Mailman 3 July 2018 - Python-ideas

Change repr of collections.OrderedDict to be more dict-like
by Miro Hrončok Aug. 2, 2018

Aug. 2, 2018

Hi, now when dicts are sorted, can we change the repr of collections.OrderedDict to look like dict? I mean from: OrderedDict([('a', '1'), ('b', '2')]) To: OrderedDict({'a': '1', 'b': '2'}) I assume the current list-of-tuples repr is there so copy pasting the repr to the interpreter results in the same OrderedDict. When dicts preserve insertion order, this now works with dict. I consider the dict-based repr much easier to read. Thoughts? -- Miro Hrončok -- Phone: +420777974800 IRC: mhroncok

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PEP 505: None-aware operators
by Steve Dower Aug. 1, 2018

Aug. 1, 2018

Possibly this is exactly the wrong time to propose the next big syntax change, since we currently have nobody to declare on it, but since we're likely to argue for a while anyway it probably can't hurt (and maybe this will become the test PEP for whoever takes the reins?). FWIW, Guido had previously indicated that he was generally favourable towards most of this proposal, provided we could figure out coherent semantics. Last time we tried, that didn't happen, so this time I've made the … [View More]semantics much more precise, have implemented and verified them, and made much stronger statements about why we are proposing these. Additional thanks to Mark Haase for writing most of the PEP. All the fair and balanced parts are his - all the overly strong opinions are mine. Also thanks to Nick Coghlan for writing PEPs 531 and 532 last time we went through this - if you're unhappy with "None" being treated as a special kind of value, I recommend reading those before you start repeating them. There is a formatted version of this PEP at https://www.python.org/dev/peps/pep-0505/ My current implementation is at https://github.com/zooba/cpython/tree/pep-505 (though I'm considering removing some of the new opcodes I added and just generating more complex code - in any case, let's get hung up on the proposal rather than the implementation :) ) Let the discussions begin! --- PEP: 505 Title: None-aware operators Version: $Revision$ Last-Modified: $Date$ Author: Mark E. Haase <mehaase(a)gmail.com>, Steve Dower <steve.dower(a)python.org> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 18-Sep-2015 Python-Version: 3.8 Abstract ======== Several modern programming languages have so-called "``null``-coalescing" or "``null``- aware" operators, including C# [1]_, Dart [2]_, Perl, Swift, and PHP (starting in version 7). These operators provide syntactic sugar for common patterns involving null references. * The "``null``-coalescing" operator is a binary operator that returns its left operand if it is not ``null``. Otherwise it returns its right operand. * The "``null``-aware member access" operator accesses an instance member only if that instance is non-``null``. Otherwise it returns ``null``. (This is also called a "safe navigation" operator.) * The "``null``-aware index access" operator accesses an element of a collection only if that collection is non-``null``. Otherwise it returns ``null``. (This is another type of "safe navigation" operator.) This PEP proposes three ``None``-aware operators for Python, based on the definitions and other language's implementations of those above. Specifically: * The "``None`` coalescing`` binary operator ``??`` returns the left hand side if it evaluates to a value that is not ``None``, or else it evaluates and returns the right hand side. A coalescing ``??=`` augmented assignment operator is included. * The "``None``-aware attribute access" operator ``?.`` evaluates the complete expression if the left hand side evaluates to a value that is not ``None`` * The "``None``-aware indexing" operator ``?[]`` evaluates the complete expression if the left hand site evaluates to a value that is not ``None`` Syntax and Semantics ==================== Specialness of ``None`` ----------------------- The ``None`` object denotes the lack of a value. For the purposes of these operators, the lack of a value indicates that the remainder of the expression also lacks a value and should not be evaluated. A rejected proposal was to treat any value that evaluates to false in a Boolean context as not having a value. However, the purpose of these operators is to propagate the "lack of value" state, rather that the "false" state. Some argue that this makes ``None`` special. We contend that ``None`` is already special, and that using it as both the test and the result of these operators does not change the existing semantics in any way. See the `Rejected Ideas`_ section for discussion on the rejected approaches. Grammar changes --------------- The following rules of the Python grammar are updated to read:: augassign: ('+=' | '-=' | '*=' | '@=' | '/=' | '%=' | '&=' | '|=' | '^=' | '<<=' | '>>=' | '**=' | '//=' | '??