A Simple Python Example

 

Overview

The following Python service prepends a greeting message to the selected text. This is a trivial example in that there is no GUI interaction, does not utilize any additional Python libraries, nor does it deal with multiple pasteboard types. It does, however, illustrate the basic points to consider when implementing services with Python.

 

A Simple Example: Prepend Greeting

The following code listing shows a complete Bellhop service script written in the Python language. You can try this yourself by simply creating a Bellhop document, adding a new Python service, and then pasting the following code directly into the editor window. Then all you have to do is activate the service using the Settings panel, and then you can invoke it from any services-aware application.

# Sample Python service to prepend a greeting to the selected text. This # service might be labeled "Prepend Greeting" in the Services menu. # The aPasteboard argument identifies the unique pasteboard name # that is reserved for the current service transaction. You will # use this name as the argument to the various methods that get # and set data from the pasteboard. def runService (aPasteboard): # read from pasteboard theString = pasteboard.readstring(aPasteboard, NSStringPboardType) # format new string by adding greeting theString = 'Greetings from Python: ' + theString # write new string back to pboard pasteboard.declaretypes(aPasteboard, [NSStringPboardType]) pasteboard.writestring(aPasteboard, NSStringPboardType, theString)

Listing 1: Sample Python service

If you are familiar with the concepts and implementation details regarding system services in Mac OS X, the above script should be straightforward. If not, you can refer to the Apple developer documentation covering System Services and the More Info section of this document.

Looking at Listing 1, the first non-comment line is

def runService (aPasteboard):

This is the entry to your service. Almost all of your runService() functions will follow a similar pattern: read pasteboard data, operate on it, and write modified pasteboard data back. The only argument to this function, aPasteboard, is the particular pasteboard that is used for this service transaction. Each service transaction is assigned a unique service pasteboard.

The following lines read the selection from the service pasteboard:

# read from pasteboard theString = pasteboard.readstring(aPasteboard, NSStringPboardType)

This line uses the Bellhop pasteboard extension for Python to get a string from the pasteboard. Notice the use of the NSStringPboardType constant to specify string type data. This is one of the global pasteboard type constants that is made available to all Bellhop Python scripts, and mimics the same symbolic constant as defined in the Cocoa frameworks.

Next we manipulate the pasteboard data to modify it. We are simply going to add a greeting:

# format new string by adding greeting theString = 'Greetings from Python:' + theString

This line simply concatenates our Python greeting and the selected text that was read from the pasteboard. The resulting string is stored so that we can write it back to the pasteboard. This is the meat of our service.

Finally, we must write the modified data back to the pasteboard so that it is made available to the requesting application:

# Write new string back to pboard pasteboard.declaretypes(aPasteboard, [NSStringPboardType]) pasteboard.writestring(aPasteboard, NSStringPboardType, theString)

These lines first inform aPasteboard (our unique service pasteboard) that we are about to provide it with string data. Then we actually write the modified string to aPasteboard. Again, notice that we are using the custom Bellhop pasteboard extension to deal with the Mac OS X pasteboard.

It is worth pointing out that we did not have to explicitly import any of the custom Bellhop Python extensions, and that we are using constants that mimic those defined in the Cocoa frameworks.

That's all there is to it! When a user invokes our "Prepend Greeting" service from the Services menu (e.g. from the TextEdit application), the operating system will automatically forward the request to Bellhop, which will in turn invoke the appropriate script's runService() function. All of your services will follow a similar pattern.