1 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 2 <html xmlns="http://www.w3.org/1999/xhtml"> 3 <head> 4 <title>Character Encodings</title> 5 <meta name="generator" 6 content="amaya 8.1a, see http://www.w3.org/Amaya/" /> 7 <link href="styles.css" rel="stylesheet" type="text/css" /> 8 </head> 9 <body> 10 <h1>Character Encodings</h1> 11 <p>When writing applications with WebStack, you should try and use 12 Python's Unicode objects as much as possible. However, there are a 13 number of places where plain Python strings can be involved:</p> 14 <ul> 15 <li><a href="parameters.html">Inspecting request parameters</a></li> 16 <li><a href="responses.html">Sending output in a response</a></li> 17 <li><a href="parameters.html">Receiving uploaded content</a></li> 18 <li><a href="state.html">Accessing cookie information</a></li> 19 <li><a href="sessions.html">Accessing session information</a></li> 20 </ul> 21 <p>When Web pages (and other types of content) are sent to and from 22 users of your application, the text will be in some kind of character 23 encoding. For example, in English-speaking environments, the US-ASCII 24 encoding is common and contains the basic letters, numbers and symbols 25 used in English, whereas in Western Europe encodings like 26 ISO-8859-1 and ISO-8859-15 are typically used, since they contain 27 additional letters and symbols in order to support other languages. 28 Often, UTF-8 is used to encode text because it covers most languages 29 simultaneously and is therefore flexible enough for many applications.</p> 30 <p>When URLs are received in applications, in order for some of the 31 request parameters to be interpreted, the situation is a bit more 32 awkward. The original text is encoded in US-ASCII but will contain 33 special numeric codes that indicate character values in the 34 original text encoding - see the <a href="parameters.html">description 35 of query strings</a> for more information.</p> 36 <h2>Recommendations</h2> 37 <dl> 38 <dt>The following recommendations should help you avoid issues with 39 incorrect characters in the Web pages (and other content) that you 40 produce:</dt> 41 </dl> 42 <h3>Use Unicode Objects for Textual Content</h3> 43 <p>Handling text in specific encodings using normal Python strings can 44 be difficult, and handling text in multiple encodings in the same 45 application can be highly error-prone. Fortunately, Python has support 46 for Unicode objects which let you think of letters, numbers, symbols 47 and all other characters in an abstract way.</p> 48 <ul> 49 <li>Convert textual content to Unicode as soon as possible (see below 50 for choosing encodings).</li> 51 <li>If you must include hard-coded messages in your application code, 52 make sure to specify the encoding using the <a 53 href="http://www.python.org/peps/pep-0263.html">standard declaration</a> 54 at the top of your source file.</li> 55 <li>Remember that the standard library <code>codecs</code> 56 module contains useful functions to access streams as if Unicode 57 objects were being transmitted; for example:</li> 58 </ul> 59 <pre>import codecs<br /><br />class MyResource:<br /><br /> encoding = "utf-8"<br /><br /> def respond(self, trans):<br /> stream = trans.get_request_stream() # only reads strings<br /> unicode_stream = codecs.getreader(self.encoding)(stream) # reads Unicode objects<br /><br /> [Some activity...]<br /><br /> out = trans.get_response_stream() # only writes strings<br /> unicode_out = codecs.getwriter(self.encoding)(out) # writes Unicode objects</pre> 60 <h3>Use Strings for Binary Content</h3> 61 <p>If you are reading and writing binary content, Unicode objects are 62 inappropriate. Make sure to open files in binary mode, where necessary.</p> 63 <h3>Use Explicit Encodings and Be Consistent</h3> 64 <p>Although WebStack has some support for detecting character encodings 65 used 66 in requests, it is often best for your application to exercise control 67 over 68 which encoding is used when <a href="parameters.html">inspecting 69 request 70 parameters</a> and when <a href="responses.html">producing responses</a>. 71 The 72 best way to do this is to decide which encoding is most suitable for 73 the data 74 presented and received in your application and then to use it 75 throughout. 76 Here is an outline of code which does this:</p> 77 <pre>from WebStack.Generic import ContentType<br /><br />class MyResource:<br /><br /> encoding = "utf-8" # We decide on "utf-8" as our chosen<br /> # encoding.<br /> def respond(self, trans):<br /> [Do various things.]<br /><br /> fields = trans.get_fields_from_body(encoding=self.encoding) # Explicitly use the encoding.<br /><br /> [Do other things with the Unicode values from the fields.]<br /><br /> trans.set_content_type(ContentType("text/html", self.encoding)) # The output Web page uses the encoding.<br /><br /> [Produce the response, making sure that self.encoding is used to convert Unicode to raw strings.]</pre> 78 <h3>Tell Encodings to Other Components</h3> 79 <p>When using other components to generate content (see <a 80 href="integrating.html">"Integrating with Other Systems"</a>), it may 81 be the case that such components will just write the generated content 82 straight to a normal stream (rather than one wrapped by a <code>codecs</code> 83 module function). In such cases, it is likely that for textual content 84 such as XML or related formats (XHTML, SVG, HTML) you will need to 85 instruct the component to use your chosen encoding; for example:</p> 86 <pre> # In the respond method, xml_document is an xml.dom.minidom.Document object...<br /> xml_document.toxml(self.encoding)</pre> 87 <p>This will then generate the appropriate characters in the output <span 88 style="font-style: italic;">and</span> specify the correct encoding 89 for the XML document.</p> 90 </body> 91 </html>