From my experience teaching Python to non-programmers, it's a huge hurdle/nightmare to teach functions/methods that modify objects in-place vs. return a value that must be reassigned. Behold Pandas's DataFrame's sort method, which has an optional `in_place` argument that defaults to *False*, which despite being a method that looks like mylist.sort(), works differently from the method on lists, but more like the sorted *function*, arrrgh! That was a fun session...
I think for consistency, having object methods that can act in-place (semantics like mylist.pop, mylist.append are nice as Stéfane suggests) only act in-place, and functions return a new object for reassignment would help new users. Maybe I'm teaching it poorly, suggestions welcome. Nick On Wed, Mar 1, 2017 at 11:26 AM, Stéfane Fermigier <sf@fermigier.com> wrote:
Definitively not, just like M. Fowler: "Meyer likes to use command-query separation absolutely, but there are exceptions. Popping a stack is a good example of a query that modifies state. Meyer correctly says that you can avoid having this method, but it is a useful idiom. So I prefer to follow this principle when I can, but I'm prepared to break it to get my pop."
What I wanted to point out is that the paragraph quoted by Stephan ("In general in Python (and in all cases in the standard library) a method that mutates an object will return None to help avoid getting the two types of operations confused. So if you mistakenly write y.sort() thinking it will give you a sorted copy of y, you’ll instead end up with None, which will likely cause your program to generate an easily diagnosed error.") doesn't seem to be true in this case.
S.
On Wed, Mar 1, 2017 at 6:23 PM, Cory Benfield <cory@lukasa.co.uk> wrote:
On 1 Mar 2017, at 10:26, Stéfane Fermigier <sf@fermigier.com> wrote:
Cf. https://martinfowler.com/bliki/CommandQuerySeparation.html
But:
l = [1,2,3] l.pop() 3 l [1, 2]
=> Not so true.
S.
This is naturally a different circumstance: pop must return the element it popped, otherwise it would just be del. Surely you aren’t suggesting that pop should return self?
Cory
-- Stefane Fermigier - http://fermigier.com/ - http://twitter.com/sfermigier - http://linkedin.com/in/sfermigier Founder & CEO, Abilian - Enterprise Social Software - http://www.abilian.com/ Chairman, Free&OSS Group / Systematic Cluster - http://www.gt-logiciel-libre.org/ Co-Chairman, National Council for Free & Open Source Software (CNLL) - http://cnll.fr/ Founder & Organiser, PyData Paris - http://pydata.fr/ --- “You never change things by fighting the existing reality. To change something, build a new model that makes the existing model obsolete.” — R. Buckminster Fuller
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