=') power: coalesce ['**' factor] coalesce: atom_expr ['??' factor] atom_expr: ['await'] atom trailer* trailer: ('(' [arglist] ')' | '[' subscriptlist ']' | '?[' subscriptlist ']' | '.' NAME | '?.' NAME) Inserting the ``coalesce`` rule in this location ensures that expressions resulting in ``None`` are natuarlly coalesced before they are used in operations that would typically raise ``TypeError``. Like ``and`` and ``or`` the right-hand expression is not evaluated until the left-hand side is determined to be ``None``. For example:: a, b = None, None def c(): return None def ex(): raise Exception() (a ?? 2 ** b ?? 3) == a ?? (2 ** (b ?? 3)) (a * b ?? c // d) == a * (b ?? c) // d (a ?? True and b ?? False) == (a ?? True) and (b ?? False) (c() ?? c() ?? True) == True (True ?? ex()) == True (c ?? ex)() == c() Augmented coalescing assignment only rebinds the name if its current value is ``None``. If the target name already has a value, the right-hand side is not evaluated. For example:: a = None b = '' c = 0 a ??= 'value' b ??= undefined_name c ??= shutil.rmtree('/') # don't try this at home, kids assert a == 'value' assert b == '' assert c == '0' and any(os.scandir('/')) Adding new trailers for the other ``None``-aware operators ensures that they may be used in all valid locations for the existing equivalent operators, including as part of an assignment target (more details below). As the existing evaluation rules are not directly embedded in the grammar, we specify the required changes here. Assume that the ``atom`` is always successfully evaluated. Each ``trailer`` is then evaluated from left to right, applying its own parameter (either its arguments, subscripts or attribute name) to produce the value for the next ``trailer``. Finally, if present, ``await`` is applied. For example, ``await a.b(c).d[e]`` is currently parsed as ``['await', 'a', '.b', '(c)', '.d', '[e]']`` and evaluated:: _v = a _v = _v.b _v = _v(c) _v = _v.d _v = _v[e] await _v When a ``None``-aware operator is present, the left-to-right evaluation may be short-circuited. For example, ``await a?.b(c).d?[e]`` is evaluated:: _v = a if _v is not None: _v = _v.b _v = _v(c) _v = _v.d if _v is not None: _v = _v[e] await _v .. note:: ``await`` will almost certainly fail in this context, as it would in the case where code attempts ``await None``. We are not proposing to add a ``None``-aware ``await`` keyword here, and merely include it in this example for completeness of the specification, since the ``atom_expr`` grammar rule includes the keyword. If it were in its own rule, we would have never mentioned it. Parenthesised expressions are handled by the ``atom`` rule (not shown above), which will implicitly terminate the short-circuiting behaviour of the above transformation. For example, ``(a?.b ?? c).d?.e`` is evaluated as:: # a?.b _v = a if _v is not None: _v = _v.b # ... ?? c if _v is None: _v = c # (...).d?.e _v = _v.d if _v is not None: _v = _v.e When used as an assignment target, the ``None``-aware operations may only be used in a "load" context. That is, ``a?.b = 1`` and ``a?[b] = 1`` will raise ``SyntaxError``. Use earlier in the expression (``a?.b.c = 1``) is permitted, though unlikely to be useful unless combined with a coalescing operation:: (a?.b ?? d).c = 1 Examples ======== This section presents some examples of common ``None`` patterns and shows what conversion to use ``None``-aware operators may look like. Standard Library ---------------- Using the ``find-pep505.py`` script[3]_ an analysis of the Python 3.7 standard library discovered up to 678 code snippets that could be replaced with use of one of the ``None``-aware operators:: $ find /usr/lib/python3.7 -name '*.py' | xargs python3.7 find-pep505.py <snip> Total None-coalescing `if` blocks: 449 Total [possible] None-coalescing `or`: 120 Total None-coalescing ternaries: 27 Total Safe navigation `and`: 13 Total Safe navigation `if` blocks: 61 Total Safe navigation ternaries: 8 Some of these are shown below as examples before and after converting to use the new operators. From ``bisect.py``:: def insort_right(a, x, lo=0, hi=None): # ... if hi is None: hi = len(a) # ... After updating to use the ``??