Discussion of Python 3.5 support

[dpgeorge]

MicroPython aims to implement the Python 3.x "standard". At the moment x is currently 4, ie we try to be compatible with CPython 3.4. It's new territory as to what to do when CPython evolves to larger version numbers. I would say we should try to follow the changes and implement them when possible/sensible.

Python 3.5 had a feature freeze on 24 May 2015 and is scheduled for final release on 13 September 2015. This ticket here is about discussing if, what and how we should upgrade uPy to version 3.5 of the language. It may be that some issues need to break off into separate tickets and that's fine but we should link to them from this one.

The PEP discussing the new features in 3.5: https://www.python.org/dev/peps/pep-0478/

A more friendly overview in the docs: https://docs.python.org/3.5/whatsnew/3.5.html

Below is a list of finalised/accepted PEPs for 3.5 grouped into their impact to MicroPython.

Extensions to the syntax:

• PEP 448 - additional unpacking generalizations; see Additional function arg unpacking (PEP 448) #5807
• PEP 465 - a new matrix multiplication operator; done in 2069c56
• PEP 492 - coroutines with async and await syntax

Extensions and changes to the runtime:

• PEP 461 - %-formatting for binary strings - tentatively done
• PEP 475 - retrying system calls that fail with EINTR - done in 9418611
• PEP 479 - change StopIteration handling inside generators - done in 3f6ffe0

Standard library changes:

• PEP 471 - os.scandir()
• PEP 485 - math.isclose(), a function for testing approximate equality - done in af5c998

Miscellaneous changes that are not relevant to MicroPython:

• PEP 441 - improved Python zip application support
• PEP 486 - make the Python Launcher aware of virtual environments
• PEP 484 - type hints (advisory only)
• PEP 488 - elimination of PYO files
• PEP 489 - redesigning extension module loading

Other Language Changes

• Added the "namereplace" error handlers. The "backslashreplace" error handlers now work with decoding and translating.
• Property docstrings are now writable. This is especially useful for collections.namedtuple() docstrings.
• Circular imports involving relative imports are now supported.

New modules

• typing
• zipapp

Changes to MicroPython built-in modules

• asyncio (many, may need another ticket)
• cmath - A new function isclose() provides a way to test for approximate equality.

• collections
  • The OrderedDict class is now implemented in C, which makes it 4 to 100 times faster.
  • OrderedDict.items(), OrderedDict.keys(), OrderedDict.values() views now support reversed() iteration.
  • The deque class now defines index(), insert(), and copy(), and supports the + and * operators.
  • Docstrings produced by namedtuple() can now be updated.
  • The UserString class now implements the __getnewargs__(), __rmod__(), casefold(), format_map(), isprintable(), and maketrans() methods to match the corresponding methods of str.

• heapq - Element comparison in merge() can now be customized by passing a key function in a new optional key keyword argument, and a new optional reverse keyword argument can be used to reverse element comparison
• io - A new BufferedIOBase.readinto1() method, that uses at most one call to the underlying raw stream's RawIOBase.read() or RawIOBase.readinto() methods.
• json - JSON decoder now raises JSONDecodeError instead of ValueError to provide better context information about the error.

