This study investigated common student errors and underlying difficulties of two groups of Chinese middle school students in an introductory Python programming course using data in the automated assessment tool (AAT) Mulberry. One group of students was from a typical middle school while the other group was from a high-ability middle school. By analyzing 8030 erroneous student programs, we identified 12 common errors of the two groups. Further analysis indicated that the two groups had similarities and differences in the difficulties of learning to program. Our findings suggest that using AATs can effectively help teachers understand student difficulties in introductory programming.