[Python-ideas] YAML (yet-another-multiline-lambda)

[Andrew Barnert]

On Oct 22, 2013, at 5:39, Steven D'Aprano <steve at pearwood.info> wrote:

> On Tue, Oct 22, 2013 at 03:10:29AM -0700, Antony Lee wrote:
> 
>> Specifically, I suggest that the "def" (and possibly the "class")
>> keyword(s) may be used in an expression context, if immediately surrounded
>> by parentheses.  
> 
> I don't think there is any need to allow class in an expression, since 
> we already have type().

But type doesn't allow you to do most of what you can do in a class definition. This is like arguing that we don't need expression def because we already have types.FunctionType.

>> The indentation of the body of the function is given by
>> the indentation of the first line after the def.
> 
> I presume that means you can't do this:
> 
> value = [1, 2, 3, (def spam(arg): do_this()
>                        do_that()
>                        ), 4, 5, 6]
> 
> I think I would be okay with that, since you can't do that with the def 
> statement either:
> 
> py> def spam(): do_this()
> ...     do_that()
>  File "<stdin>", line 2
>    do_that()
>    ^
> IndentationError: unexpected indent
> 
> 
> I suppose, like the statement version, a single-line function body could 
> be inline:
> 
> value = [1, 2, 3, (def spam(arg): do_this()), 4, 5, 6]
> 
> which would then make lambda redundant.
> 
> In each of your examples, you have the body indented further to the 
> right than the def. Is that mandatory, or would you allow something like 
> this?
> 
> value = [1, 2, 3, (def spam(arg):
>    do_this()
>    do_that()
>    if condition():
>        do_something_else()
>    ),
>    4, 5, 6,
>    ]
> 
> That is, relative to the def itself, the body is outdented.
> 
> 
> I can't see any reason to justify prohibiting the above in the language, 
> although I'd probably frown upon it in style-guides. I think that should 
> be written as:
> 
> value = [1, 2, 3, 
>         (def spam(arg):
>              do_this()
>              do_that()
>              if condition():
>                  do_something_else()
>              ),
>         4, 5, 6,
>         ]
> 
> sort of thing. But I don't think that having the first indent be to the 
> left of the def should be prohibited. However, that can lead to some 
> pretty ugly, and misleading, constructions:
> 
> def spam(arg, func=(def ham(a,
>                            x, 
>                            y, 
>                            z):
>    fe(a)
>    fi(x)
>    fo(y)
>    fum(z)
>    ),
>        ):
>    ...
> 
> 
> I'm not sure if there is a question buried in this, or just an 
> observation. I hardly ever miss having multi-line lambdas, and I fear 
> that they will make code harder to read and understand.
> 
> 
> -- 
> Steven
> _______________________________________________
> Python-ideas mailing list
> Python-ideas at python.org
> https://mail.python.org/mailman/listinfo/python-ideas