• math
  • Two new constants have been added to the math module: inf and nan.
  • A new function isclose() provides a way to test for approximate equality.
  • A new gcd() function has been added. The fractions.gcd() function is now deprecated.
• os
  • The new scandir() function returning an iterator of DirEntry objects has been added.
  • The urandom() function now uses the getrandom() syscall on Linux 3.17 or newer, and getentropy() on OpenBSD 5.6 and newer, removing the need to use /dev/urandom and avoiding failures due to potential file descriptor exhaustion.
  • New get_blocking() and set_blocking() functions allow getting and setting a file descriptor's blocking mode (O_NONBLOCK.)
  • There is a new os.path.commonpath() function returning the longest common sub-path of each passed pathname.
• re
  • References and conditional references to groups with fixed length are now allowed in lookbehind assertions.
  • The number of capturing groups in regular expressions is no longer limited to 100.
  • The sub() and subn() functions now replace unmatched groups with empty strings instead of raising an exception.
  • The re.error exceptions have new attributes, msg, pattern, pos, lineno, and colno, that provide better context information about the error
• socket
  • Functions with timeouts now use a monotonic clock, instead of a system clock.
  • A new socket.sendfile() method allows sending a file over a socket by using the high-performance os.sendfile() function on UNIX, resulting in uploads being from 2 to 3 times faster than when using plain socket.send().
  • The socket.sendall() method no longer resets the socket timeout every time bytes are received or sent. The socket timeout is now the maximum total duration to send all data.
  • The backlog argument of the socket.listen() method is now optional. By default it is set to SOMAXCONN or to 128, whichever is less.
• ssl
  • Memory BIO Support
  • Application-Layer Protocol Negotiation Support
  • There is a new SSLSocket.version() method to query the actual protocol version in use.
  • The SSLSocket class now implements a SSLSocket.sendfile() method.
  • The SSLSocket.send() method now raises either the ssl.SSLWantReadError or ssl.SSLWantWriteError exception on a non-blocking socket if the operation would block. Previously, it would return 0.
  • The cert_time_to_seconds() function now interprets the input time as UTC and not as local time, per RFC 5280. Additionally, the return value is always an int.
  • New SSLObject.shared_ciphers() and SSLSocket.shared_ciphers() methods return the list of ciphers sent by the client during the handshake.
  • The SSLSocket.do_handshake(), SSLSocket.read(), SSLSocket.shutdown(), and SSLSocket.write() methods of the SSLSocket class no longer reset the socket timeout every time bytes are received or sent.
  • The match_hostname() function now supports matching of IP addresses.
• sys
  • A new set_coroutine_wrapper() function allows setting a global hook that will be called whenever a coroutine object is created by an async def function. A corresponding get_coroutine_wrapper() can be used to obtain a currently set wrapper.
  • A new is_finalizing() function can be used to check if the Python interpreter is shutting down.

• time The monotonic() function is now always available.

(Changes to non-built-in modules will need to be documented elsewhere.)

The above list should be edited if/when progress is made on a given feature.

[pfalcon]

We "by definition" have "%-formatting for binary strings", I'm not sure however what details PEP 461 may have.

[pfalcon]

retrying system calls that fail with EINTR

is potentially interesting, because it's boring to handle it in each app, and actually nobody handles it in each app, unless faces issues, and then it's likely users, and not an author. So, doing that PEP kinda saves from such situations. But then again it may be just adding more bloat than necessary. Need to read PEP, and actually, all PEPs (in detail, again, or at all).

[dpgeorge]

I'm not sure however what details PEP 461 may have.

We are missing: "%b" for interpolating bytes objects, "%a" for repr of bytes, "%c" in uPy truncates to 7-bit char but should be full 8-bit char, and we don't support % operator on bytearray object.

[roger-]

Type hints sound like they could be useful.

Personally I'd like to see unpacking generalizations and async/await support.

[dpgeorge]

I'd like to see unpacking generalizations

I just had a read of that PEP and it looks like it might be some effort, and a decent increase in code size, to implement that. It allows things like lst = [*a, b, *c] and foo(*a, b, *c).

[pfalcon]

Running uPy testsuite against CPython3.5b3 produces few warnings/errors.

[dpgeorge]

Running uPy testsuite against CPython3.5b3 produces few warnings/errors.

A few too many, or few enough to easily fix them? Do you have any examples of failures?

[pfalcon]

I built 3.5b3, and that managed to install itself as system-wide python3, so I scratched ny head for a bit while testsuite fails. As soon as I figured that out, I switched it back and didn't look back, and don't think it's a priority before 3.5 release. So, just FYI ;-).

[pfalcon]

3.5.0 was released today.

[dpgeorge]

CPy 3.5 is now distributed with Arch Linux so that's what I'll be using. There are only a few errors (mostly related to new , * syntax).

[dpgeorge]

@pfalcon you'll be pleased to know that regex splitting with an empty regex is on track for deprecation. Running tests/extmod/ure_split.py with CPy 3.5 gives a FutureWarning for these cases. So this means the dirty magic is hopefully gone!

[dpgeorge]

Tests now pass with CPy 3.5; see 34f26ea.