Python – UnboundLocalError on local variable when reassigned after first use

pythonscopevariables

The following code works as expected in both Python 2.5 and 3.0:

a, b, c = (1, 2, 3)

print(a, b, c)

def test():
    print(a)
    print(b)
    print(c)    # (A)
    #c+=1       # (B)
test()

However, when I uncomment line (B), I get an UnboundLocalError: 'c' not assigned at line (A). The values of a and b are printed correctly. This has me completely baffled for two reasons:

Why is there a runtime error thrown at line (A) because of a later statement on line (B)?
Why are variables a and b printed as expected, while c raises an error?

The only explanation I can come up with is that a local variable c is created by the assignment c+=1, which takes precedent over the "global" variable c even before the local variable is created. Of course, it doesn't make sense for a variable to "steal" scope before it exists.

Could someone please explain this behavior?

Best Answer

Python treats variables in functions differently depending on whether you assign values to them from inside or outside the function. If a variable is assigned within a function, it is treated by default as a local variable. Therefore, when you uncomment the line you are trying to reference the local variable c before any value has been assigned to it.

If you want the variable c to refer to the global c = 3 assigned before the function, put

global c

as the first line of the function.

As for python 3, there is now

nonlocal c

that you can use to refer to the nearest enclosing function scope that has a c variable.

Short Answer

Use $this to refer to the current object. Use self to refer to the current class. In other words, use $this->member for non-static members, use self::$member for static members.

Full Answer

Here is an example of correct usage of $this and self for non-static and static member variables:

<?php
class X {
    private $non_static_member = 1;
    private static $static_member = 2;

    function __construct() {
        echo $this->non_static_member . ' '
           . self::$static_member;
    }
}

new X();
?>

Here is an example of incorrect usage of $this and self for non-static and static member variables:

<?php
class X {
    private $non_static_member = 1;
    private static $static_member = 2;

    function __construct() {
        echo self::$non_static_member . ' '
           . $this->static_member;
    }
}

new X();
?>

Here is an example of polymorphism with $this for member functions:

<?php
class X {
    function foo() {
        echo 'X::foo()';
    }

    function bar() {
        $this->foo();
    }
}

class Y extends X {
    function foo() {
        echo 'Y::foo()';
    }
}

$x = new Y();
$x->bar();
?>

Here is an example of suppressing polymorphic behaviour by using self for member functions:

<?php
class X {
    function foo() {
        echo 'X::foo()';
    }

    function bar() {
        self::foo();
    }
}

class Y extends X {
    function foo() {
        echo 'Y::foo()';
    }
}

$x = new Y();
$x->bar();
?>

The idea is that $this->foo() calls the foo() member function of whatever is the exact type of the current object. If the object is of type X, it thus calls X::foo(). If the object is of type Y, it calls Y::foo(). But with self::foo(), X::foo() is always called.

From http://www.phpbuilder.com/board/showthread.php?t=10354489:

By http://board.phpbuilder.com/member.php?145249-laserlight

Objective-c – How to pass data back from detail view controller to uitableview

You need to make use of custom delegates. Create a protocol in your detailview and implement it in your rootview.Pass the selected string as parameter to delegate method and from the delegate method, display it in your textfield.

//something like this
@interface detailViewController

// protocol declaration 
@protocol myDelegate
@optional
   -(void)selectedValueIs:(NSString *)value;

// set it as the property
@property (nonatomic, assign) id<myDelegate> selectedValueDelegate;

// in your implementation class synthesize it and call the delegate method
@implementation detailViewController
@synthesize selectedValueDelegate
// in your didselectRowAtIndex method call this delegate method
- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
 {

      [self  selectedValueDelegate])selectedValueIs:valueString] ;
      [self.navigationController popViewControllerAnimated:YES];

  }




@end

// In your rootViewController conform to this protocol and then set the delegate

         detailViewCtrlObj.selectedValueDelegate=self;
//Implement  the delegate Method 
     -(void)selectedValueIs:(NSString *)value{
       {
            // do whatever you want with the value string
       }

Best Answer

Related Solutions

Php – When should I use ‘self’ over ‘$this’

Short Answer

Full Answer

Objective-c – How to pass data back from detail view controller to uitableview

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