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129 lines
5.7 KiB
129 lines
5.7 KiB
:mod:`uio` -- input/output streams
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==================================
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.. module:: uio
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:synopsis: input/output streams
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|see_cpython_module| :mod:`python:io`.
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This module contains additional types of `stream` (file-like) objects
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and helper functions.
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Conceptual hierarchy
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--------------------
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.. admonition:: Difference to CPython
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:class: attention
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Conceptual hierarchy of stream base classes is simplified in MicroPython,
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as described in this section.
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(Abstract) base stream classes, which serve as a foundation for behavior
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of all the concrete classes, adhere to few dichotomies (pair-wise
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classifications) in CPython. In MicroPython, they are somewhat simplified
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and made implicit to achieve higher efficiencies and save resources.
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An important dichotomy in CPython is unbuffered vs buffered streams. In
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MicroPython, all streams are currently unbuffered. This is because all
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modern OSes, and even many RTOSes and filesystem drivers already perform
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buffering on their side. Adding another layer of buffering is counter-
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productive (an issue known as "bufferbloat") and takes precious memory.
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Note that there still cases where buffering may be useful, so we may
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introduce optional buffering support at a later time.
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But in CPython, another important dichotomy is tied with "bufferedness" -
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it's whether a stream may incur short read/writes or not. A short read
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is when a user asks e.g. 10 bytes from a stream, but gets less, similarly
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for writes. In CPython, unbuffered streams are automatically short
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operation susceptible, while buffered are guarantee against them. The
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no short read/writes is an important trait, as it allows to develop
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more concise and efficient programs - something which is highly desirable
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for MicroPython. So, while MicroPython doesn't support buffered streams,
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it still provides for no-short-operations streams. Whether there will
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be short operations or not depends on each particular class' needs, but
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developers are strongly advised to favor no-short-operations behavior
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for the reasons stated above. For example, MicroPython sockets are
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guaranteed to avoid short read/writes. Actually, at this time, there is
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no example of a short-operations stream class in the core, and one would
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be a port-specific class, where such a need is governed by hardware
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peculiarities.
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The no-short-operations behavior gets tricky in case of non-blocking
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streams, blocking vs non-blocking behavior being another CPython dichotomy,
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fully supported by MicroPython. Non-blocking streams never wait for
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data either to arrive or be written - they read/write whatever possible,
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or signal lack of data (or ability to write data). Clearly, this conflicts
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with "no-short-operations" policy, and indeed, a case of non-blocking
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buffered (and this no-short-ops) streams is convoluted in CPython - in
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some places, such combination is prohibited, in some it's undefined or
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just not documented, in some cases it raises verbose exceptions. The
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matter is much simpler in MicroPython: non-blocking stream are important
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for efficient asynchronous operations, so this property prevails on
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the "no-short-ops" one. So, while blocking streams will avoid short
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reads/writes whenever possible (the only case to get a short read is
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if end of file is reached, or in case of error (but errors don't
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return short data, but raise exceptions)), non-blocking streams may
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produce short data to avoid blocking the operation.
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The final dichotomy is binary vs text streams. MicroPython of course
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supports these, but while in CPython text streams are inherently
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buffered, they aren't in MicroPython. (Indeed, that's one of the cases
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for which we may introduce buffering support.)
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Note that for efficiency, MicroPython doesn't provide abstract base
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classes corresponding to the hierarchy above, and it's not possible
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to implement, or subclass, a stream class in pure Python.
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Functions
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---------
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.. function:: open(name, mode='r', **kwargs)
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Open a file. Builtin ``open()`` function is aliased to this function.
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All ports (which provide access to file system) are required to support
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*mode* parameter, but support for other arguments vary by port.
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Classes
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-------
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.. class:: FileIO(...)
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This is type of a file open in binary mode, e.g. using ``open(name, "rb")``.
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You should not instantiate this class directly.
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.. class:: TextIOWrapper(...)
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This is type of a file open in text mode, e.g. using ``open(name, "rt")``.
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You should not instantiate this class directly.
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.. class:: StringIO([string])
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.. class:: BytesIO([string])
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In-memory file-like objects for input/output. `StringIO` is used for
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text-mode I/O (similar to a normal file opened with "t" modifier).
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`BytesIO` is used for binary-mode I/O (similar to a normal file
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opened with "b" modifier). Initial contents of file-like objects
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can be specified with *string* parameter (should be normal string
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for `StringIO` or bytes object for `BytesIO`). All the usual file
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methods like ``read()``, ``write()``, ``seek()``, ``flush()``,
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``close()`` are available on these objects, and additionally, a
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following method:
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.. method:: getvalue()
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Get the current contents of the underlying buffer which holds data.
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.. class:: StringIO(alloc_size)
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.. class:: BytesIO(alloc_size)
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Create an empty `StringIO`/`BytesIO` object, preallocated to hold up
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to *alloc_size* number of bytes. That means that writing that amount
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of bytes won't lead to reallocation of the buffer, and thus won't hit
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out-of-memory situation or lead to memory fragmentation. These constructors
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are a MicroPython extension and are recommended for usage only in special
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cases and in system-level libraries, not for end-user applications.
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.. admonition:: Difference to CPython
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:class: attention
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These constructors are a MicroPython extension.
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