=`` augmented assignment statement:: def insort_right(a, x, lo=0, hi=None): # ... hi ??= len(a) # ... From ``calendar.py``:: encoding = options.encoding if encoding is None: encoding = sys.getdefaultencoding() optdict = dict(encoding=encoding, css=options.css) After updating to use the ``??`` operator:: optdict = dict(encoding=encoding ?? sys.getdefaultencoding(), css=options.css) From ``dis.py``:: def _get_const_info(const_index, const_list): argval = const_index if const_list is not None: argval = const_list[const_index] return argval, repr(argval) After updating to use the ``?[]`` and ``??`` operators:: def _get_const_info(const_index, const_list): argval = const_list?[const_index] ?? const_index return argval, repr(argval) From ``inspect.py``:: for base in object.__bases__: for name in getattr(base, "__abstractmethods__", ()): value = getattr(object, name, None) if getattr(value, "__isabstractmethod__", False): return True After updating to use the ``?.`` operator (and deliberately not converting to use ``any()``):: for base in object.__bases__: for name in base?.__abstractmethods__ ?? (): if object?.name?.__isabstractmethod__: return True From ``os.py``:: if entry.is_dir(): dirs.append(name) if entries is not None: entries.append(entry) else: nondirs.append(name) After updating to use the ``?.`` operator:: if entry.is_dir(): dirs.append(name) entries?.append(entry) else: nondirs.append(name) jsonify ------- This example is from a Python web crawler that uses the Flask framework as its front-end. This function retrieves information about a web site from a SQL database and formats it as JSON to send to an HTTP client:: class SiteView(FlaskView): @route('/site/<id_>', methods=['GET']) def get_site(self, id_): site = db.query('site_table').find(id_) return jsonify( first_seen=site.first_seen.isoformat() if site.first_seen is not None else None, id=site.id, is_active=site.is_active, last_seen=site.last_seen.isoformat() if site.last_seen is not None else None, url=site.url.rstrip('/') ) Both ``first_seen`` and ``last_seen`` are allowed to be ``null`` in the database, and they are also allowed to be ``null`` in the JSON response. JSON does not have a native way to represent a ``datetime``, so the server's contract states that any non-``null`` date is represented as an ISO-8601 string. Without knowing the exact semantics of the ``first_seen`` and ``last_seen`` attributes, it is impossible to know whether the attribute can be safely or performantly accessed multiple times. One way to fix this code is to replace each conditional expression with an explicit value assignment and a full ``if``/``else`` block:: class SiteView(FlaskView): @route('/site/<id_>', methods=['GET']) def get_site(self, id_): site = db.query('site_table').find(id_) first_seen_dt = site.first_seen if first_seen_dt is None: first_seen = None else: first_seen = first_seen_dt.isoformat() last_seen_dt = site.last_seen if last_seen_dt is None: last_seen = None else: last_seen = last_seen_dt.isoformat() return jsonify( first_seen=first_seen, id=site.id, is_active=site.is_active, last_seen=last_seen, url=site.url.rstrip('/') ) This adds ten lines of code and four new code paths to the function, dramatically increasing the apparent complexity. Rewriting using the ``None``-aware attribute operator results in shorter code with more clear intent:: class SiteView(FlaskView): @route('/site/<id_>', methods=['GET']) def get_site(self, id_): site = db.query('site_table').find(id_) return jsonify( first_seen=site.first_seen?.isoformat(), id=site.id, is_active=site.is_active, last_seen=site.last_seen?.isoformat(), url=site.url.rstrip('/') ) Grab ---- The next example is from a Python scraping library called `Grab <https://github.com/lorien/grab/blob/4c95b18dcb0fa88eeca81f5643c0ebfb114bf72… ab/upload.py>`_:: class BaseUploadObject(object): def find_content_type(self, filename): ctype, encoding = mimetypes.guess_type(filename) if ctype is None: return 'application/octet-stream' else: return ctype class UploadContent(BaseUploadObject): def __init__(self, content, filename=None, content_type=None): self.content = content if filename is None: self.filename = self.get_random_filename() else: self.filename = filename if content_type is None: self.content_type = self.find_content_type(self.filename) else: self.content_type = content_type class UploadFile(BaseUploadObject): def __init__(self, path, filename=None, content_type=None): self.path = path if filename is None: