Mailman 3 June 2017 - Python-ideas

Re: [Python-ideas] [Python-Dev] Language proposal: variable assignment in functional context
by Steven D'Aprano June 26, 2017

June 26, 2017

Welcome Robert. My response below. Follow-ups to Python-Ideas, thanks. You'll need to subscribe to see any further discussion. On Fri, Jun 16, 2017 at 11:32:19AM +0000, Robert Vanden Eynde wrote: > In a nutshell, I would like to be able to write: > y = (b+2 for b = a + 1) I think this is somewhat similar to a suggestion of Nick Coghlan's. One possible syntax as a statement might be: y = b + 2 given: b = a + 1 https://www.python.org/dev/peps/pep-3150/ In mathematics, I might write: y = b + 2 where b = a + 1 although of course I wouldn't do so for anything so simple. Here's a better example, the quadratic formula: -b ± √Δ x = ───────── 2a where Δ = b² - 4ac although even there I'd usually write Δ in place. > Python already have the "functional if", lambdas, list comprehension, > but not simple assignment functional style. I think you mean "if *expression*" rather than "functional if". The term "functional" in programming usually refers to a particular paradigm: https://en.wikipedia.org/wiki/Functional_programming -- Steve

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A suggestion for a do...while loop
by Markus Meskanen June 26, 2017

June 26, 2017

I'm a huge fan of the do...while loop in other languages, and it would often be useful in Python too, when doing stuff like: while True: password = input() if password == ...: break I've seen the pep 315 which got rejected, and I believe these two suggestions were mostly focused on: 1. do ... while <condition>: <body> 2. do: <body> while <condition> But both were rejected for valid reasons: 1. It makes little sense to have the while at the top, since it might need to use variables from within the body. That's the whole point of the do...while loop 2. There's no other syntax like this in Python, where you'd need a closing unindentation. Also it's using the existing "while" keyword with a different syntax than the current one What I'd like to suggest is a different approach, with an existing syntax found in functions. At first it might sound silly, but I belive it makes sense after a while: do: <body> Now bare with me a second, this is similar to function's def <header>: <body> And similar to how you can exit functions with `return`, you could also exit the `do` loop with some keyword, such as `until`. This makes it very similar to how `return` behaves in a function: do: password = input() until password == secret_password Now before you say "but return can be anywhere in the function, and there can be multiple of them", I suggest we do the same for "until". It would be like a break, but with a condition: do: password = input('Password: ') until password == secret_password # This line only gets printed if until failed print('Invalid password, try again!') print('You have successfully signed in!') The keywords are obviously a subject to change, I've also been tinkering with repeat/until, do/breakif, repeat/breakon, etc. Any thoughts, is this complete madness?

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Re: [Python-ideas] Allow function to return multiple values
by Mikhail V June 25, 2017

June 25, 2017

joannah nanjekye wrote: > [...] > >Today I was writing an example snippet for the book and needed to write a >function that returns two values something like this: > >def return_multiplevalues(num1, num2): > return num1, num2 > > I noticed that this actually returns a tuple of the values which I did not >want in the first place.I wanted python to return two values in their own >types so I can work with them as they are but here I was stuck with working >around a tuple. It was quite puzzling at first what was the actual idea but probably I can guess why this question came up by you. It seems to me (I am just intuitively guessing that) that you were about to write a procedure which operates on global variables. If so you should use the keyword "global" for that. E.g. if you want to work with the variables defined in other part of the code you can simply do it: x = 0 y = 0 def move(): global x, y x = x + 10 y = y + 20 move() print (x,y) This function will change x and y (global variables in this case). Note that without the line with the "global" statement this will not work. Another typical usage is initialising variables inside a procedure: def init_coordinates(): global x,y x=0 y=0 init_coordinates() print (x,y) So for some reason it seemed to me that you are trying to do something like that. >My proposal is we provide a way of functions returning multiple values. >This has been implemented in languages like Go and I have found many cases >where I needed and used such a functionality. I wish for this convenience >in python so that I don't have to suffer going around a tuple. So if using globals as in the above examples you certinly don't have to suffer going around a tuple. Mikhail

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Restore the __members__ behavior to python3 for C extension writers
by Barry Scott June 20, 2017

