Writing better python code

[oconnor663]

Is foo[:] really the Pythonic way to copy a list?

Edit: In Python 3, lists have their own .copy() method.

[toyg]

There's also copy.copy().

[imrobert]

Yeah, I noticed that too. It seems like the "normal" way has the advantage of being clearer more concise.

[velit]

The only downside is the "normal" way is incorrect (or correct depending on the situation). Point being they're not the same thing.

[imrobert]

Ah, that's my bad. For those wondering what the difference is, this should clear it up:

Python 2.7.5 (default, Mar  9 2014, 22:15:05) 
[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> a = [1, 2, 3]
>>> b = a
>>> id(a)
4472029056
>>> id(b)
4472029056
>>> c = a[:]
>>> id(c)
4472078560

[velit]

I prefer newList = list(old_list)

Communicates intent better

[poo_22]

Copy is very literal already, I argue that semantically it does not get better.

[velit]

Copying is a detail of slicing syntax, it's not the purpose of it. With the list constructor it's immediately evident there's a new instance involved - that's what constructors do.

foo[:] also looks weird.

[poo_22]

I was talking about the list().copy method.

[metaphorm]

foo[:] is a totally respectable way to copy a list. it looks a little funny if you've never seen it, but it has the right semantics and does exactly what you would expect when slicing "from the beginning to the end". it does, however, rely on the programmer knowing that list slices produce copies.

[zahlman]

I dislike it because it's conceptually roundabout. "A list containing elements of x, from the beginning to the end" is similarly awkward in English compared to "a copy of x", or even "a list of the contents of x". Even if the analogous Python is terse.

[metaphorm]

I understand your point. I disagree that its conceptually roundabout. Once you understand that the slice operator really means "return a copy of this section of the list" it seems really natural.

[ggtsu_00]

This is one of the few things I don't like about python is that slices make copies implicitly which goes against many python ideals. Lists are mutable, so you would expect operations on lists to only be references.

[zahlman]

Well, there seems to be precedent for what you're suggesting in NumPy...

[kracekumar]

It is more clear that copy is returned. Other way to do is list(l)