self.filename = os.path.split(path)[1] else: self.filename = filename if content_type is None: self.content_type = self.find_content_type(self.filename) else: self.content_type = content_type This example contains several good examples of needing to provide default values. Rewriting to use conditional expressions reduces the overall lines of code, but does not necessarily improve readability:: class BaseUploadObject(object): def find_content_type(self, filename): ctype, encoding = mimetypes.guess_type(filename) return 'application/octet-stream' if ctype is None else ctype class UploadContent(BaseUploadObject): def __init__(self, content, filename=None, content_type=None): self.content = content self.filename = (self.get_random_filename() if filename is None else filename) self.content_type = (self.find_content_type(self.filename) if content_type is None else content_type) class UploadFile(BaseUploadObject): def __init__(self, path, filename=None, content_type=None): self.path = path self.filename = (os.path.split(path)[1] if filename is None else filename) self.content_type = (self.find_content_type(self.filename) if content_type is None else content_type) The first ternary expression is tidy, but it reverses the intuitive order of the operands: it should return ``ctype`` if it has a value and use the string literal as fallback. The other ternary expressions are unintuitive and so long that they must be wrapped. The overall readability is worsened, not improved. Rewriting using the ``None`` coalescing operator:: class BaseUploadObject(object): def find_content_type(self, filename): ctype, encoding = mimetypes.guess_type(filename) return ctype ?? 'application/octet-stream' class UploadContent(BaseUploadObject): def __init__(self, content, filename=None, content_type=None): self.content = content self.filename = filename ?? self.get_random_filename() self.content_type = content_type ?? self.find_content_type(self.filename) class UploadFile(BaseUploadObject): def __init__(self, path, filename=None, content_type=None): self.path = path self.filename = filename ?? os.path.split(path)[1] self.content_type = content_type ?? self.find_content_type(self.filename) This syntax has an intuitive ordering of the operands. In ``find_content_type``, for example, the preferred value ``ctype`` appears before the fallback value. The terseness of the syntax also makes for fewer lines of code and less code to visually parse, and reading from left-to-right and top-to-bottom more accurately follows the execution flow. Rejected Ideas ============== The first three ideas in this section are oft-proposed alternatives to treating ``None`` as special. For further background on why these are rejected, see their treatment in `PEP 531 <https://www.python.org/dev/peps/pep-0531/>`_ and `PEP 532 <https://www.python.org/dev/peps/pep-0532/>`_ and the associated discussions. No-Value Protocol ----------------- The operators could be generalised to user-defined types by defining a protocol to indicate when a value represents "no value". Such a protocol may be a dunder method ``__has_value__(self)` that returns ``True`` if the value should be treated as having a value, and ``False`` if the value should be treated as no value. With this generalization, ``object`` would implement a dunder method equivalent to this:: def __has_value__(self): return True ``NoneType`` would implement a dunder method equivalent to this:: def __has_value__(self): return False In the specification section, all uses of ``x is None`` would be replaced with ``not x.__has_value__()``. This generalization would allow for domain-specific "no-value" objects to be coalesced just like ``None``. For example the ``pyasn1`` package has a type called ``Null`` that represents an ASN.1 ``null``:: >>> from pyasn1.type import univ >>> univ.Null() ?? univ.Integer(123) Integer(123) Similarly, values such as ``math.nan`` and ``NotImplemented`` could be treated as representing no value. However, the "no-value" nature of these values is domain-specific, which means they *should* be treated as a value by the language. For example, ``math.nan.imag`` is well defined (it's ``0.0``), and so short-circuiting ``math.nan?.imag`` to return ``math.nan`` would be incorrect. As ``None`` is already defined by the language as being the value that represents "no value", and the current specification would not preclude switching to a protocol in the future (though changes to built-in objects would not be compatible), this idea is rejected for now. Boolean-aware operators ----------------------- This suggestion is fundamentally the same as adding a no-value protocol, and so the discussion above also applies. Similar behavior to the ``??