Is python's "set" stable? -

-

the question arose when answering question (there).

when iterate several times on python set (without changing between calls), can assume return elements in same order? , if not, rationale of changing order ? deterministic, or random? or implementation defined?

and when call same python program repeatedly (not random, not input dependent), same ordering sets?

the underlying question if python set iteration order depends on algorithm used implement sets, or on execution context?

there's no formal guarantee stability of sets (or dicts, matter.) however, in cpython implementation, long nothing changes set, items produced in same order. sets implemented open-addressing hashtables (with prime probe), inserting or removing items can change order (in particular, when triggers resize, reorganizes how items laid out in memory.) can have 2 identical sets nonetheless produce items in different order, example:

>>> s1 = {-1, -2} >>> s2 = {-2, -1} >>> s1 == s2 true >>> list(s1), list(s2) ([-1, -2], [-2, -1])

unless you're have same set , nothing touched inbetween 2 iterations, it's best not rely on staying same. making seemingly irrelevant changes to, say, functions call inbetween produce hard find bugs.


Comments

Post a Comment

Popular posts from this blog

c++ - Convert big endian to little endian when reading from a binary file -

-
this question has answer here: how convert between big-endian , little-endian values in c++? 28 answers i've been looking around how convert big-endian little-endians. didn't find solve problem. seem there's many way can conversion. anyway following code works ok in big-endian system. how should write conversion function work on little-endian system well? this homework, since systems @ school running big-endian system. it's got curious , wanted make work on home computer also #include <iostream> #include <fstream> using namespace std; int main() { ifstream file; file.open("file.bin", ios::in | ios::binary); if(!file) cerr << "not able read" << endl; else { cout << "opened" << endl; int i_var; double d_var; while(!file.eof()) {...
Read more

C#: Application without a window or taskbar item (background app) that can still use Console.WriteLine() -

-
i'm trying create application adheres following: no taskbar no console or form window can utilize console.writeline() . (i.e. if executes app command prompt write console.) the problem if create windows form (or wpf) application can have there no taskbar, console or window show up, console.writeline() nothing. if create console app, writes console, can't figure out how hide (and if did hide it, write command prompt window?)... how do this? just create standard console application. callers responsibility hide window caused program (from how run c# console application console hidden ): system.diagnostics.processstartinfo start = new system.diagnostics.processstartinfo(); start.filename = dir + @"\myprocesstostart.exe"; start.windowstyle = system.diagnostics.processwindowstyle.hidden;
Read more

unicode - Are email addresses allowed to contain non-alphanumeric characters? -

-
i'm building website using `django. website have significant users non-english speaking countries. i want know if there technical restrictions on types of characters email address contain. are email addresses allowed contain english alphabets, numbers, "_", "@" , "."? are allowed contain non-english alphabets "é" or "ü"? are allowed contain chinese or japanese or other unicode characters? email address consists of 2 parts local before @ , domain goes after. rules these parts different: for local part can use ascii: latin letters - z - z digits 0 - 9 special characters !#$%&'*+-/=?^_`{|}~ dot ., not first or last, , not in sequence space , "(),:;<>@[] characters allowed restrictions (they allowed inside quoted string, backslash or double-quote must preceded backslash) plus since 2012 can use international characters above u+007f , encoded as utf-8 . domain part more res...
Read more