mpy

mpy is the term used in MicroPython to denote the bytecode format and the associated file extension for precompiled modules. Files with the .mpy extension are produced from Python source files (.py) by the mpy-cross compiler and are intended to be run by MicroPython runtimes on microcontrollers and other constrained devices.

The primary purpose of compiling to .mpy is to reduce startup time and memory usage. Precompiled modules load more quickly than parsing and compiling Python source at runtime, which helps devices with limited RAM and storage. The .mpy format is version- and sometimes board-specific, meaning an .mpy generated for a particular MicroPython major.minor version is generally designed to run on runtimes built for that same version.

Using mpy typically involves compiling modules with mpy-cross and then transferring the resulting .mpy files to the device’s filesystem or freezing them into the firmware. Once on the device, the interpreter can import these modules in the same way as regular Python modules. MicroPython can also freeze modules into the firmware image during the build process, eliminating the need for separate .mpy files on the device.

Limitations include compatibility constraints: not all Python features are available in MicroPython, and .mpy files are not universally portable across different MicroPython versions. Developers must ensure that the .mpy files match the target MicroPython version and, when possible, the device architecture. The .mpy approach is one of several strategies for deploying code on microcontrollers, alongside pure Python sources and firmware-freezing.

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