`` operator can be achieved with an ``or`` expression, however ``or`` checks whether its left operand is false-y and not specifically ``None``. This approach is attractive, as it requires fewer changes to the language, but ultimately does not solve the underlying problem correctly. Assuming the check is for truthiness rather than ``None``, there is no longer a need for the ``??`` operator. However, applying this check to the ``?.`` and ``?[]`` operators prevents perfectly valid operations applying Consider the following example, where ``get_log_list()`` may return either a list containing current log messages (potentially empty), or ``None`` if logging is not enabled:: lst = get_log_list() lst?.append('A log message') If ``?.`` is checking for true values rather than specifically ``None`` and the log has not been initialized with any items, no item will ever be appended. This violates the obvious intent of the code, which is to append an item. The ``append`` method is available on an empty list, as are all other list methods, and there is no reason to assume that these members should not be used because the list is presently empty. Further, there is no sensible result to use in place of the expression. A normal ``lst.append`` returns ``None``, but under this idea ``lst?.append`` may result in either ``[]`` or ``None``, depending on the value of ``lst``. As with the examples in the previous section, this makes no sense. As checking for truthiness rather than ``None`` results in apparently valid expressions no longer executing as intended, this idea is rejected. Exception-aware operators ------------------------- Arguably, the reason to short-circuit an expression when ``None`` is encountered is to avoid the ``AttributeError`` or ``TypeError`` that would be raised under normal circumstances. As an alternative to testing for ``None``, the ``?.`` and ``?[]`` operators could instead handle ``AttributeError`` and ``TypeError`` raised by the operation and skip the remainder of the expression. This produces a transformation for ``a?.b.c?.d.e`` similar to this:: _v = a try: _v = _v.b except AttributeError: pass else: _v = _v.c try: _v = _v.d except AttributeError: pass else: _v = _v.e One open question is which value should be returned as the expression when an exception is handled. The above example simply leaves the partial result, but this is not helpful for replacing with a default value. An alternative would be to force the result to ``None``, which then raises the question as to why ``None`` is special enough to be the result but not special enough to be the test. Secondly, this approach masks errors within code executed implicitly as part of the expression. For ``?.``, any ``AttributeError`` within a property or ``__getattr__`` implementation would be hidden, and similarly for ``?[]`` and ``__getitem__`` implementations. Similarly, simple typing errors such as ``{}?.ietms()`` could go unnoticed. Existing conventions for handling these kinds of errors in the form of the ``getattr`` builtin and the ``.get(key, default)`` method pattern established by ``dict`` show that it is already possible to explicitly use this behaviour. As this approach would hide errors in code, it is rejected. ``None``-aware Function Call ---------------------------- The ``None``-aware syntax applies to attribute and index access, so it seems natural to ask if it should also apply to function invocation syntax. It might be written as ``foo?()``, where ``foo`` is only called if it is not None. This has been deferred on the basis of the proposed operators being intended to aid traversal of partially populated hierarchical data structures, *not* for traversal of arbitrary class hierarchies. This is reflected in the fact that none of the other mainstream languages that already offer this syntax have found it worthwhile to support a similar syntax for optional function invocations. A workaround similar to that used by C# would be to write ``maybe_none?.__call__(arguments)``. If the callable is ``None``, the expression will not be evaluated. (The C# equivalent uses ``?.Invoke()`` on its callable type.) ``?`` Unary Postfix Operator ---------------------------- To generalize the ``None``-aware behavior and limit the number of new operators introduced, a unary, postfix operator spelled ``?`` was suggested. The idea is that ``?