June 20, 2017

I have been trying to get dir(c_ext_obj) to work for PyCXX as the method used with python2 was removed in python3, namely use the list of names returned from the __members__ attribute. I have failed to find a simple replacement for the python3 API. It seems that I have implement __dir__. But to do that I need to know what dir() will return and add the member variables to the answer. I have been able to figure out what is necessary to write such code. No one on python users or python dev responded to an earlier query on this subject. Would it be possible to simply put back the support for the __members__ attribute in python3? Or provide a API call to get the list that dir() would produce for my object. Barry

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the error that raises an AttributeError should be passed to __getattr__
by Jason Maldonis June 20, 2017

June 20, 2017

Hi everyone, A while back I had a conversation with some folks over on python-list. I was having issues implementing error handling of `AttributeError`s using `__getattr__`. My problem is that it is currently impossible for a `__getattr__` in Python to know which method raised the `AttributeError` that was caught by `__getattr__` if there are nested methods. For example, we cannot tell the difference between `A.x` not existing (which would raise an AttributeError) and some attribute inside `A.x` not existing (which also raises an AttributeError). This is evident from the stack trace that gets printed to screen, but `__getattr__` doesn't get that stack trace. I propose that the error that triggers an `AttributeError` should get passed to `__getattr__` (if `__getattr__` exists of course). Then, when handling errors, users could dig into the problematic error if they so desire. What do you think? Best, Jason

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Run length encoding
by Neal Fultz June 19, 2017

June 19, 2017

Hello python-ideas, I am very new to this, but on a different forum and after a couple conversations, I really wished Python came with run-length encoding built-in; after all, it ships with zip, which is much more complicated :) The general idea is to be able to go back and forth between two representations of a sequence: [1,1,1,1,2,3,4,4,3,3,3] and [(1, 4), (2, 1), (3, 1), (4, 2), (3, 3)] where the first element is the data element, and the second is how many times it is repeated. I wrote an encoder/decoder in about 20 lines ( https://github.com/nfultz/rle.py/blob/master/rle.py ) and would like to offer it for the next version; I think it might fit in nicely in the itertools module, for example. I am curious about your thoughts. Best, -Neal

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ImportError raised for a circular import
by Barry Scott June 19, 2017

June 19, 2017

Recently I fell into the trap of creating a circular import and yet again it took time to figure out what was wrong. I'm wondering why the python import code does not detect this error and raise an exception. I took a look at the code and got as far as figuring out that I would need to add the detection to the python 3 import code. Unless I missed something I cannot get the detection without modifying the core code as I could see no way to hook the process cleanly. Is it reasonable idea to add this detection to python? I am willing to work on a patch. Barry

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Continuation of `__name__` or a builtin function for general name getting
by Alireza Rafiei June 19, 2017

June 19, 2017

Hi all, I'm not sure whether this idea has been discussed before or not, so I apologize in advanced if that's the case. Consider the behavior: >>> f = lambda: True > >>> f.__name__ > '<lambda>' > >>> x = f > >>> x.__name__ > '<lambda>' I'm arguing the behavior above is too brittle/limited and, considering that the name of the attribute is `__name__`, not entirely consistent with Python's AST. Consider: >>> f = lambda: True > >>> x = f At the first line, an ast.Assign would be created whose target is an ast.Name whose `id` is `f`. At the second line, an ast.Assign would be created whose target is an ast.Name whose `id` is `x`. However, as you can see `__name__` special method returns 'lambda' in both cases (just like it was defined https://docs.python.org/3/library/stdtypes.html#definition.__name__), whereas I think either it should have returned '<lambda>' and 'x' or a new function/attribute should exist that does so and more. For example, consider: >>> x_1 = 1 > >>> x_2 = 1 > >>> x_3 = 1 > >>> x_4 = x_1 > >>> for i in [x_1, x_2, x_3, x_4]: > >>> print(i) > 1 > 1 > 1 > 1 Now assume such a function exist and is called `name`. Then: >>> name(1) > '1' > >>> name("Something") > "Something" > >>> name(x_1) > 'x_1' > >>> name(x_4) > 'x_4' > >>> name(x_5) > 'x_5' # Or an Exception! > >>> def itername(collection): > >>> for i in map(lambda x: name(x), collection): > >>> yield i > >>> > >>> for i in [x_1, x_2, x_3, x_4]: > >>> print(i, name(i)) > 1, 'i' > 1, 'i' > 1, 'i' > 1, 'i' > >>> for i in itername([x_1, x_2, x_3, x_4]): > >>> print(i) > 'x_1' > 'x_2' > 'x_3' > 'x_4' I think above example gives an idea of the behavior I'm proposing. I can implement it in a hacky way by parsing the block into ast and going through all the nodes that have an `id` or a `name` or `asname` etc, but this behavior wouldn't be pretty for run-time cases e.g. `itername` in the example above. Anyway I'd appreciate any input. P.S. I must confess that I can't think of a single case where having this function is the only way to do the job, if the job is sufficiently different from the specification of said function. It'd be only for the sake of convenience, and continuation of what already was there with `__name__`.

