A Python Interpreter Written in Python

Byterun is a Python interpreter implemented in Python, following a structure similar to CPython to help understand the core of an interpreter.

The processing flow first goes through lexing, parsing, and compiling to create a code object and bytecode, and the interpreter executes commands in the subsequent stage.

The core concept is the stack machine.

• Instructions start out very simply, like LOAD_VALUE, ADD_TWO_VALUES, and PRINT_ANSWER.
• LOAD_VALUE finds a value from the list of constants and pushes it onto the stack.
• ADD_TWO_VALUES pops two values from the stack, adds them, and pushes the result back onto the stack.
• PRINT_ANSWER pops the final value from the stack and prints it.

This design works even with a small instruction set that executes something like 7 + 5, as in the example, and extending the same structure can handle more complex calculations. Also, because constants and instructions are kept separate, the same value can be reused multiple times.

After that, variables are added.

• STORE_NAME stores a value under a name.
• LOAD_NAME reads a value from a name.
• Variable bindings are kept in the interpreter's environment dictionary.

Finally, here's a summary of why this structure is useful.

• Python is often called an "interpreted language," but it actually goes through a compilation step.
• Byterun prioritizes clarity and simplicity over speed, making it suitable for learning.
• It shows that the essence of a Python interpreter can be explained at a smaller scale than one might think.