Dynamic loader

There are two aspects of Pmw, unrelated to megawidgets, that require special attention. Firstly, Pmw is made up of many sub-modules, potentially making access to its various classes and functions cumbersome for the user. Secondly, Pmw is regularly being modified and added to, thus requiring the release of new versions. Therefore, techniques for making access to the sub-modules easy and efficient and for dealing with the different versions have been developed. These techniques are incorporated into the dynamic loader which Pmw creates when it is first imported.

The first purpose of the loader is to give access to all Pmw classes and functions through a single entry point, the Pmw. prefix. For example, to access the ComboBox class (which resides in one of the sub-modules of Pmw), you just have to use Pmw.ComboBox. Without the loader, this would be a more complicated reference, such as, hypothetically, Pmw.PmwComboBox.Combobox.

The second purpose of the loader is to delay the importing of the sub-modules until they are needed. This improves the startup time of applications which only use a few Pmw megawidgets. It also allows more megawidgets to be added to the library without slowing down applications which do not use them.

The third purpose of the loader is to allow a script using Pmw to specify which version of Pmw it requires. This allows an application to continue working correctly even after newer releases of Pmw have been made which are not compatible with the version expected by the application. Several versions of Pmw can be installed at once, with the actual version used being specified by each application. In addition, the loader can be configured to search in one or more alpha versions of Pmw. These versions may contain new megawidgets, or new versions of existing megawidgets, that are currently not in the base releases.

Several functions are available to set and query the version of Pmw being used. These are Pmw.setversion() and Pmw.setalphaversions() which specify the version and alpha versions (if any) to use for this session; Pmw.version() which returns the version(s) being used by this session; and Pmw.installedversions() which returns the version(s) of Pmw currently installed. These are described in the Pmw functions reference manual.

When Pmw is first imported, an instance of PmwLoader is created and placed into sys.modules['Pmw']. From that point on, any reference to attributes of the Pmw 'module' is handled by the loader. The real Pmw package is stored in sys.modules['_Pmw'].

The loader searches the Pmw package base directory for sub-directories with the prefixes Pmw_ and Alpha_, which contain Pmw base releases and alpha releases. The version numbers are given by the part of the directory name following the prefix. These versions are available for use and are those returned by the Pmw.installedversions function. The current version is set to the base release with the greatest version number. When the first reference to a Pmw class or function is made, the loader reads the files named Pmw.def in the current base version directory and also in the alpha directories (if any). These files list all the classes and functions supported by the version. Pmw attributes are first searched for in the alpha directories and then in the base version directory. The first directory which supports the reference is used. In this way, alpha versions override base versions.

The directory Alpha_99_9_example contains a simple example of how to structure an alpha version. The following code can be used to request that the alpha version be used and then creates an instance of a new megawidget defined in the alpha version.

 import Pmw
 Pmw.setalphaversions('99.9.example')

 # Create a standard message dialog using the base Pmw version.
 ordinary = Pmw.MessageDialog(
     message_text = 'Ordinary\nPmw Dialog')

 # Create an example dialog using the alpha Pmw version.
 alpha = Pmw.AlphaExample()