but how to emulate the style of a Python library.

I presume Python's has native support for iterators. Perl 6 has lazy lists, but Perl 5's doesn't, and nothing's going to make for work with lazy lists short of overriding for/foreach. Tied arrays definitely can't achieve that.

There are a few modules that provide iterator-aware looping primitives. Search for "iterator" on CPAN.

With for(@tied_array) we also doesn't need to calculate the length of the list in advance

That's not true.

Whether it's called for every loop pass or not, it's still called before the first pass, so you still need to know the size up front. And FETCHSIZE must always return a correct value because it's called once in other situations.

In fact, I consider it a bug that FETCHSIZE is called more than once in that situation.

Ironically, if you circumvent the optimisation of for (@a) by using for ((), @a), you actually get something faster for tied arrays since the latter only calls FETCHSIZE once. 

BEGIN {
    package My::TiedArray;

    use strict;
    use warnings;

    use Tie::Array qw( );

    our @ISA = 'Tie::StdArray';

    sub TIEARRAY {
        my $class = shift;
        bless [@_], $class
    }

    sub FETCHSIZE {
        my $self = shift;
        print("FETCHSIZE\n");
        return $self->SUPER::FETCHSIZE(@_);
    }

    $INC{"My/TiedArray.pm"} = 1;
}

use strict;
use warnings;
use feature qw( say );

use My::TiedArray qw( );

tie my @a, My::TiedArray::, qw( a b c );

say "Before loop";
for (@a) {
   say "In Loop";
}

say "";

say "Before loop";
for ((), @a) {
   say "In Loop";
}

[download]
Before loop
FETCHSIZE
In Loop
FETCHSIZE
In Loop
FETCHSIZE
In Loop
FETCHSIZE

Before loop
FETCHSIZE
In Loop
In Loop
In Loop

[download]

In reply to Re^9: Getting for() to accept a tied array in one statement by ikegami
in thread Getting for() to accept a tied array in one statement by perlancar

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.