Mailman 3 February 2021 - Python-ideas

Make filter() and map() return function if iterable is not passed
by Henry Harutyunyan Feb. 11, 2021

Feb. 11, 2021

One of my students gave me an interesting question about why `filter` and `map` do not return a reusable function in case the iterable is not passed. With the current design ``` In [1]: a = filter(lambda x: x > 60) --------------------------------------------------------------------------- TypeError Traceback (most recent call last) <ipython-input-1-ad0178d4fce0> in <module> ----> 1 a = filter(lambda x: x > 60) TypeError: filter expected 2 … [View More]

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Re: Make UTF-8 mode more accessible for Windows users.
by Eryk Sun Feb. 11, 2021

Feb. 11, 2021

On 2/6/21, Christopher Barker <pythonchb(a)gmail.com> wrote: > On Sat, Feb 6, 2021 at 11:47 AM Eryk Sun <eryksun(a)gmail.com> wrote: > >> Relative to the installation, "python.cfg" should only be found in the >> same directory as the base executable, not its parent directory. > > OK, my mistake — I thought that was already the case with pyvenv.cfg. > Though I don’t get why it matters. Chiefly, I don't want to overload "pyvenv.cfg" with new behavior that's … [View More]

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Re: Generic NamedTuples
by Peter Ludemann Feb. 10, 2021

Feb. 10, 2021

I use dataclass (with frozen=True) instead of NamedTuple - and I can do mixins. https://docs.python.org/3/library/dataclasses.html

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Re: Very strange problem in python
by nate lust Feb. 10, 2021

Feb. 10, 2021

This is not really a support mailing list, it is intended for ideas about modifying the python language or its libraries. However after a quick look, the line tt = arr2 is not creating a copy of array2. It creates a new name bound to the same underlying object, an alias if you will. You will need to use some sort of copy function (such as np.copy) if you want an independent array. If you are interested in learning more, I suggest you look into how python implements variables. The FAQ ( https:/… [View More]

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Very strange problem in python
by Priya Singh Feb. 10, 2021

Feb. 10, 2021

def closset_mtach(check, arr, itr): id_min = [] for ii in range(itr): id_min_tmp = np.argmin( abs(arr - check) ) id_min.append(id_min_tmp) arr[id_min_tmp] = float('-inf') id_min = np.array(id_min) return id_min def get_CS_id(arr1, arr2, tol, itr): tt = arr2 for ii in range(5,6): id_com = np.where( abs(arr1[ii] - tt) < tol )[0] if np.size(id_com) >= itr: mm = closset_mtach(arr1[ii], tt[id_com], itr) … [View More]

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warning when importing a file from directoty
by Jonatan Feb. 9, 2021

Feb. 9, 2021

I see many people who have their project something like this: ``` directory/ some_module.py main.py ``` and in main.py they do ``` from some_module import some_function ``` while they have a library named `some_module` that is installed from pip, and they can't figure out why Python says that there is no such function. I would recommend adding a warning like "ImportWarning: The same module exists in this folder and site-packages a well." so people won't be confused why this function … [View More]

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class(obj) could return obj.__class__
by Hans Ginzel Feb. 9, 2021

Feb. 9, 2021

Please, consider class(obj) to return obj.__class__ consistenly with dir(), vars(), repr(), str(),… >>> class c: pass >>> o = c() >>> o.__class__ <class '__main__.c'> >>> class(o) File "<stdin>", line 1 class(o) ^ SyntaxError: invalid syntax H.

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Generic NamedTuples
by Ben Avrahami Feb. 8, 2021

Feb. 8, 2021

In both python 3.7 and 3.8, typing.NamedTuple had some strange behaviour regarding multiple inheritance. This behaviour was solved in 3.9 (bpo-36517), but this fix also barred me from a feature very close to my heart: the generic named tuple ``` T = TypeVar('T') class LLNode(NamedTuple, Generic[T]): value: T next: Optional[LLNode[T]] ``` bpo-36517 was committed because any additional features of other classes would be missing from the NamedTuple class at runtime. But Generic has no … [View More]

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What's up with assignment expression and tuples?
by Paul Sokolovsky Feb. 8, 2021

Feb. 8, 2021

Hello, Everyone knows how hard to find a compelling usecase for the assignment expression operator (":=", colloquially "walrus operator"). https://www.python.org/dev/peps/pep-0572/ examples never felt compelling and we all remember the split it caused. I finally found a usecase where *not* using assignment expression is *much* worse than using it. I'm working on SSA (Static Single Assignment, https://en.wikipedia.org/wiki/Static_single_assignment_form) conversion of Python programs, where … [View More]

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Re: [Python-Dev] Re: What's up with assignment expression and tuples?
by Paul Sokolovsky Feb. 8, 2021

Feb. 8, 2021

Hello, Thanks for the reply. On Fri, 5 Feb 2021 13:32:25 -0500 Terry Reedy <tjreedy(a)udel.edu> wrote: > On 2/5/2021 2:51 AM, Paul Sokolovsky wrote: > > > https://www.python.org/dev/peps/pep-0572/ > ... > > >>>> ((a, b) := (1, 2)) > > File "<stdin>", line 1 > > SyntaxError: cannot use assignment expressions with tuple > > ---- > > > > Why this accidental syntactic gap? > > As should be clear from … [View More]reading "Differences between assignment > expressions and assignment statements", this 'gap' in entirely > intentional, not accidental. *All* elaborations of 'name := > expression' are listed and rejected as outside the scope of the > proposal, which was to keep one reference to the expression value for > later use. At least some of these elaborations were suggested and > rejected during the voluminous discussion. And looking back now, that seems like intentionally added accidental gap in the language (https://en.wikipedia.org/wiki/Accidental_gap). Similar to artificially limiting decorator syntax, which was already un-limited. But seems, there're no "lessons learned", and there's now need to wait a decade again before fixing that? > The principal a.e. use in conditional expressions is testing for > non-nullness. Your > > > while ((a1, b1) := phi([a0, a2], [b0, b2]))[0] < 5: > > a2 = a1 + 1 > > b2 = b1 + 1 > > is an unusual and very specific use. Well, many people were thinking (and I bet still think) that ":=" itself is very unusual case. But if it's in, why not make it consistent with the assignment statement and unleash the full power of it? > You want to have your tuple > (for subscripting for testing) and eat it too (by unpacking). That's good characterization, thanks. And Python syntax alone would allow to do that, if not extra-syntactical limitations put on top of it. > One can instead separate unpacking from testing a couple of ways > > while (tup := phi([a0, a2], [b0, b2]))[0] < 5: > a2, b2 = tup > a2 = a1 + 1 > b2 = b1 + 1 > > > while True: > a1, b1 = phi([a0, a2], [b0, b2]) > if a1 >= 5: break > a2 = a1 + 1 > b2 = b1 + 1 Right, but I started my original email with "I finally found a usecase where *not* using assignment expression is *much* worse than using it." Both conversions above apply additional disturbances to the original program, beyond pure SSA conversion, which is quite a disturbance on its own. I was really excited that Python 3.7+ would be *the* language which would allow to express SSA conversion faithfully on the source form of the high-level language (usually SSA is applied to low-level assembly-like intermediate representation). But oops, accidental gap, and despite all the Python advanceness, still need to apply workarounds as with other mundane languages. [] -- Best regards, Paul mailto:pmiscml@gmail.com [View Less]

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