Mailman 3 April 2019 - Python-ideas

Using rightarrow "->" for typing annotation of functions
by Vaibhav Karve April 26, 2019

April 26, 2019

(Note: This idea is about a particular static typecheking (typing?) annotation syntax). The idea is that currently the use of the "->" (right arrow) is restricted to only function definition annotation. Can we extent it to declaration of type for functions even outside their definitions? Example: Currently we write: f: Callable[[int, Dict[str, int]], str] # declaring the type of some fake function This would be much cleaner if we could write: f: int -> Dict[str, int] -> str … [View More]

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Add a `dir_fd` parameter to `os.truncate`?
by Sebastian M. Ernst April 25, 2019

April 25, 2019

Hi everyone, many methods in `os` have a `dir_fd` parameter, for instance `unlink` [1]: ```python os.unlink(path, *, dir_fd=None) ``` The `dir_fd` parameter [2] allows it to describe paths relative to directory descriptors. Otherwise, relative paths are relative to the current working directory. The implementation of `truncate` in `os` does not have this parameter [3]: ```python os.truncate(path, length) ``` On POSIX-like systems for instance, the `os` module actually imports [4] this … [View More]

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Re: [Python-ideas] What are the strong use cases for str.rindex()? (John Lin)
by Thautwarm Zhao April 25, 2019

April 25, 2019

IMO, there're lots of use cases in parsing related stuffs, which requires rindex a lot, say, when you have generated a tokenizer which might across multiple lines: line 8: X """ line 9: line 10: """ In this case, we need to extract 2 tokens X and , a multiline whitespace string. After getting each token we're to compute/update the current column and line number. If the line number gets advanced then we use rindex('\n') to help with updating the new column number, otherwise, col_offset += … [View More]len(current_token) . However, the reason why we don't need list.rindex but do for str.rindex is simple I'd say: str is immutable and has no O(1) reverse method. On the other hand, when it comes to list, you can use list.index after list.reverse, and after a bunch of operations you can resume the state by invoking list.reverse again. On Wed, Apr 24, 2019, 12:11 AM <python-ideas-request(a)python.org wrote: > Send Python-ideas mailing list submissions to > python-ideas(a)python.org > > To subscribe or unsubscribe via the World Wide Web, visit > https://mail.python.org/mailman/listinfo/python-ideas > or, via email, send a message with subject or body 'help' to > python-ideas-request(a)python.org > > You can reach the person managing the list at > python-ideas-owner(a)python.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Python-ideas digest..." > Today's Topics: > > 1. What are the strong use cases for str.rindex()? (???) > > > > ---------- Forwarded message ---------- > From: "林自均" <johnlinp(a)gmail.com> > To: python-ideas(a)python.org > Cc: > Bcc: > Date: Tue, 23 Apr 2019 14:44:25 +0800 > Subject: [Python-ideas] What are the strong use cases for str.rindex()? > Hi all, > > I found that there are str.index() and str.rindex(), but there is only > list.index() and no list.rindex(). So I filed the issue > https://bugs.python.org/issue36639 to provide list.rindex(). However, the > issue was rejected and closed with the comment: > > > There were known, strong use cases for str.rindex(). The list.rindex() > method was intentionally omitted. AFAICT no compelling use cases have > arisen, so we should continue to leave it out. In general, we don't grow > the core APIs unnecessarily. > > However, I am not sure what the known, strong use cases for str.rindex() > are. Why doesn't the strong use cases apply on list.rindex()? Could anyone > give me some examples? Thanks. > > Best, > John Lin > _______________________________________________ > Python-ideas mailing list > Python-ideas(a)python.org > https://mail.python.org/mailman/listinfo/python-ideas > [View Less]

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CPython Bytecode Assembler
by Batuhan Osman Taşkaya April 24, 2019

April 24, 2019

Hello, Currently it is hard to assemble cpython bytecode without help of 3rd party libraries (like: vstinner/bytecode). I'm proposing an assembler to standard library and an API to cpython's peephole optimizer. Also an interface like `ast.NodeVisitor` and `ast.NodeTransformer` for bytecode objects will may be handy. It would help if you are doing; - Runtime patching - Specific optimizations at bytecode level

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contains_any_in and contains_all_in
by João Matos April 24, 2019

April 24, 2019

Hello, If we want to check if a string contains any/all of several other strings we have to use several or/and conditions or any/all. For any: if ('string1' in master_string or 'string2' in master_string or 'string3' in master_string): or if any(item in master_string for item in ['string1', 'string2', 'string3']): For all: if ('string1' in master_string and 'string2' in master_string and'string3' in master_string): or if all(item in master_string for item in ['string1',… [View More]

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Catching the return value of a generator at the end of a for loop
by Stefano Borini April 17, 2019

April 17, 2019

given the following code def g(): yield 2 yield 3 return 6 for x in g(): print(x) The output is obviously 2 3 As far as I know, there is currently no way to capture the StopIteration value when the generator is used in a for loop. Is it true? If not, would a syntax like: for x in g() return v: print(x) print(v) # prints 6 be useful? It would be syntactic sugar for (corner cases omitted) def g(): yield 2 yield 3 return 6 it = iter(g()) while True: try:… [View More]

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Logical tracebacks
by Antoine Pitrou April 16, 2019

April 16, 2019

Hello, I apologize because I'm only going to throw a very vague idea and I don't currently have time or motivation to explore it myself. But I think it may prove interesting for other people and perhaps spur some concrete actionable proposal. With the growing complexity of Python software stacks, the length of tracebacks is continuously growing and is frequently making debugging errors and issues more tedious than it should be. This is a language-agnostic problem. Java software is often … [View More]

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Syntax for allowing extra keys when unpacking a dict as keyword arguments
by Viktor Roytman April 14, 2019

April 14, 2019

Currently, unpacking a dict in order to pass its items as keyword arguments to a function will fail if there are keys present in the dict that are invalid keyword arguments: >>> def func(*, a): ... pass ... >>> func(**{'a': 1, 'b': 2}) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: func() got an unexpected keyword argument 'b' The standard approach I have encountered in this scenario is to pass … [View More]

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Re: [Python-ideas] Syntax for allowing extra keys when unpacking a dict as keyword arguments
by Viktor Roytman April 12, 2019

April 12, 2019

> > That seems to me to be quite different issue. Just throwing invalid stuff > on the ground in this scenario will avoid a crash but lose data. This seems > much worse to me than the crash. > Throwing it away does seem extreme. Maybe something that indicates what's left over? In other words: result, leftover_kwargs = func(<some syntax for this>kwargs) or result = func(<some syntax for this>kwargs) assert kwargs == {'whatever is': 'left over'}

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Re: [Python-ideas] Syntax for allowing extra keys when unpacking a dict as keyword arguments
by Lucas Bourneuf April 12, 2019

April 12, 2019

Hello ! I made *fief*, a small python package allowing just that using a decorator: from fief import filter_effective_parameters as fief @fief def func(a, b): # some implementation # and then, use it as you want to: func(**MY_BIG_CONFIG_DICT_WITH_MANY_WEIRD_KEYS) The code is quite simple. You may want to use it, with modifications (i didn't touch the code for months, maybe years ; it could probably be improved now). Link: https://github.com/aluriak/fief The … [View More]

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