This paper presents a design-based research which focuses on the design and the evaluation of the Pyrates online application. This serious game aims to introduce Python programming language supporting the transition from block-based languages. The layout of Pyrates' learning environment is inspired from beneficial features of block-based programming editors. In order to evaluate this design, the application has been tested in eight classrooms with French 10-th grade students (n = 240). Self-generated activity traces have been collected (n = 69, 701) and supplemented by a qualitative online survey. The data analysis shows that some of the design choices conduct to the expected effects. The creation of a "programming memo" (synthesized documentation) allows the discovery of algorithmic notions while offering a reference support for the Python syntax. The ease of copy-paste from this memo limits keyboarding. The integration of a syntax analyzer designed for beginners gives students a high level of autonomy in handling errors. However, other choices have rather deleterious impacts. For instance, the creation of a control panel for program executions proves to be dedicated to a trialand-error programming approach or to "notional bypassing" strategies.