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Make functools.singledispatch register method return original function.
by Mital Ashok June 18, 2017

June 18, 2017

Right now, an example for single dispatch would be: from functools import singledispatch @singledispatch def fun(arg, verbose=True): if verbose: print("Let me just say,", end=" ") print(arg) @fun.register(int) def _(arg, verbose=True): if verbose: print("Strength in numbers, eh?", end=" ") print(arg) @fun.register(list) def _(arg, verbose=True): if verbose: print("Enumerate this:") for i, elem in enumerate(arg): print(i, elem) But this makes a useless _ function, that should either be deleted or ignored. For properties, a common pattern is this: class Foo: @property def bar(self): return self._bar @bar.setter def bar(self, value): self._bar = value So I'm suggesting that @function.register for single dispatch functions returns the same function, so you would end up with something like: @singledispatch def fun(arg, verbose=True): if verbose: print("Let me just say,", end=" ") print(arg) @fun.register(int) def fun(arg, verbose=True): if verbose: print("Strength in numbers, eh?", end=" ") print(arg) And to get back the old behaviour, where you can get the function being decorated, just call it afterwards: @singledispatch def fun(arg, verbose=True): if verbose: print("Let me just say,", end=" ") print(arg) def used_elsewhere(arg, verbose=True): if verbose: print("Strength in numbers, eh?", end=" ") print(arg) fun.register(int)(used_elsewhere) But this goes against what a single-dispatch function is, so I think this is a minor enough use case to not worry about.

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Re: [Python-ideas] [Python-Dev] Language proposal: variable assignment in functional context
by Erik June 16, 2017

June 16, 2017

[cross-posted to python-ideas] Hi Robert, On 16/06/17 12:32, Robert Vanden Eynde wrote: > Hello, I would like to propose an idea for the language but I don't know > where I can talk about it. Can you please explain what the problem is that you are trying to solve? > In a nutshell, I would like to be able to write: > y = (b+2 for b = a + 1) The above is (almost) equivalent to: y = (a+1)+2 I realize the parentheses are not required, but I've included them because if your example mixed operators with different precedence then they might be necessary. Other than binding 'b' (you haven't defined what you expect the scope of that to be, but I'll assume it's the outer scope for now), what is it about the form you're proposing that's different? > Or in list comprehension: > Y = [b+2 for a in L for b = a+1] > > Which can already be done like this: > Y = [b+2 for a in L for b in [a+1]] Y = [(a+1)+2 for a in L] > Which is less obvious, has a small overhead (iterating over a list) and > get messy with multiple assignment: > Y = [b+c+2 for a in L for b,c in [(a+1,a+2)]] > > New syntax would allow to write: > Y = [b+c+2 for a in L for b,c = (a+1,a+2)] Y = [(a+1)+(a+2)+2 for a in L] > My first example (b+2 for b = a+1) can already be done using ugly syntax > using lambda > > y = (lambda b: b+2)(b=a+1) > y = (lambda b: b+2)(a+1) > y = (lambda b=a+1: b+2)() > > Choice of syntax: for is good because it uses current keyword, and the > analogy for x = 5 vs for x in [5] is natural. > > But the "for" loses the meaning of iteration. > The use of "with" would maybe sound more logical. > > Python already have the "functional if", lambdas, list comprehension, > but not simple assignment functional style. Can you present an example that can't be re-written simply by reducing the expression as I have done above? Regards, E.

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