Hi Michael, thank you so much for your answer!
I just started to learn how Python and its syntax work, so I apologize in advance if I’m going to ask you and the other users silly questions 
So, the code I posted here has some differences from the original code you wrote back in 2015.
First of all, I implemented your code to my template following the instructions you gave @paulaluegi on the Google Forum. Everything worked fine on the visual side, but not for the collection of the IRI.
Then, I searched some more information here on the forum, because I wanted to display a line of dashes to cover the words without revealing the number of letters of each word of the sentence. I followed the instructions I found in the threads started by @tjmccormick5 https://discourse.psychopy.org/t/self-paced-reading-space-doesnt-always-move-window/4075 and @Kyung https://discourse.psychopy.org/t/how-do-i-get-moving-window-presentation-in-self-paced-reading-task/2250. I edited my code and tried to run the experiment: again, I achieved my goal on the visual side by masking all the words, but I couldn’t collect any IRI value except for the IRI_0.
I think that I definitely wrote something wrong while editing my code 
Again, thank you for your help Michael, I really appreciate it!