What is Python? How the Interpreter Works and How to Write "Hello World" in Python

Python is a high-level, general-purpose programming language known for its simplicity, readability, and extensive ecosystem of libraries and frameworks. It is an interpreted language, meaning that Python code is executed by an interpreter at runtime rather than being compiled ahead of time. In this article, we‘ll take an in-depth look at what Python is, how the Python interpreter works under the hood, and write our first "Hello World" program in Python.

What is Python?

Python is a dynamically-typed, object-oriented programming language first released by Guido van Rossum in 1991. Its design philosophy emphasizes code readability and makes heavy use of whitespace to delimit code blocks rather than curly braces or keywords. Python supports multiple programming paradigms including structured, object-oriented and functional programming.

Some of Python‘s key features include:

• Simple, easy to learn syntax that emphasizes readability
• High-level data types and dynamic typing
• Automatic memory management and garbage collection
• Large standard library and extensive third-party ecosystem
• Interactive mode for testing snippets of code
• Portability across operating systems

Python is widely used for a variety of applications such as web development, scientific computing, artificial intelligence, data analysis, automation, and more. It has become especially popular for data science and machine learning due to libraries like NumPy, Pandas, and scikit-learn.

How the Python Interpreter Works

When you run a Python program, the code is processed by the Python interpreter. While there are several implementations of Python, the reference implementation is known as CPython and is written in the C programming language. Let‘s take a look at how CPython executes your Python code step-by-step.

Lexical Analysis

The first step is lexical analysis, where the interpreter reads the source code character by character and breaks it down into a sequence of lexical tokens. This is done according to the rules specified by the Python grammar.

During this phase, the lexer identifies items like keywords (e.g. if, for, while), operators (+, -, *, /), literal values (e.g. 123, 3.14, "hello"), and special symbols like parentheses, brackets, and braces. It discards whitespace and comments.

If the lexer encounters any invalid characters or malformed tokens, it will raise a SyntaxError and halt interpretation. Otherwise, it outputs an ordered stream of valid tokens to be parsed.

Parsing

The next phase is parsing, which takes the sequence of tokens and generates an abstract syntax tree (AST) based on the Python grammar. The AST is a hierarchical tree structure where each node represents a construct in the Python syntax.

The parser uses a technique called recursive descent parsing to build the AST. It starts at the root of the grammar and recursively parses each non-terminal symbol into its constituent parts using the order and logic defined by the grammar‘s production rules.

For example, consider the following code:

x = 42
y = x + 5

The AST would break this down into an assignment statement node with the target x and the literal value 42, and another assignment statement node with the target y and a binary operation node combining the variable x and literal 5 with the + operator.

Building the full AST requires multiple passes through the token stream. If the parser encounters a grammatical error at any point, it will raise a SyntaxError just like the lexer.

Compilation to Bytecode

Once the complete AST is generated, the interpreter traverses the tree and emits bytecode for each node. Bytecode is a low-level, platform-independent representation of the Python code designed to be executed by the Python virtual machine.

Each bytecode instruction consists of an opcode specifying the operation to be performed and any arguments. For example, common instructions include:

• LOAD_CONST: Pushes a constant value onto the stack
• LOAD_FAST: Loads a local variable onto the stack
• STORE_FAST: Stores a value from the stack into a local variable
• BINARY_ADD: Pops the top two values from the stack, adds them, and pushes the result
• RETURN_VALUE: Returns the top value from the stack

These low-level instructions are much more efficient for the interpreter to execute compared to walking the AST each time. The bytecode is cached on disk in .pyc files so that subsequent executions can skip the lexing, parsing and compilation steps if the .py file hasn‘t changed.

Execution in the Python VM

The final step is executing the compiled bytecode in the Python virtual machine (PVM). The PVM is a stack-based interpreter that reads instructions one at a time and performs the corresponding operations.

The PVM maintains a call stack of currently executing code blocks (frames), a set of global and local namespaces for storing variables, and a heap for allocating objects. It also handles memory management by periodically garbage collecting objects that are no longer referenced.

Each bytecode instruction manipulates the state of the PVM in some way. For example, LOAD_CONST pushes a value onto the stack, STORE_FAST pops a value off the stack and stores it as a local variable, and BINARY_ADD pops two values, adds them, and pushes the result.

Execution continues until the end of the bytecode is reached or an exception is raised. Any output is written to standard output and the program terminates.

This is just a high-level overview of how the Python interpreter works – there are many more details and optimizations, but these are the key phases all Python code goes through. Understanding this process can help you reason about the performance characteristics of your code.

Writing "Hello World" in Python

Now that we‘ve covered what Python is and how it works under the hood, let‘s write our first Python program. We‘ll start with the classic "Hello World" example:

print("Hello, World!")

Let‘s break this down:

• print is a built-in Python function that outputs text to the console. It takes one or more arguments and writes them to standard output followed by a newline character.

• "Hello, World!" is a string literal, denoted by the quotation marks. It‘s a sequence of characters that gets passed as an argument to print.

To run this code, save it as a file with a .py extension, open a terminal, and run it with the python command:

$ python hello.py
Hello, World!

You should see the text "Hello, World!" printed to the console. Congratulations, you just wrote and ran your first Python program!

This simple example demonstrates some key aspects of Python syntax:

• Statements don‘t need to be terminated with a semicolon
• Parentheses are used to denote function calls and to group expressions
• Literal values can be used directly without declaring a type
• Indentation is used to delimit code blocks (there are no curly braces)

Of course, there‘s a lot more to learn to take full advantage of Python. But equipped with an understanding of what Python is, how the interpreter works, and the basic "Hello World" program, you‘re well on your way to becoming a proficient Pythonista.

Conclusion

In this article, we took a deep dive into Python and its interpreter. We covered:

• What Python is and what it‘s commonly used for
• How the CPython interpreter executes Python code in four phases: lexing, parsing, bytecode compilation, and execution in the Python virtual machine
• The key steps and data structures used by each phase of the interpreter
• How to write, save, and run a "Hello World" program in Python

While we‘ve only scratched the surface of what Python can do, I hope this gives you a solid foundation to build on as you continue learning. Python is a powerful and versatile language with a lot to offer, so keep exploring and practicing. The official Python documentation and tutorials are great resources to continue your journey.