Mutable default values for function parameters

Erik Max Francis max at alcyone.com
Thu Oct 4 19:46:44 CEST 2001


sebastien wrote:

> I'd like to point something that is said in §4.7.1 of the tutorial:
> 
> "Important warning: The default value is evaluated only once. This
> makes a difference when the default is a mutable object such as a list
> or dictionary"
> 
> I think that this point is really difficult to understand. That means
> that parameters with mutable default value are global to all the calls
> of the function:
	...
> 
> This is a really confusing and I always fall in this sort of trap.
> Maybe a bad neuronal connection;-)

I avoid this problem by simply never using a mutable object as a default
argument, but instead always using a sentinel value that's substituted
in the function body:

	def sample(x, d = None):
	    if d is None:
	        d = []
	    d.append(x)
	    print d

To be honest, I can't remember ever running into this pitfall, since
I've used this pattern ever since I started using Python, and happened
to be made aware of the rule before encountering the wrong end of it.

-- 
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