python - What do (lambda) function closures capture? -


recently started playing around python , came around peculiar in way closures work. consider following code:

adders=[0,1,2,3]  in [0,1,2,3]:    adders[i]=lambda a: i+a  print adders[1](3)

it builds simple array of functions take single input , return input added number. functions constructed in for loop iterator i runs 0 3. each of these numbers lambda function created captures i , adds function's input. last line calls second lambda function 3 parameter. surprise output 6.

i expected 4. reasoning was: in python object , every variable essential pointer it. when creating lambda closures i, expected store pointer integer object pointed i. means when i assigned new integer object shouldn't effect created closures. sadly, inspecting adders array within debugger shows does. lambda functions refer last value of i, 3, results in adders[1](3) returning 6.

which make me wonder following:

  • what closures capture exactly?
  • what elegant way convince lambda functions capture current value of i in way not affected when i changes value?

your second question has been answered, first:

what closure capture exactly?

scoping in python dynamic and lexical. closure remember name , scope of variable, not object it's pointing to. since functions in example created in same scope , use same variable name, refer same variable.

edit: regarding other question of how overcome this, there 2 ways come mind:

  1. the concise, not strictly equivalent way one recommended adrien plisson. create lambda argument, , set argument's default value object want preserved.

  2. a little more verbose less hacky create new scope each time create lambda:

    >>> adders = [0,1,2,3] >>> in [0,1,2,3]: ...     adders[i] = (lambda b: lambda a: b + a)(i) ...      >>> adders[1](3) 4 >>> adders[2](3) 5

    the scope here created using new function (a lambda, brevity), binds argument, , passing value want bind argument. in real code, though, have ordinary function instead of lambda create new scope:

    def createadder(x):     return lambda y: y + x adders = [createadder(i) in range(4)]

Comments

Post a Comment

Popular posts from this blog

inversion of control - Autofac named registration constructor injection -

does autofac support specifying registration name in components' constructors? example: ninject's namedattribute . you need use autofac.extras.attributed package on top achieve this so let's have 1 interface , 2 classes: public interface ihello { string sayhello(); } public class englishhello : ihello { public string sayhello() { return "hello"; } } public class frenchhello : ihello { public string sayhello() { return "bonjour"; } } then have consumer class, in want select instance injected: public class helloconsumer { private readonly ihello helloservice; public helloconsumer([withkey("en")] ihello helloservice) { if (helloservice == null) { throw new argumentnullexception("helloservice"); } this.helloservice = helloservice; } public string sayhello() { return this.helloservice.sayhello()...
Read more

ios - Change Storyboard View using Seague -

i have following code checks connection server, , depending on want change views if no connection found. nsstring *connect = [nsstring stringwithcontentsofurl:[nsurl urlwithstring:@"http://myserver.com"] encoding:nsutf8stringencoding error:nil]; if (connect == null) { [self performseguewithidentifier:@"network_failure" sender:self]; } else { // other code here } my problem not change views. have added storyboard segue between 2 views identifier of network_failure . not have navigation controller or that, problem? thanks help! no not problem. first of all, in objectivec exists nil object oriented null . must use that. so: if(connect == nil) or better: if(!connect) so due fact nil not same of null , probably code does't enter in if scope . you have been able discover breakpoint. anyway: be sure set trigger segue (line) 2 view controller , , name network_failure (that not best name. should instead view1toview2 ) as, m...
Read more

verilog - Systemverilog dynamic casting issues -

i've code snippet following in testbench function void write_to_port( my_data_type_base data ); my_data_type_extended data_ext; if(!$cast(data_ext, data)); `uvm_error(get_type_name(), "failed cast"); `uvm_info(get_name(), $psprintf("data_ext :\n%s", data_ext.sprint()), uvm_medium) // write data_ext out port.... endfunction when run it, i'm getting uvm_error "failed cast." i'm not quire sure why $cast not returning 1. can see, i'm printing out extended class data item after casting uvm_info. can see it's being cast properly. if don't use $cast if condition, don't runtime error. isn't coding practice use if dynamic cast check if $cast returning 1 ? what might reason behind cast not returning 1 in above case? i think semicolon on line 'if' not belong? i think consumes if statement, , uvm_error executes regardless of how if evaluates: if(!$cast(data_ext, data)); <- no semico...
Read more