What is a PY file?

PY files contain Python source code — a high-level, interpreted programming language created by Guido van Rossum and first released in 1991. Python is designed for readability, using indentation to define code blocks instead of curly braces. It is consistently ranked the most popular programming language globally, driven by its dominance in data science, machine learning, automation, and web development.

Python’s philosophy is captured in the Zen of Python: “Readability counts,” “Simple is better than complex,” and “There should be one — and preferably only one — obvious way to do it.” PY files run on the CPython interpreter by default, with no compilation step required.

How to open PY files

• Python interpreter — python script.py to execute from the terminal
• VS Code (Windows, macOS, Linux) — IntelliSense, debugging, virtual environment support
• PyCharm (Windows, macOS, Linux) — The most feature-rich Python IDE
• IDLE — Python’s built-in editor, installed with Python
• Jupyter Notebook / JupyterLab — Interactive environment popular in data science

Technical specifications

Common use cases

• Data science: Pandas, NumPy, matplotlib, Polars for data analysis and visualization
• Machine learning / AI: TensorFlow, PyTorch, scikit-learn, Hugging Face
• Web development: Django (full-stack), Flask (microframework), FastAPI (async APIs)
• Automation: Scripts for file processing, web scraping (BeautifulSoup, Playwright), task scheduling
• Scientific computing: SciPy, Astropy, BioPython for domain-specific research
• DevOps: Ansible playbooks, AWS Lambda functions, CLI tools

Python code example

# Type hints (optional but recommended)
def word_count(text: str) -> dict[str, int]:
    words = text.lower().split()
    counts: dict[str, int] = {}
    for word in words:
        counts[word] = counts.get(word, 0) + 1
    return counts

# List comprehension
squares = [x**2 for x in range(10) if x % 2 == 0]

# Async function
import asyncio

async def fetch_data(url: str) -> str:
    # ... async HTTP call
    pass

Virtual environments

Python projects use virtual environments to isolate dependencies:

python -m venv .venv          # Create environment
source .venv/bin/activate     # Activate (Unix)
.venv\Scripts\activate        # Activate (Windows)
pip install -r requirements.txt

This prevents version conflicts between projects. Modern tools like uv handle environments and dependencies significantly faster than pip.

Python 2 vs Python 3

Python 2 reached end-of-life in January 2020 and should never be used for new projects. Python 3 introduced breaking changes: print became a function, strings are Unicode by default, and integer division returns floats. Any .py file that starts with print "hello" (no parentheses) is Python 2 code and requires migration.

Security considerations

Python scripts executed from untrusted sources can run arbitrary system commands. Use subprocess with a list of arguments (not a shell string) to prevent shell injection. When using pickle for serialization, never deserialize data from untrusted sources — pickle can execute arbitrary code during deserialization. Prefer JSON or msgpack for data interchange with external systems.

Type hints and static analysis

Python’s optional type hint system (introduced in Python 3.5, mature in 3.10+) allows tools like mypy, pyright, and Pylance (VS Code) to catch type errors before runtime. Type hints do not affect execution — they are purely for developer tooling. Modern Python codebases increasingly use type hints for better IDE support and fewer runtime bugs.