docs/uio: Describe differences between uPy an CPy stream hierarchy.

docs/library/uio.rst

@@ -7,6 +7,71 @@
 This module contains additional types of stream (file-like) objects
 and helper functions.
 
+Conceptual hierarchy
+--------------------
+
+.. admonition:: Difference to CPython
+   :class: attention
+
+   Conceptual hierarchy of stream base classes is simplified in MicroPython,
+   as described in this section.
+
+(Abstract) base stream classes, which serve as a foundation for behavior
+of all the concrete classes, adhere to few dichotomies (pair-wise
+classifications) in CPython. In MicroPython, they are somewhat simplified
+and made implicit to achieve higher efficiencies and save resources.
+
+An important dichotomy in CPython is unbuffered vs buffered streams. In
+MicroPython, all streams are currently unbuffered. This is because all
+modern OSes, and even many RTOSes and filesystem drivers already perform
+buffering on their side. Adding another later of buffering is counter-
+productive (an issue known as "bufferbloat") and spends precious memory.
+Note that there still cases where buffering may be useful, so we may
+introduce optional buffering support at a later time.
+
+But in CPython, another important dichotomy is tied with "bufferedness" -
+it's whether a stream may incur short read/writes or not. A short read
+is when a user asks e.g. 10 bytes from a stream, but gets less, similarly
+for writes. In CPython, unbuffered streams are automatically short
+operation susceptible, while buffered are guarantee against them. The
+no short read/writes is an important traits, as it allows to develop
+more concise and efficient programs - something which is highly desirable
+for MicroPython. So, while MicroPython doesn't support buffered streams,
+it still provides for no-short-operations streams. Whether there will
+be short operations or not depends on each particular class' needs, but
+developers are strongly advised to favor no-short-operations behavior
+for the reasons stated above. For example, MicroPython sockets are
+guaranteed to avoid short read/writes. Actually, at this time, there is
+no example of a short-operations stream class in the core, and one would
+be a port-specific class, where such a need is governed by hardware
+peculiarities.
+
+The no-short-operations behavior gets tricky in case of non-blocking
+streams, blockedness vs non-blockedness being another CPython dichotomy,
+fully supported by MicroPython. Non-blocking streams never wait for
+data either to arrive or be written - they read/write whatever possible,
+or signal lack of data (or ability to write data). Clearly, this conflicts
+with "no-short-operations" policy, and indeed, a case of non-blocking
+buffered (and this no-short-ops) streams is convoluted in CPython - in
+some places, such combination is prohibited, in some it's undefined or
+just not documented, in some cases it raises verbose exceptions. The
+matter is much simpler in MicroPython: non-blocking stream are important
+for efficient asynchronuous operations, so this property prevails on
+the "no-short-ops" one. So, while blocking streams will avoid short
+reads/writes whenever possible (the only case to get a short read is
+if end of file is reached, or in case of error (but errors don't
+return short data, but raise exceptions)), non-blocking streams may
+produce short data to avoid blocking the operation.
+
+The final dichotomy is binary vs text streams. MicroPython of course
+supports these, but while in CPython text streams are inherently
+buffered, they aren't in MicroPython. (Indeed, that's one of the cases
+for which we may introduce buffering support.)
+
+Note that for efficiency, MicroPython doesn't provide abstract base
+classes corresponding to the hierarchy above, and it's not possible
+to implement, or subclass, a stream class in pure Python.
+
 Functions
 ---------