Commenter(object): |
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230 |
... |
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231 |
|
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def format_comments(self, comments): |
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if not comments: |
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return '' |
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text = [] |
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text.append('<hr>') |
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text.append('<h2><a name="comment-area"></a>Comments (%s):</h2>' % len(comments)) |
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for comment in comments: |
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text.append('<h3><a href="%s">%s</a> at %s:</h3>' % ( |
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html_escape(comment['homepage']), html_escape(comment['name']), |
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241 |
time.strftime('%c', comment['time']))) |
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# Susceptible to XSS attacks!: |
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text.append(comment['comments']) |
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return ''.join(text) |
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245 |
|
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246 |
We put in a header (with an anchor we'll use later), and a section for |
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247 |
each comment. Note that ``html_escape`` is the same as ``cgi.escape`` |
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and just turns ``&`` into ``&``, etc. |
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249 |
|
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250 |
Because we put in some text without quoting it is susceptible to a |
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251 |
`Cross-Site Scripting |
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<http://en.wikipedia.org/wiki/Cross-site_scripting>`_ attack. Fixing |
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that is beyond the scope of this tutorial; you could quote it or clean |
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254 |
it with something like `lxml.html.clean |
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<http://codespeak.net/lxml/lxmlhtml.html#cleaning-up-html>`_. |
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256 |
|
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257 |
Accepting Comments |
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258 |
------------------ |
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259 |
|
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260 |
All of those pieces *display* comments, but still no one can actually |
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261 |
make comments. To handle this we'll take a little piece of the URL |
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262 |
space for our own, everything under ``/.comments``, so when someone |
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POSTs there it will add a comment. |
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264 |
|
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265 |
When the request comes in there are two parts to the path: |
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``SCRIPT_NAME`` and ``PATH_INFO``. Everything in ``SCRIPT_NAME`` has |
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already been parsed, and everything in ``PATH_INFO`` has yet to be |
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268 |
parsed. That means that the URL *without* ``PATH_INFO`` is the path |
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269 |
to the middleware; we can intercept anything else below |
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270 |
``SCRIPT_NAME`` but nothing above it. The name for the URL without |
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``PATH_INFO`` is ``req.application_url``. We have to capture it early |
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272 |
to make sure it doesn't change (since the WSGI application we are |
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273 |
wrapping may update ``SCRIPT_NAME`` and ``PATH_INFO``). |
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274 |
|
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275 |
So here's what this all looks like: |
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276 |
|
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277 |
.. code-block:: |
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278 |
|
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279 |
class Commenter(object): |
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280 |
... |
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281 |
|
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282 |
def __call__(self, environ, start_response): |
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283 |
req = Request(environ) |
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284 |
if req.path_info_peek() == '.comments': |
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return self.process_comment(req)(environ, start_response) |
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286 |
# This is the base path of *this* middleware: |
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287 |
base_url = req.application_url |
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288 |
resp = req.get_response(self.app) |
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289 |
if resp.content_type != 'text/html' or resp.status_int != 200: |
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290 |
# Not an HTML response, we don't want to |
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291 |
# do anything to it |
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292 |
return resp(environ, start_response) |
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293 |
# Make sure the content isn't gzipped: |
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294 |
resp.decode_content() |
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295 |
comments = self.get_data(req.url) |
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296 |
body = resp.body |
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297 |
body = self.add_to_end(body, self.format_comments(comments)) |
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298 |
body = self.add_to_end(body, self.submit_form(base_url, req)) |
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299 |
resp.body = body |
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300 |
return resp(environ, start_response) |
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301 |
|
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302 |
``base_url`` is the path where the middleware is located (if you run |
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303 |
the example server, it will be ``http://localhost:PORT/``). We use |
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304 |
``req.path_info_peek()`` to look at the next segment of the URL -- |
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305 |
what comes after base_url. If it is ``.comments`` then we handle it |
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306 |
internally and don't pass the request on. |
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307 |
|
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308 |
We also put in a little guard, ``resp.decode_content()`` in case the |
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309 |
application returns a gzipped response. |
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310 |
|
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311 |
Then we get the data, add the comments, add the *form* to make new |
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312 |
comments, and return the result. |
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313 |
|
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314 |
submit_form |
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315 |
~~~~~~~~~~~ |
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316 |
|
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317 |
Here's what the form looks like: |
