A system without a (full-fledged) OS, for example an MCU-based system. When running on a baremetal system, MicroPython effectively becomes its user-facing OS with a command interpreter (REPL).
callee-owned tuple
A tuple returned by some builtin function/method, containing data which is valid for a limited time, usually until next call to the same function (or a group of related functions). After next call, data in the tuple may be changed. This leads to the following restriction on the usage of callee-owned tuples - references to them cannot be stored. The only valid operation is extracting values from them (including making a copy). Callee-owned tuples is a MicroPython-specific construct (not available in the general Python language), introduced for memory allocation optimization. The idea is that callee-owned tuple is allocated once and stored on the callee side. Subsequent calls don’t require allocation, allowing to return multiple values when allocation is not possible (e.g. in interrupt context) or not desirable (because allocation inherently leads to memory fragmentation). Note that callee-owned tuples are effectively mutable tuples, making an exception to Python’s rule that tuples are immutable. (It may be interesting why tuples were used for such a purpose then, instead of mutable lists - the reason for that is that lists are mutable from user application side too, so a user could do things to a callee-owned list which the callee doesn’t expect and could lead to problems; a tuple is protected from this.)
GPIO port
interned string
A string referenced by its (unique) identity rather than its address. Interned strings are thus can be quickly compared just by their identifiers, instead of comparing by content. The drawbacks of interned strings are that interning operation takes time (proportional to the number of existing interned strings, i.e. becoming slower and slower over time) and that the space used for interned strings is not reclaimable. String interning is done automatically by MicroPython compiler and runtimer when it’s either required by the implementation (e.g. function keyword arguments are represented by interned string id’s) or deemed beneficial (e.g. for short enough strings, which have a chance to be repeated, and thus interning them would save memory on copies). Most of string and I/O operations don’t produce interned strings due to drawbacks described above.

MicroPython is (usually) distributed as a single executable/binary file with just few builtin modules. There is no extensive standard library comparable with CPython. Instead, there is a related, but separate project micropython-lib which provides implementations for many modules from CPython’s standard library. However, large subset of these modules require POSIX-like environment (Linux, FreeBSD, MacOS, etc.; Windows may be partially supported), and thus would work or make sense only with MicroPython Unix port. Some subset of modules is however usable for baremetal ports too.


MicroPython port
MicroPython Unix port
Also known as a “file-like object”. An object which provides sequential read-write access to the underlying data. A stream object implements a corresponding interface, which consists of methods like read(), write(), readinto(), seek(), flush(), close(), etc. A stream is an important concept in MicroPython, many I/O objects implement the stream interface, and thus can be used consistently and interchangeably in different contexts. For more information on streams in MicroPython, see uio module.
(Literally, “micro pip”). A package manage for MicroPython, inspired by CPython’s pip, but much smaller and with reduced functionality. upip runs both on Unix port and on baremetal ports (those which offer filesystem and networking support).