Organizing code files

Python appears early in this syllabus because it sits at the intersection of simplicity and capability. It lets us write small, readable programs while still scaling up to real systems, including those that generate content, process data, or act as agents. Before we write anything meaningful, we need a clear picture of what Python actually is and what happens when we ask it to run.

What Python is as a programming language and runtime

Python is both a programming language and a runtime environment. As a language, it defines how we express instructions using keywords, symbols, and structure. As a runtime, it provides the machinery that understands those instructions and carries them out.

This pairing matters because Python code does not stand alone. It always relies on the Python runtime to make sense of what we have written and to turn text into behavior.

The role of the Python interpreter

The Python runtime is delivered primarily through the Python interpreter. The interpreter is a program that reads Python source code, understands its meaning, and executes it.

Rather than translating an entire program into another form ahead of time, the interpreter works directly with the source code. It reads instructions, evaluates them, and produces results as it goes. This model is a key part of Python’s interactive and flexible nature.

Python source files and the .py file extension

Python programs are usually stored in plain text files with a .py extension. These files contain Python statements written in a specific order.

The .py extension signals that the file contains Python source code. When we ask Python to run a file, the interpreter loads the contents of that file and treats it as a Python program.

How Python executes code top to bottom

Python executes code in the order it appears, starting at the top of the file and moving downward. Each statement is handled in sequence unless control structures later change that flow.

This top-to-bottom execution model makes program behavior easy to reason about. The order of statements on the page closely matches the order in which actions occur when the program runs.

What it means to “run” a Python program

To run a Python program means to ask the Python interpreter to read a source file and execute its contents. The interpreter processes each statement and performs the actions those statements describe.

When the interpreter reaches the end of the file, the program stops. The result of running a Python program is whatever observable effects those statements produced, such as printed output, generated files, or updated data.

Conclusion

At this point, we know what Python is, how the interpreter fits into the picture, and what happens when a Python program runs. We are oriented to the basic execution model and the role of source files, which gives us a solid footing for actually running Python code in practice.