On Thu, Mar 30, 2017 at 04:23:05PM +0200, Pavol Lisy wrote:
On 3/30/17, Nick Coghlan <ncoghlan@gmail.com> wrote:
On 30 March 2017 at 19:18, Markus Meskanen <markusmeskanen@gmail.com> wrote:
Hi Pythonistas,
yet again today I ended up writing:
d = [[0] * 5 for _ in range(10)]
d = [[0]*5]*10 # what about this?
That doesn't do what you want. It's actually a common "gotcha", since it makes ten repetitions of the same five element list, not ten *copies*. py> d = [[0]*5]*10 py> d[0][0] = 9999 py> print(d) [[9999, 0, 0, 0, 0], [9999, 0, 0, 0, 0], [9999, 0, 0, 0, 0], [9999, 0, 0, 0, 0], [9999, 0, 0, 0, 0], [9999, 0, 0, 0, 0], [9999, 0, 0, 0, 0], [9999, 0, 0, 0, 0], [9999, 0, 0, 0, 0], [9999, 0, 0, 0, 0]] A slightly unfortunate conflict between desires: on the one hand, we definitely don't want * making copies of its arguments; on the other hand, that makes it less useful for initialising multi-dimensional (nested) lists. But then, nested lists ought to be rare. "Flat is better than nested."
Simplified repeating could be probably useful in interactive mode.
I'm sorry, did you just suggest that language features should behave differently in interactive mode than non-interactive mode? If so, that's a TERRIBLE idea. The point of interactive mode is to try out syntax and code and see what it does, before using it in non- interactive scripts. If things behave differently, people will be left confused why the *exact same line of code* works differently in a script and when they try it interactively. It is bad enough that the interactive interpreter includes a handful of features that make it different from non-interactive. I've been caught out repeatedly by the "last evaluated result" variable _ changing when the destructor method __del__ runs. That's another unavoidable case: adding extra, necessary functionality to the interactive interpreter, namely the ability to access the last evaluated result, neccessarily holds onto a reference to that last result. But that's easy enough to reason about, once you remember what's going on. But changing the behaviour of the language itself is just a bad idea. -- Steve