`` might return a special object that could would override dunder methods that return ``self``. For example, ``foo?`` would evaluate to ``foo`` if it is not ``None``, otherwise it would evaluate to an instance of ``NoneQuestion``:: class NoneQuestion(): def __call__(self, *args, **kwargs): return self def __getattr__(self, name): return self def __getitem__(self, key): return self With this new operator and new type, an expression like ``foo?.bar[baz]`` evaluates to ``NoneQuestion`` if ``foo`` is None. This is a nifty generalization, but it's difficult to use in practice since most existing code won't know what ``NoneQuestion`` is. Going back to one of the motivating examples above, consider the following:: >>> import json >>> created = None >>> json.dumps({'created': created?.isoformat()})`` The JSON serializer does not know how to serialize ``NoneQuestion``, nor will any other API. This proposal actually requires *lots of specialized logic* throughout the standard library and any third party library. At the same time, the ``?`` operator may also be **too general**, in the sense that it can be combined with any other operator. What should the following expressions mean?:: >>> x? + 1 >>> x? -= 1 >>> x? == 1 >>> ~x? This degree of generalization is not useful. The operators actually proposed herein are intentionally limited to a few operators that are expected to make it easier to write common code patterns. Built-in ``maybe`` ------------------ Haskell has a concept called `Maybe <https://wiki.haskell.org/Maybe>`_ that encapsulates the idea of an optional value without relying on any special keyword (e.g. ``null``) or any special instance (e.g. ``None``). In Haskell, the purpose of ``Maybe`` is to avoid separate handling of "something" and nothing". A Python package called `pymaybe <https://pypi.org/p/pymaybe/>`_ provides a rough approximation. The documentation shows the following example:: >>> maybe('VALUE').lower() 'value' >>> maybe(None).invalid().method().or_else('unknown') 'unknown' The function ``maybe()`` returns either a ``Something`` instance or a ``Nothing`` instance. Similar to the unary postfix operator described in the previous section, ``Nothing`` overrides dunder methods in order to allow chaining on a missing value. Note that ``or_else()`` is eventually required to retrieve the underlying value from ``pymaybe``'s wrappers. Furthermore, ``pymaybe`` does not short circuit any evaluation. Although ``pymaybe`` has some strengths and may be useful in its own right, it also demonstrates why a pure Python implementation of coalescing is not nearly as powerful as support built into the language. The idea of adding a builtin ``maybe`` type to enable this scenario is rejected. Just use a conditional expression --------------------------------- Another common way to initialize default values is to use the ternary operator. Here is an excerpt from the popular `Requests package <https://github.com/kennethreitz/requests/blob/14a555ac716866678bf17e43e2323… a8149f5/requests/models.py#L212>`_:: data = [] if data is None else data files = [] if files is None else files headers = {} if headers is None else headers params = {} if params is None else params hooks = {} if hooks is None else hooks This particular formulation has the undesirable effect of putting the operands in an unintuitive order: the brain thinks, "use ``data`` if possible and use ``[]`` as a fallback," but the code puts the fallback *before* the preferred value. The author of this package could have written it like this instead:: data = data if data is not None else [] files = files if files is not None else [] headers = headers if headers is not None else {} params = params if params is not None else {} hooks = hooks if hooks is not None else {} This ordering of the operands is more intuitive, but it requires 4 extra characters (for "not "). It also highlights the repetition of identifiers: ``data if data``, ``files if files``, etc. When written using the ``None`` coalescing operator, the sample reads:: data = data ?? [] files = files ?? [] headers = headers ?? {} params = params ?? {} hooks = hooks ?? {} References ========== .. [1] C# Reference: Operators (https://msdn.microsoft.com/en-us/library/6a71f45d.aspx) .. [2] A Tour of the Dart Language: Operators (https://www.dartlang.org/docs/dart-up-and-running/ch02.html#operators) .. [3] Associated scripts (https://github.com/python/peps/tree/master/pep-0505/) Copyright ========= This document has been placed in the public domain. [View Less]

