PyQt connect inside for loop vs. separate calls results in different behavior

Question

I\'m building a plotting UI that lets a user generate multiple plots from loaded data sets. As part of this the user can add marker lines to their plots to examine (x, y) va

Answer 1

When you connect a signal to a lambda function, the contents of the lambda function are evaluated when the signal is emitted, not when the signal is connected. As such, the variables you use (marker_one and marker_two) always point to the objects created in the last iteration of the loop.

One simple solution is to explicitly pass in marker_one and marker_two as default arguments to variables of the same name, in the signature of the lambda function:

lambda marker_one=marker_one: self.update_marker_vals(marker_one, "Marker One")
lambda marker_two=marker_two: self.update_marker_vals(marker_two, "Marker Two")

There are several useful answers relating to a very similar problem here, specifically the answer by ekhumoro, if you would like to know more (my answer to that question my also be of use, although ekhumoro's solution is cleaner)