MicroPython .mpy files

MicroPython defines the concept of an .mpy file which is a binary container file format that holds precompiled code, and which can be imported like a normal .py module. The file foo.mpy can be imported via import foo, as long as foo.mpy can be found in the usual way by the import machinery. Usually, each directory listed in sys.path is searched in order. When searching a particular directory foo.py is looked for first and if that is not found then foo.mpy is looked for, then the search continues in the next directory if neither is found. As such, foo.py will take precedence over foo.mpy.

These .mpy files can contain bytecode which is usually generated from Python source files (.py files) via the mpy-cross program. For some architectures an .mpy file can also contain native machine code, which can be generated in a variety of ways, most notably from C source code.

Versioning and compatibility of .mpy files

A given .mpy file may or may not be compatible with a given MicroPython system. Compatibility is based on the following:

  • Version of the .mpy file: the version of the file must match the version supported by the system loading it.

  • Sub-version of the .mpy file: if the .mpy file contains native machine code then the sub-version of the file must match the version support by the system loading it. Otherwise, if there is no native machine code in the .mpy file, then the sub-version is ignored when loading.

  • Small integer bits: the .mpy file will require a minimum number of bits in a small integer and the system loading it must support at least this many bits.

  • Native architecture: if the .mpy file contains native machine code then it will specify the architecture of that machine code and the system loading it must support execution of that architecture’s code.

If a MicroPython system supports importing .mpy files then the sys.implementation._mpy field will exist and return an integer which encodes the version (lower 8 bits), features and native architecture.

Trying to import an .mpy file that fails one of the first four tests will raise ValueError('incompatible .mpy file'). Trying to import an .mpy file that fails the native architecture test (if it contains native machine code) will raise ValueError('incompatible .mpy arch').

If importing an .mpy file fails then try the following:

  • Determine the .mpy version and flags supported by your MicroPython system by executing:

    import sys
    sys_mpy = sys.implementation._mpy
    arch = [None, 'x86', 'x64',
        'armv6', 'armv6m', 'armv7m', 'armv7em', 'armv7emsp', 'armv7emdp',
        'xtensa', 'xtensawin'][sys_mpy >> 10]
    print('mpy version:', sys_mpy & 0xff)
    print('mpy sub-version:', sys_mpy >> 8 & 3)
    print('mpy flags:', end='')
    if arch:
        print(' -march=' + arch, end='')
  • Check the validity of the .mpy file by inspecting the first two bytes of the file. The first byte should be an uppercase ‘M’ and the second byte will be the version number, which should match the system version from above. If it doesn’t match then rebuild the .mpy file.

  • Check if the system .mpy version matches the version emitted by mpy-cross that was used to build the .mpy file, found by mpy-cross --version. If it doesn’t match then recompile mpy-cross from the Git repository checked out at the tag (or hash) reported by mpy-cross --version.

  • Make sure you are using the correct mpy-cross flags, found by the code above, or by inspecting the MPY_CROSS_FLAGS Makefile variable for the port that you are using.

The following table shows the correspondence between MicroPython release and .mpy version.

MicroPython release

.mpy version

v1.22.0 and up


v1.20 - v1.21.0




v1.12 - v1.18




v1.9.3 - v1.10


v1.9 - v1.9.2


v1.5.1 - v1.8.7


For completeness, the next table shows the Git commit of the main MicroPython repository at which the .mpy version was changed.

.mpy version change

Git commit

6.1 to 6.2


6 to 6.1


5 to 6


4 to 5


3 to 4


2 to 3


1 to 2


0 to 1


initial version 0


Binary encoding of .mpy files

MicroPython .mpy files are a binary container format with code objects (bytecode and native machine code) stored internally in a nested hierarchy. The code for the outer module is stored first, and then its children follow. Each child may have further children, for example in the case of a class having methods, or a function defining a lambda or comprehension. To keep files small while still providing a large range of possible values it uses the concept of a variably-encoded-unsigned-integer (vuint) in many places. Similar to utf-8 encoding, this encoding stores 7 bits per byte with the 8th bit (MSB) set if one or more bytes follow. The bits of the unsigned integer are stored in the vuint in LSB form.

The top-level of an .mpy file consists of three parts:

  • The header.

  • The global qstr and constant tables.

  • The raw-code for the outer scope of the module. This outer scope is executed when the .mpy file is imported.

You can inspect the contents of a .mpy file by using mpy-tool.py, for example (run from the root of the main MicroPython repository):

$ ./tools/mpy-tool.py -xd myfile.mpy

The header

The .mpy header is:




value 0x4d (ASCII ‘M’)


.mpy major version number


native arch and minor version number (was feature flags in older versions)


number of bits in a small int

The global qstr and constant tables

An .mpy file contains a single qstr table, and a single constant object table. These are global to the .mpy file, they are referenced by all nested raw-code objects. The qstr table maps internal qstr number (internal to the .mpy file) to the resolved qstr number of the runtime that the .mpy file is imported into. This links the .mpy file with the rest of the system that it executes within. The constant object table is populated with references to all constant objects that the .mpy file needs.




number of qstrs


number of constant objects

qstr data

encoded constant objects

Raw code elements

A raw-code element contains code, either bytecode or native machine code. Its contents are:




type, size and whether there are sub-raw-code elements

code (bytecode or machine code)


number of sub-raw-code elements (only if non-zero)

sub-raw-code elements

The first vuint in a raw-code element encodes the type of code stored in this element (the two least-significant bits), whether this raw-code has any children (the third least-significant bit), and the length of the code that follows (the amount of RAM to allocate for it).

Following the vuint comes the code itself. Unless the code type is viper code with relocations, this code is constant data and does not need to be modified.

If this raw-code has any children (as indicated by a bit in the first vuint), following the code comes a vuint counting the number of sub-raw-code elements.

Finally any sub-raw-code elements are stored, recursively.