Statistics has become widely adopted for low costs, minimal staffing needs, and accurate data interpretations. However, university students often struggle with learning statistics due to the complexity. To solve it, we have studied its self-learning with Python in the web-based Python Programming Assistant Learning System (PLAS). In this study, we present the Comment Insertion Problem (CIP) to help students learn inferential statistics by themselves. CIP instances ask inserting appropriate comments into the blank sections of the given Python source code. For evaluations, we generated five CIP instances on inferential statistics, and assigned them to the second-year undergraduate students at Universitas Pembangunan Nasional Veteran Jawa Timur, Indonesia, where the students successfully solved them.