Analogue of devar in Python -


when writing python code, find myself wanting behavior similar lisp's defvar. basically, if variable doesn't exist, want create , assign particular value it. otherwise, don't want anything, , in particular, don't want override variable's current value.

i looked around online , found suggestion:

try:     some_variable except nameerror:     some_variable = some_expensive_computation()

i've been using , works fine. however, me has of code that's not paradigmatically correct. code 4 lines, instead of 1 required in lisp, , requires exception handling deal that's not "exceptional."

the context i'm doing interactively development. i'm executing python code file frequently, improve it, , don't want run some_expensive_computation() each time so. arrange run some_expensive_computation() hand every time start new python interpreter, i'd rather automated, particularly code can run non-interactively. how season python programmer achieve this?

i'm using winxp sp3, python 2.7.5 via anaconda 1.6.2 (32-bit), , running inside spyder.

it's bad idea rely on existence or not of variable having meaning. instead, use sentinel value indicate variable not set appropriate value. none common choice kind of sentinel, though may not appropriate if possible output of expensive computation.

so, rather current code, this:

# on in program some_variable = none  # later: if some_variable none:     some_variable = some_expensive_computation()  # use some_variable here

or, version none significant value:

_sentinel = object() some_variable = _sentinel # means doesn't have meaningful value  # later if some_variable _sentinel:     some_variable = some_expensive_computation()

Comments

Post a Comment

Popular posts from this blog

java.util.scanner - How to read and add only numbers to array from a text file -

i'm learning java , have question regarding reading file want read numbers file contains strings well. here example of file: 66.56 "3 java 3-43 5-42 2.1 1 and here coding: public class test { public static void main (string [] args){ if (0 < args.length) { file x = new file(args[0]); try{ scanner in = new scanner( new fileinputstream(x)); arraylist<double> test = new arraylist<>(); while(in.hasnext()){ if(in.hasnextdouble()){ double f=in.nextdouble(); test.add(f);} else {in.next();} } catch(ioexception e) { system.err.println("exception during reading: " + e); } } my problem add 66.56,2.1 , 1 doesn't add 3 after "3 or ignores 3-43 , 5-42 can tell me how skip strings , add doubles here? thanks all said 3 ie; "3, 3-43 , 4-42 strings either u read string , split , check number @ " , - or put in space between characters , integers. jvm after compilation tre
Read more

rewrite - Trouble with Wordpress multiple custom querystrings -

i'm working on adding multiple querystring , doesn't seem work. i've found codes seems work others might i'm missing something. i want like: http://mydomain.com/board/?getyear=2013&getsometext2=sometext to http://mydomain.com/board/2013/sometext/ i added code below functions.php file. doesn't seem work should though. this works. echoing querystring gets both variable without problem. http://mydomain.com/board/2013/sometext/ but doesn't. tested querystring vis isset() seems nothing being set/get http://mydomain.com/board/2013/ function add_query_vars($vars) { $vars[] = "getyear"; $vars[] = "getsometext2"; return $vars; } add_filter('query_vars', 'add_query_vars'); function add_rewrite_rules($arules) { $anewrules = array('board/([^/]+)/([^/]+)/?$' => 'index.php?pagename=board&getyear=$matches[1]&getsometext2=$matches[2]'); $arules = $a
Read more

php - Accessing static methods using newly created $obj or using class Name -

see class definition below: i using 5.3.9 version of php class a{ static function ab(){ echo "static function ab<br>"; } public function xy(){ echo "public function xy<br>"; } } $obj = new a(); $obj->ab(); a::ab(); both functions call give same output without error static function ab static function ab how possible static method can called class object? because static method calls using class name only?! now difference between accessing these 2 ways call static method ? referring php.net website declaring class properties or methods static makes them accessible without needing instantiation of class. property declared static can not accessed instantiated class object (though static method can). a big difference is because static methods callable without instance of object created, pseudo-variable $this not available inside method declared static. refer page php.net/manual/en/
Read more