inserting colon automatically enters first suggestion from a drop down list

Hi all, 

I am not so much having an issue more than a feature i would like to turn of but not entirely sure what this is called!

I used the ' : ' a lot when coding (for example 0:6, 8:14 etc...) 

Every time i begin to type the first number, an automatic drop down suggestion comes up and when i type in the ' : ' it automatically enters the first suggestion (y_train in this case)

I am really sorry as i am unsure about what this feature is. Is there a way to turn this of in preferences?

thank you!