If a programmer understands the language, then the programmer can understand what the program is doing and why it is producing the observed output. The programmer can determine a proper fix or devise meaningful experiments to better understand what values are being manipulated at run time.
Unfortunately, not all students are taught these simple, effective techniques. I have been horrified to see instructors (even at my own institution!) teach bad habits by telling students, "The only way to know what this program does is to run it." Many programming textbooks and websites are just as bad: they don't explain the programming model, or they do so in vague English.
The Python Evaluation Rules document gives precise semantics for much of the Python language. It presents step-by-step rules for executing a Python program. Every skilled programmer uses such rules to reason about the effects of Python code. This document helps beginners to become experts more quickly. With this knowledge, a programmer finds it easier to write correct code, debug incorrect code, and read unfamiliar code.
The document applies to both Python 2 and Python 3. It does not cover every possible Python expression, but does cover the most frequently-used ones.
This document has been in use for 5 years, with great success. I recently moved to a new GitHub repository, where source code is available and you can submit bug reports or make pull requests.