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App and packages are compiled to .pyc by default

note

This guide is accurate as of Flet 0.86.0. Later releases might add new APIs or additional migration paths.

The breaking changes and deprecations index lists the guides created for each release.

Summary​

flet build now compile your app and the installed packages to .pyc by default (via python -m compileall -b, with the original .py files removed from the bundle). Previously compilation was off unless you opted in with --compile-app / --compile-packages or [tool.flet.compile] in pyproject.toml.

Shipping bytecode removes per-launch recompilation. The win is largest on mobile, where pure Python is imported from a stored, in-place zip (zipimport) that cannot cache bytecode back to disk — so without precompiled .pyc, every module is recompiled from source on every cold start. On a mid-range Android device this costs roughly 1–2 seconds of startup, scaling with the number of modules imported.

Background​

Compilation has always been available behind the --compile-app / --compile-packages flags; it just defaulted off. With the move to in-place zipimport packaging on Android, leaving it off meant a recurring cold-start tax on every launch, so the default was flipped on.

Compiled web builds were verified to load correctly in Pyodide: the standalone CPython used to compile and the Pyodide runtime share the same minor version (e.g. 3.14), and CPython keeps the .pyc magic number stable across patch releases of a minor version, so the bytecode is accepted.

Migration guide​

Most apps need no changes — builds simply start faster.

If you relied on uncompiled builds (to keep .py source in the bundle for debugging, to let a build complete despite syntax errors, or to speed up iterative builds), opt out explicitly.

Opt out via the CLI flag​

The flags are now argparse.BooleanOptionalAction, so each gets a --no- form (the original --compile-app / --compile-packages still work):

flet build apk --no-compile-app --no-compile-packages

Opt out in pyproject.toml​

[tool.flet.compile]
app = false
packages = false

Per-platform overrides also work, e.g. [tool.flet.android.compile]. The resolution order is CLI flag → [tool.flet.<platform>.compile] → [tool.flet.compile] → default (true).

Timeline​

  • Changed in: 0.86.0

References​