On Mon, 4 May 2020 at 19:13, Lewis Ball
I had a similar solution with a decorator using inspect.getfullargspec but it is pretty fiddly and it is pretty easy to miss some of the edge cases. Having a standard implementation would definitely take care of this. And I imagine static analysis tools would be able to cope with it, they do a good job with all of the current language features!
I like this idea of having a decorator for that. Since the decorator can apply the arguments to the instance without actually fiddling into `locals` or on `__init__` code. It could also be used _with_ dataclasses, suplying the assignment boiler plate for when one wants to have an explicit __init__ method. (yes, there is post_init, but as seem recently in a thread here, it is tough to get it working cooperatively) But I suppose @autoassign def __init__(self, a, b, flag1, flag2, etcetera): ... could live in "functools" without causing great harm.
On Mon, 4 May 2020, 22:11 Steele Farnsworth,
wrote: I agree that dataclasses are for a slightly different use case.
It looks like this could be implemented as a decorator using the functionality afforded by `inspect.signature`, though what I've come up with so far is a bit clunky because you have to account for parameters that could be positional or keyword and assigning default values for missing arguments.
If this were added, I assume that static analysis tools would need to be updated to account for the assumption that each instance has attributes with the same names that appear in the `__init__` signature, and I have no idea what that would entail. It would probably pose a similar issue for automated refactoring.
On Mon, May 4, 2020 at 4:48 PM Lewis Ball
wrote: I did think about data classes and although I haven't really used them much they do seem to be for a different use case, for example they don't support keyword-only args or positional-only args. I'm not sure if there are any other differences. Maybe a data class which supported kW-only args and pos-only args would suit my use case.
On Mon, 4 May 2020, 21:19 Henk-Jaap Wagenaar,
wrote: You are not the first to have this idea. Unless I am mistaken you might find what you are looking for in dataclasses which were added in Python 3.7:
https://docs.python.org/3/library/dataclasses.html
On Mon, 4 May 2020 at 19:06, Lewis Ball
wrote: Hi All,
First of all, if this is something which has been discussed in the past the please point me in the right direction.
Problem:
When creating classes in Python, I find myself writing the __init__ method in a very similar way a lot of the time, that is: ``` def __init__(self, argument_1, argument_2, argument_3=None): self.argument_1 = argument_1 self.argument_2 = argument_2 self.argument_3 = argument_3 # then maybe some other attribute setting and logic follows ```
Every argument of __init__ gets a corresponding attribute with the same name. This means that each `argument_i` has been typed 3 times, which seems overly-verbose as well as being easy to mistype. This pattern is easy to find in various popular python libraries, and in some it is actually enforced. For example, I do quite a bit of work with classifiers using the sklearn estimator API, and for various reasons sklearn enforce this pattern for an __init__ (see here if interested).
Here is an example of this pattern from the standard library (from textwrap.TextWrapper): ``` def __init__(self, width=70, initial_indent="", subsequent_indent="", expand_tabs=True, replace_whitespace=True, fix_sentence_endings=False, break_long_words=True, drop_whitespace=True, break_on_hyphens=True, tabsize=8, *, max_lines=None, placeholder=' [...]'): self.width = width self.initial_indent = initial_indent self.subsequent_indent = subsequent_indent self.expand_tabs = expand_tabs self.replace_whitespace = replace_whitespace self.fix_sentence_endings = fix_sentence_endings self.break_long_words = break_long_words self.drop_whitespace = drop_whitespace self.break_on_hyphens = break_on_hyphens self.tabsize = tabsize self.max_lines = max_lines self.placeholder = placeholder ```
With a quick scan of the top 50 or so most used python packages, 1 in 4 __init__ methods that takes arguments has the line `self.argument_i = argument_i` for every single argument, with several of them having 10+ arguments.
Suggestion:
A new built-in called something like `assign()` which would assign every single __init__ arg to a corresponding attribute. e.g. the snippet from above could be rewritten to: ``` def __init__(self, argument_1, argument_2, argument_3=None): assign() # other init logic goes here ```
This could alternatively be implemented as a decorator, like so ``` @assign def __init__(self, argument_1, argument_2, argument_3=None): # other init logic goes here ``` but then this requires a `pass` if no other logic is needed inside the __init__. There may also be some other syntax for this which would be even easier to use.
Is this something that others would find useful?
Thanks,
Lewis _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-leave@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/VLI3DO... Code of Conduct: http://python.org/psf/codeofconduct/
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