Package Startup Configuration with .start Files is part of Python 3.15 changes and migration. This lesson focuses on the exact problem it solves, the behavior you can observe, and the limits to understand before applying it in your own project.
Python 3.15 adds .start files as explicit startup entry points. Code is invoked when the interpreter processes site packages, even if the package is never imported later.
What You Will Learn
- Identify the purpose of Package Startup Configuration with .start Files and when it is useful.
- Understand the worked example and what happens to each input.
- Recognize the relevant compatibility, safety or correctness limitation.
- Apply the technique to a small exercise and verify the result.
Before You Start
This lesson discusses a capability introduced or revised in Python 3.15. When a demonstration checks for a feature or presents commented new syntax, run it under Python 3.15 or newer to exercise that exact capability. Do not assume an older interpreter supports the code.
How It Works
Python 3.15 adds .start files as explicit startup entry points. Code is invoked when the interpreter processes site packages, even if the package is never imported later.
Focus on the public contract first: what goes in, what comes out, what may fail, and which state changes. Internal implementation details are useful only when they explain a measured behavior or guide a safe design decision.
Step-by-Step Example
The following self-contained example is intentionally small enough to read line by line. Comments containing another programming language or a terminal command are illustrative and must not be executed as Python source.
# example.start in site-packages (Python 3.15+):
# mypackage.startup:initialize
print('A .start line specifies module:function for a no-argument callable.')Understanding the Example
- The print call exposes the result or a specific property of the operation. Compare it with your prediction rather than memorizing the line.
- Read the code from top to bottom and track how the input becomes a result; any exception is part of the documented contract.
- The important rule is: Python 3.15 adds .start files as explicit startup entry points. Code is invoked when the interpreter processes site packages, even if the package is never imported later.
Expected behavior: Python 3.15 adds .start files as explicit startup entry points. Code is invoked when the interpreter processes site packages, even if the package is never imported later. Exact timing values, platform paths, interpreter internals and native library availability may differ between machines.
Practical Use and Design Choices
Check the feature against the interpreter that actually runs your program and against supported dependency versions. Isolate version-specific examples and keep regression tests for older behavior when backward compatibility matters.
A migration is complete only when installation, tests, deployment, monitoring and rollback have been exercised—not merely when the new syntax parses.
A Second Scenario to Consider
Imagine using the same technique with an unavailable feature, an unexpected input or a larger workload. Before editing the code, identify which assumption of the first example no longer holds. Decide whether to reject the input, choose a safe fallback or change the design.
For this specific lesson: Document a harmless idempotent startup hook before testing.
Important Limitations
Startup hooks can slow every process and must not access unavailable dependencies.
Environment requirement: Python 3.15+. Keep compatibility checks and platform restrictions visible to users before they copy the code.
Practice Exercise
- Document a harmless idempotent startup hook before testing.
- Write down the expected result before running a changed case.
- Add a test for one normal case and one invalid or boundary case. Record the observed result and explain any difference.
Common Beginner Mistakes
- Startup hooks can slow every process and must not access unavailable dependencies.
- Startup hooks can slow every process and must not access unavailable dependencies.
- Applying Package Startup Configuration with .start Files without checking input types, resource ownership, side effects or version support.
Key Takeaways
- Python 3.15 adds .start files as explicit startup entry points. Code is invoked when the interpreter processes site packages, even if the package is never imported later.
- The worked example demonstrates the central behavior of Package Startup Configuration with .start Files using visible results and bounded input.
- A useful next step is: Document a harmless idempotent startup hook before testing.