Newbie question - default values of a function

Dennis Benzinger Dennis.Benzinger at gmx.net
Wed Dec 22 23:18:06 CET 2004


Bulba! wrote:
> [...]
> But why the following happens?
> 
> 
>>>>def j(a,i=0):
> 
> ... 	i=i+1 
> ... 	return (a, i)
> ... 
> 
>>>>j(2)
> 
> (2, 1)
> 
>>>>j(3)
> 
> (3, 1)
> 
>>>>j(5)
> 
> (5, 1)
> 
> 
> From Language Reference:
> 
> http://www.python.org/doc/2.3.4/ref/function.html
> 
> "Default parameter values are evaluated when the function definition
> is executed. This means that the expression is evaluated once, when
> the function is defined, and that that same ``pre-computed'' value is
> used for each call. This is especially important to understand when a
> default parameter is a mutable object, such as a list or a dictionary:
> if the function modifies the object (e.g. by appending an item to a
> list), the default value is in effect modified. "
> 
> 
> Surely the variable i is a mutable object?
> [...]

No, i is not mutable!
i is a name for a number object which is _not_ mutable
(http://www.python.org/doc/2.4/ref/types.html).

The line i=i+1 creates a new number object with the value 1 and gives it
the name i.


Bye,
Dennis



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