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Re: [Python-ideas] slice[] to get more complex slices
by Jeroen Demeyer July 31, 2018

July 31, 2018

On 2018-07-23 12:24, Jeroen Demeyer wrote: > Another solution that nobody has mentioned (as far as I know) is to add > additional syntax to the language for that. For example, one could say > that (1:3) could be used to construct slice(1, 3) directly. The > parentheses are required to avoid confusion with type hints. I'm not a > Python language expert, but I don't think that type hints can occur > inside parentheses like that. And this could be extended to tuples (1:3, 2:4) … [View More]

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Idea: msgfmt.py and pygettext..py should be -m executable modules
by Miro Hrončok July 30, 2018

July 30, 2018

Currently, the documentation (as well as plenty of other places on the Internet, such as Stack Overflow answers) mentions msgfmt.py and pygettext.py. See https://docs.python.org/3/library/gettext.html > (Python also includes pure-Python versions of these programs, called > pygettext.py and msgfmt.py; some Python distributions will install > them for you. pygettext.py is similar to xgettext, but only > understands Python source code and cannot handle other programming > … [View More]

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Re: [Python-ideas] As-do statements/anonymous blocks in python
by James Lu July 29, 2018

July 29, 2018

By passing a function to another function I meant passing a code block as an inline function to a function call. The do statement is simply the arguments the function is called with Brackets = optional as expr [do comma-separated-expressions]: block means evaluate expr, then call the result of the expression. If do is present, call it with the argument list after do. Otherwise, call it with no arguments. > If you don't have practical uses, this proposal has ZERO chance of being > … [View More]

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Can we add "zip and assert equal length" to the standard library?
by Peter O'Connor July 28, 2018

July 28, 2018

I find that about 90% of the time I want want to zip iterators together, I expect them to be the same length and want to throw an exception if they aren't. Yet there is not currently a solution for this in the standard library for this, and as a result I always have to take this function everywhere I go: def zip_equal(*iterables): """ Zip and raise exception if lengths are not equal. Taken from solution by Martijn Pieters, here: http://stackoverflow.com/… [View More]

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Idea: Deferred Default Arguments?
by Peter O'Connor July 28, 2018

July 28, 2018

Often when programming I run into a situation where it would be nice to have "deferred defaults". Here is an example of what I mean: def subfunction_1(a=2, b=3, c=4): return a+b*c def subfunction_2(d=5, e=6, f=7): return d*e+f def main_function(a=2, b=3, c=4, d=5, e=6, f=7): return subfunction_1(a=a, b=b, c=c) + subfunction_2(d=d, e=e, f=f) Here you can see that I had to redefine the defaults in the main_function. In larger codebases, I find bugs often … [View More]

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Type annotations in TypeError messages - Improved error reporting
by Karthikeyan July 27, 2018

