Python in a Nutshell, 4th Edition

Chapter 26. v3.7 to v3.n Migration

This book spans several versions of Python and covers some substantial (and still evolving!) new features, including:

• Order-preserving dicts

• Type annotations

• := assignment expressions (informally called “the walrus operator”)

• Structural pattern matching

Individual developers may be able to install each new Python version as it is released, and solve compatibility issues as they go. But for Python developers working in a corporate environment or maintaining a shared library, migrating from one version to the next involves deliberation and planning.

This chapter deals with the changing shape of the Python language, as seen from a Python programmer’s viewpoint. (There have been many changes in Python internals as well, including to the Python C API, but those are beyond the scope of this chapter: for details, see the “What’s New in Python 3.n” sections of each release’s online documentation.)

Significant Changes in Python Through 3.11

Most releases have a handful of significant new features and improvements that characterize that release, and it is useful to have these in mind as high-level reasons for targeting a particular release. Table 26-1 details only major new features and breaking changes in versions 3.6–3.11¹ that are likely to affect many Python programs; see the Appendix for a more complete list.

Table 26-1. Significant changes in recent Python releases

Version	New features	Breaking changes
3.6	dicts preserve order ...