{"id":2115,"date":"2016-09-11T07:21:32","date_gmt":"2016-09-11T05:21:32","guid":{"rendered":"https:\/\/www.opengis.ch\/?p=2115"},"modified":"2020-04-29T16:05:13","modified_gmt":"2020-04-29T14:05:13","slug":"updating-pyqt-signals-that-use-lambda-in-qgis-with-2to3","status":"publish","type":"post","link":"https:\/\/www.opengis.ch\/it\/2016\/09\/11\/updating-pyqt-signals-that-use-lambda-in-qgis-with-2to3\/","title":{"rendered":"Updating PyQt signals that use lambda in QGIS with 2to3"},"content":{"rendered":"

Just for the sake of documenting things, when running qgis 2to3 on a plugin I encountered a tricky situation regarding signals.<\/p>\n

MYQGISDIR\/scripts\/2to3 -f signals -w my\/plugin\/path<\/pre>\n
The original code:<\/p>\n
extra_arg = \"my test argument\"\nQObject.connect(\n  action,\n  SIGNAL( \"triggered()\"),\n  lambda extra_arg=my_arg: show_extra_arg(extra_arg))\ndef do_load_project(extra_arg):\n  print extra_arg # \"my test argument\"\n<\/pre>\n
The generated code:<\/p>\n
extra_arg = \"test_arg\"\naction.triggered.connect(\n  lambda extra_arg=my_arg: show_extra_arg(extra_arg))\ndef do_load_project(extra_arg):\n  print extra_arg # False<\/pre>\n
so in do_load_project<\/em> we get False instead of “my test argument”<\/em>, why?
\nwell due to a subtle difference in the generated code. in the original code we had the signature triggered()<\/em>\u00a0which has no arguments, so in our lambda extra_arg<\/em>\u00a0gets passed my_arg<\/em>.
\nin the generated code,\u00a0triggered<\/em>\u00a0actually has an optional param checked<\/em> [1] which if emitted gets passed to extra_arg<\/em>\u00a0causing the problem.
\nThe correct code (note the additional argument in the lambda definition)<\/p>\n
extra_arg = \"test_arg\"\naction.triggered.connect(\n  lambda checked, extra_arg=my_arg: show_extra_arg(extra_arg))\ndef do_load_project(extra_arg):\n  print extra_arg # False<\/pre>\n
some reference:
\n[0] https:\/\/pyqt.sourceforge.net\/Docs\/PyQt5\/signals_slots.html<\/a>
\n[1] https:\/\/doc.qt.io\/qt-4.8\/qaction.html#triggered<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"
Just for the sake of documenting things, when running qgis 2to3 on a plugin I encountered a tricky situation regarding signals. MYQGISDIR\/scripts\/2to3 -f signals -w my\/plugin\/path The original code: extra_arg = “my test argument” QObject.connect( action, SIGNAL( “triggered()”), lambda extra_arg=my_arg: show_extra_arg(extra_arg)) def do_load_project(extra_arg): print extra_arg # “my test argument” The […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_themeisle_gutenberg_block_has_review":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_post_was_ever_published":false},"categories":[37,16],"tags":[125],"class_list":["post-2115","post","type-post","status-publish","format-standard","hentry","category-pyqt","category-qgis-plugins","tag-qgis-org"],"jetpack_featured_media_url":"","jetpack-related-posts":[{"id":1668,"url":"https:\/\/www.opengis.ch\/it\/2016\/09\/07\/using-threads-in-qgis-python-plugins\/","url_meta":{"origin":2115,"position":0},"title":"Using threads in QGIS python plugins","author":"Marco Bernasocchi","date":"7 Settembre 2016","format":false,"excerpt":"Here an example on how to work with threads in a consistent and clean manner in QGIS python plugins","rel":"","context":"In "Python"","block_context":{"text":"Python","link":"https:\/\/www.opengis.ch\/it\/category\/programming\/python\/"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":2127,"url":"https:\/\/www.opengis.ch\/it\/2016\/09\/19\/qgis2-compatibility-plugin\/","url_meta":{"origin":2115,"position":1},"title":"QGIS2 compatibility plugin","author":"Marco Bernasocchi","date":"19 Settembre 2016","format":false,"excerpt":"Lately I've been spending time porting a bigger plugin from QGIS 2.8 to 3 while maintaining 2.8 compatibility. 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