July 27, 2018

Python 3 has support for type annotations. Normally when we call a function with too few arguments then it returns a TypeError message with required arguments name but it gives no information about the type of the variables when there are type annotations for the function. I have added a little patch to return type annotations for the parameters and return type when there are type errors. This doesn't affect the existing code but improves on the code that has type annotations. My C skills … [View More]

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Add function readbyte to asyncio.StreamReader
by Jörn Heissler July 27, 2018

July 27, 2018

Hello, I'm implementing a protocol where I need to read individual bytes until a condition is met (value & 0x80 == 0). My current approach is: value = (await reader.readexactly(1))[0] To speed this up, I propose that a new function is added to asyncio.StreamReader: value = await reader.readbyte() I duplicated readexactly and stripped out some parts. Below code appears to work: async def readbyte(self): if self._exception is not None: raise self._exception … [View More]

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As-do statements/anonymous blocks in python
by James Lu July 27, 2018

July 27, 2018

I'm open to any changes or criticism. ``` import atexit as atexit.register: # ...do various cleanup tasks... print('Goodbye') # is approximately equivalent to => import atexit def _(): # ...do various cleanup tasks... print('Goodbye') atexit.register(_) # flask example @app.route("/") def hello(): return "Hello World!" # is approximately equivalent to => as app.route('/'): return "Hello World!" @app.route('/user/<username>') def show_user_profile(username)… [View More]: # show the user profile for that user return 'User %s' % username as app.route('/user/<username>') do username: return "Hello World!" def print_sorted(iterable, block): sorted(iterable, ) l = [1, 2, 3, 4, 'spam'] as l.sort(key=%) do obj: return str(obj) # multiple arguments as spam do a, b: ... ``` ## `%` function call syntax Calling a function with a single percent in place of an argument creates a new function. ``` lumberjack(15, %) # is equivalent to the expression lambda x: lumberjack(15, %) ``` Using `*` instead of `%` could also be possible. ``` import threading, time def interval(seconds, block): def target(): while True: time.sleep(seconds) if block(): # stop looping if block returns True break threading.Thread(target=target).start() as interval(5, %): print("chirp") # => chirp every 5 seconds on a seperate thread as threading.Timer(5, %): print("hi") # => say "hi" in 5 seconds ``` ## `^` currying function definition syntax? I'm not sure this is necessary or a good idea. ``` def interval(seconds, ^block): def target(): while True: time.sleep(seconds) if block(): # stop looping if block returns True break threading.Thread(target=target).start() # is aprroximately equivalent to def interval(seconds, block=None): def inner(block): def target(): while True: time.sleep(seconds) if block(): break threading.Thread(target=target).start() if block == None: def outer(block): return inner(block) else: return inner(block) as interval(5): print('chirp') # equivalent to interval(5)(lambda: print('chirp')) ``` ### Lazy evaluation of chained `%` calls? This would allow things like: ``` start_on_new_thread = threading.Thread(target=%).start() def bong(): while True: time.sleep(6*60) print('BONG') start_on_new_thread(bong) # alternatively as start_on_new_thread: while True: time.sleep(6*60) print('BONG') ``` ## As-do statements in classes ``` class M(): def __init__(self): self.time = 5 as interval(self.time, %): print('chirp') ``` I'm not sure if this should be valid, and I'd like the community's input on when as-do statements should be bound when as-do. ## Proposed Type Hinting Syntax ``` as app.route('/user/<username>') do (username: str): return "Hello World!" ``` Alternatives: ``` as app.route('/user/<username>') do username: str% return "Hello World!" # like objective-c as app.route('/user/<username>') do username: str^ return "Hello World!" # like coffeescript as app.route('/user/<username>') do username: str -> return "Hello World!" ``` I’m not totally sure of practical uses, but I’m imagining it would make passing a function to another function much more convenient. In React, you pass an argument called `render` to a `FlatList`, and `render` renders an item of the list. `FlatList` is a scrollable list that handles unloading off-screen items and loading items that will appear in the scroll box soon. [View Less]

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