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318 |
|
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319 |
.. code-block:: |
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320 |
|
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321 |
class Commenter(object): |
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322 |
... |
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323 |
|
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324 |
def submit_form(self, base_path, req): |
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325 |
return '''<h2>Leave a comment:</h2> |
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326 |
<form action="%s/.comments" method="POST"> |
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327 |
<input type="hidden" name="url" value="%s"> |
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328 |
<table width="100%%"> |
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329 |
<tr><td>Name:</td> |
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330 |
<td><input type="text" name="name" style="width: 100%%"></td></tr> |
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331 |
<tr><td>URL:</td> |
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332 |
<td><input type="text" name="homepage" style="width: 100%%"></td></tr> |
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333 |
</table> |
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334 |
Comments:<br> |
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335 |
<textarea name="comments" rows=10 style="width: 100%%"></textarea><br> |
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336 |
<input type="submit" value="Submit comment"> |
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337 |
</form> |
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338 |
''' % (base_path, html_escape(req.url)) |
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339 |
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340 |
Nothing too exciting. It submits a form with the keys ``url`` (the |
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341 |
URL being commented on), ``name``, ``homepage``, and ``comments``. |
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342 |
|
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343 |
process_comment |
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344 |
~~~~~~~~~~~~~~~ |
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345 |
|
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346 |
If you look at the method call, what we do is call the method then |
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347 |
treat the result as a WSGI application: |
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348 |
|
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349 |
.. code-block:: |
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350 |
|
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351 |
return self.process_comment(req)(environ, start_response) |
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352 |
|
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353 |
You could write this as: |
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354 |
|
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355 |
.. code-block:: |
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356 |
|
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357 |
response = self.process_comment(req) |
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358 |
return response(environ, start_response) |
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359 |
|
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360 |
A common pattern in WSGI middleware that *doesn't* use WebOb is to |
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361 |
just do: |
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362 |
|
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363 |
.. code-block:: |
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364 |
|
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365 |
return self.process_comment(environ, start_response) |
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366 |
|
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367 |
But the WebOb style makes it easier to modify the response if you want |
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368 |
to; modifying a traditional WSGI response/application output requires |
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369 |
changing your logic flow considerably. |
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370 |
|
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371 |
Here's the actual processing code: |
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372 |
|
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373 |
.. code-block:: |
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374 |
|
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375 |
from webob import exc |
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376 |
from webob import Response |
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377 |
|
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378 |
class Commenter(object): |
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379 |
... |
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380 |
|
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381 |
def process_comment(self, req): |
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382 |
try: |
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383 |
url = req.params['url'] |
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384 |
name = req.params['name'] |
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385 |
homepage = req.params['homepage'] |
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386 |
comments = req.params['comments'] |
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387 |
except KeyError, e: |
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388 |
resp = exc.HTTPBadRequest('Missing parameter: %s' % e) |
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389 |
return resp |
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390 |
data = self.get_data(url) |
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391 |
data.append(dict( |
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392 |
name=name, |
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393 |
homepage=homepage, |
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394 |
comments=comments, |
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395 |
time=time.gmtime())) |
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396 |
self.save_data(url, data) |
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397 |
resp = exc.HTTPSeeOther(location=url+'#comment-area') |
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398 |
return resp |
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|
399 |
|
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400 |
We either give a Bad Request response (if the form submission is |
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401 |
somehow malformed), or a redirect back to the original page. |
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|
402 |
|
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|
403 |
The classes in ``webob.exc`` (like ``HTTPBadRequest`` and |
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|
404 |
``HTTPSeeOther``) are Response subclasses that can be used to quickly |
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405 |
create responses for these non-200 cases where the response body |
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406 |
usually doesn't matter much. |
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|
407 |
|
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|
408 |
Conclusion |
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|
409 |
---------- |
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|
410 |
|
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|
411 |
This shows how to make response modifying middleware, which is |
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412 |
probably the most difficult kind of middleware to write with WSGI -- |
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|
413 |
modifying the request is quite simple in comparison, as you simply |
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|
414 |
update ``environ``. |