Python Virtual Machine (PVM)

• Python Virtual Machine (PVM) is the runtime engine of Python
• PVM executes Python bytecode
• PVM is an interpreter of the compiled bytecode
• PVM is also known as the Python Interpreter
• The term "Python Virtual Machine" refers to an abstract concept rather than a concrete software
• The Python interpreter implements this abstract virtual machine

How Python Virtual Machine Works

• Python code compiles into bytecode initially
• Bytecode consists of instructions that the PVM can understand
• PVM then executes the bytecode
• Compilation is implicit and automatic when the .py file executes
• The .py file compiles to .pyc, containing the bytecode
• .pyc files cache for later use, improving startup time
• When a Python script runs, the interpreter checks for a compiled .pyc version
• If the .pyc file exists and is newer than the .py file, the interpreter loads and executes bytecode from the .pyc file
• If a .pyc file does not exist or is older than the .py file, the interpreter compiles the .py file to bytecode and saves it in a new .pyc file (if possible) before executing it
• Compilation involves lexical analysis, parsing, and code generation
• Lexical analysis breaks source code into tokens
• Parsing builds an abstract syntax tree (AST) from tokens
• Code generation translates the AST into Python bytecode

Bytecode

• Python bytecode consists of numeric codes that represent instructions that are stack-oriented
• Each bytecode instruction performs a specific action
• Actions include pushing data onto the stack, performing arithmetic operations, and calling functions
• Bytecode is platform-independent, which allows Python code to run on any OS with a Python interpreter
• Disassembling Python bytecode is possible using the dis module in Python
• The dis module allows inspection of the underlying operations
• Bytecode is an intermediate representation between source code and machine code

Python Interpreter Implementations

• CPython is the reference implementation of Python (written in C) which compiles Python code to bytecode and then interprets it
• CPython includes a garbage collector for memory management, and a global interpreter lock (GIL)
• GIL allows only one thread to hold control of the Python interpreter
• Jython is a Python implementation that runs on the Java Virtual Machine (JVM)
• Jython compiles Python code to Java bytecode, which allows Python code to interact with Java classes and libraries
• IronPython is a Python implementation for the .NET framework
• IronPython enables Python code to interact with .NET libraries
• PyPy is a Python implementation using a just-in-time (JIT) compiler
• PyPy can execute Python code faster than CPython in many cases and is written in RPython, a subset of Python