O’Reilly has a nice article about programming proficiency written by Gregory Brown.

The traits of a proficient programmer
Bridging the gap between competence and proficiency
https://www.oreilly.com/ideas/the-traits-of-a-proficient-programmer

I think he describes the T-shaped professional that human resources folks love so much.

Pick a small number of specific skills you’re simply good but not great at, and then develop competence to an extreme, almost obsessive degree.

Yep, that’s a T.