QGIS run python script upon print composer export

Question

I'm looking for a way to execute a python script upon exporting a print composer. Specifically, I want it to run once for each page exported. (logging some data and possibly updating a database).

So far I've created a custom expression that writes a log to a file and returns an empty string. I created an off-page label that runs the expression.

The problem is, this runs every time the page refreshes so it's running a bunch just paging through the atlas.

Trying to get it to run only on export, I put it in the 'export filename' field but that too runs every time the page refreshes.

Is there some way I can either force it to only run on export, or in the python code detect if the page is being exported before running?

edit: here's the test code I'm running so far:

def logTest(logitem):
 global logFileName
 f = open(logFileName, 'a')
 f.write("{}\n".format(logitem))
 f.close()
@qgsfunction(args='auto', group='Territory')
def postData(postItem, feature, parent):
 logTest(str(postItem))
 return u''

and my expression to test it is simply:

postData("id")

• user52245: Doesn't seem possible. Created a feature request here: hub.qgis.org/issues/15726
• Vincent_v_E: Could you make a workaround based on catching the QgsComposerMap.preparedForAtlas() signal?
• user52245: Thanks for your suggestion. I don't know how to do that but I will research.
• underdark: @JesseMcMillan Please post the feature request as an answer.

Answer 1

Not currently possible. Feature request here: https://issues.qgis.org/issues/15726