Jim Hoskins

I wanted to post a quick guide to the special ruby syntax for literals that utilize the % (percent) symbol. Most beginners guides to ruby leave out an explanation of these forms of literals, but many ruby coders use them. When someone encounters them for the first time it is almost impossible to figure out what they mean. (Try searching Google for “%w”)

Ruby has special syntax for making strings, arrays and system commands easier to write. They allow you to use different characters as delimiters so you can minimize escaping in your literals.

The syntax

The syntax for the % literals is a percent symbol (%) a letter which defines what kind of literal it is (Q, q, w, x, r) a delimiter,  the content, and the closing delimiter.

The delimiter can be any character, and is defined as whatever is immediately after the letter in the syntax. For example %Q!content! , the delimiter is the ! and it surrounds the content. There are special cases when the delimiter is { or (, the closing delimiter will be } or ) respectively.

%Q and %q (Percent Q): Strings

%Q!Some String of “Characters”! <==> ” Some String of \”Characters\” “

%Q is the equivalent to a double-quoted ruby string. #{expression} evaluation works just like in double-quoted strings, even if you use %Q{} as your delimiter!

You can also leave off the Q and it will have the same functionality. I recommend leaving the Q in to be more clear.

%q!Some String of “Characters”! <==> ‘Some String of Characters’

The %q is just like %Q, but acts the same as a single-quoted string. Whatever is inside the delimiters is returned as a string.

You can remember %Q is for strings because it acts like Quotes.

More info here: http://docs.huihoo.com/ruby/ruby-man-1.4/syntax.html#string

%W (Percent W): Arrays

%W(North South East West) <==> ["North", "South", "East", "West"]

%W (and %w) allow you to create an Array of strings without using quotes and commas.

The delimiter rules are the same as strings, but typically parentheses are used. The content inside the delimiters are split by white-space, and put into an array. This is great if you have a hard coded list of single word strings.

You can remember %W is by thinking of it as a White-space divided Array.

More info here : http://docs.huihoo.com/ruby/ruby-man-1.4/syntax.html#array

%x (Percent x): System Execution

%x{ ls /usr/local } <==> `ls /usr/local`

%x allows you to call system commands, equivilent to wrapping the command in `s (grave accents). The benefit of the $x{} syntax is you don’t have to escape your accents in commands that use them.

You can remember to use X because it eXecutes a command.

More info here: http://docs.huihoo.com/ruby/ruby-man-1.4/syntax.html#command

%r (Percent r): Regular Expressions

%r{/usr/bin/} <==> /\/user\/bin\//

%r is really handy for regular expressions that contain /s (forward slashes) which are the default delimiter for regular expressions and have to be escaped.

Remember to use %r with regular expressions.

More info here: http://docs.huihoo.com/ruby/ruby-man-1.4/syntax.html#regexp

I hope this information is helpful. Please leave a comment if this helped or if I left something out.

Many applications require the use of password based authentication. In many web applications, this is done using cookies/sessions, and the login mechanism is an HTML form that is submitted to the application. If you are trying to build a RESTful web service, or any service where this client may be automated, It becomes useful to utilize HTTP authentication.

It may not be immediately obvious from the CherryPy documentation, but CherryPy includes all the tools needed to implement HTTP authentication, both Digest and Basic. You can see what the CherryPy website has documented about authentication at the CherryPy Builtin Tools Page. The best documentation for this is available in the book: CherryPy Essentials: Rapid Python Web Application Development

Basic vs Digest

HTTP supports two types of authentication, basic and digest. To a user accessing a protected page through a web browser, both types look the same. The browser will prompt the user for a username and password. The difference is how the client (browser) sends the password to the server.

With basic authentication, the client sends the password in clear-text, so any malicious attacker who can see the request can see the password. With digest authentication, the client does not send the password, but rather a digest based on the password and other factors. The server will also compute the digest using the known correct password. If the digest sent by the client matches the one calculated by the server, authentication succeeds.

The catch with digest authentication is the users’ passwords must be stored stored in clear-text on the server** in order to calculate the digest. On the flipside, basic authentication must send the password in clear-text through the network. If you are going to use basic authentication, it is recommended you use SSL to prevent the password from being sniffed.

** This is not entirely true, The digest algorithm allows you to store a partially digested password on the server side, however CherryPy has no easy way to specify this.

Digest Authentication

The following code creates a simple application with a public page and a secure page using digest authentication. The server configuration defines that the /secure url is to be protected with digest authentication (via the tools.digest_auth Tool available in CherryPy)

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