Options for the Tk option database

May 03, 2001, 8:41 PM — In the first installment of this series, we mentioned three books as references for the use of the option database in its specific bindings to the Perl, Python, and Tcl languages (see the Resources section at the end of this article for pointers to this and other information). It was an oversight not to recommend Mark Harrison and Michael McLennan's Effective Tcl/Tk Programming at the same time. As master programmer and Expect inventor Don Libes wrote, "Their book is a gem and is the one that finally taught me to use the option database. They use options throughout the entire book -- which makes sense, since their whole book is about 'effective programming.'" Start with section 8.1, "Adding Polish to Your Application."

Effective Tcl makes good reading

Effective Tcl/Tk Programming is good reading for any Tk user, including those who rely on PerlTk and Tkinter. Last month we recommended John Grayson's book on Tkinter to anyone scripting graphical user interfaces, whatever the toolkit. Although Grayson's examples are exclusively in Python, his design principles transcend any particular language. Similarly, Harrison and McLennan communicate coding ideas that you'll find useful no matter what language you're using. For example, their presentation of Tk correctly emphasizes the power and flexibility of the built-in canvas and text widgets.

Effective Tcl/Tk Programming also gives a clear exposition on the use of a script as a configuration file to be evaluated. Sections 4.7.8, "Saving a Drawing," and 7.6.6.3, "Using Tcl Commands to Express Data," explore the notion that "Tcl commands are a powerful way of expressing data." Our last column gave several other examples showing that this holds true for commands in any scripting language.

Another strength of Harrison and McLennan's book is its presentation in chapter 7 of the event-based programming style that we've covered in past Regular Expressions columns.

Back to (optional) work

Recall from the first two installments of this series that Tk uses the option command for all actions taken against the option database. We've already looked at the option add and option readfile subcommands. In this column, we'll explain option clear and option get.

option clear resets all assignments made to the option database by previous option add or option readfile commands. The idea is to return to a fresh wish process, one initialized only with the platform-specific resource database values. Tk implements this by dropping all values, and, at the time of the next use of the resource database, re-reading the external ~/.Xdefaults. This can surprise newcomers. An example helps: if .Xdefaults in your home directory contains the line:

     *Button.background: cyan

and you run the script:

     pack [button .b1 -text Button1]
     pack [button .b2 -text Button2 -bg red]
     pack [button .b3 -text Button3]
     option add *Button.background blue
     option add *Button.foreground yellow
     pack [button .b4 -text Button4]
     option clear
     pack [button .b5 -text Button5]
     option clear
     pack [button .b6 -text Button6]
     pack [button .b7 -text Button7]
     option clear
     pack [button .b8 -text Button8]

through a Tk processor, you'll see a display like this one: