19.1 Modules and Imports

Before you start

Work through the earlier fundamentals chapters. Use a Python 3 interpreter and run each example locally.

Core concepts

A Python file can be imported as a module. Imported top-level code executes on first import in a process, so keep network calls and CLI parsing out of module scope. The if __name__ == "__main__" guard runs an entry point only when the file is executed directly.

Group related modules in a package, commonly using an __init__.py file. Prefer clear imports and avoid names such as csv.py or logging.py that shadow standard-library modules. Use a project virtual environment for third-party dependencies.

Worked example

# Save as health.py
def is_healthy(status):
    return status == "healthy"

def main():
    print(is_healthy("healthy"))

if __name__ == "__main__":
    main()

Mini lab

Save the example as health.py. In another file, import is_healthy and assert that critical returns False. Importing the module must not print anything.

# Save beside health.py as check_health.py
from health import is_healthy

assert is_healthy("critical") is False
print("check passed")

Knowledge check

What does the main guard prevent?

Common mistake

Circular imports often indicate responsibilities are mixed. Move shared definitions to a small independent module.

Completion checklist

• Run the example and explain its output.
• Complete the mini lab without copying the solution.
• Test an edge case and explain how the code handles it.
• Answer the knowledge check in your own words.

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