Object Mentor Blog: Python's Mutable Default Problem

Python's Mutable Default Problem

Tim Ottinger — Thu, 22 May 2008 22:16:00 +0000

Today I was perusing a very fine python article all professional python programmers should read. I had to grin when I saw one of my favorite python quirks/flaws.

Python only evaluates default parameters when it defines a function, leading to surprising side effects (example from saifoo):

def function(item, stuff = []):
    stuff.append(item)
    print stuff

function(1)
# prints '[1]'

function(2)
# prints '[1,2]' !!!

The grin on my face was because I was bit by this one again last Tuesday. You just don’t want to make a mutable object a default parameter. The solution I used was close to the solution from saifoo:

Saifoo.net:

def function(item, stuff=None):
   if stuff is None:
      stuff = []
   stuff.append(item)
   print stuff

This, of course, gives the correct behavior because it creates a new empty list each time it is called without a second parameter.

My solution actually combines some advice contained earlier in the article with the solution given for this problem:

Mine

def function(item, stuff=None):
   stuff = stuff or []
   stuff.append(item)
   print stuff

It’s not necessarily prettier, and I struggle with whether it is more obvious (to a Python programmer) or not. I fear it may be clever (a word I only use in the pejorative sense), but it is also clean-looking to me.

The point remains, though, that the mutable default parameter quirk is an ugly corner worth avoiding, and I suspect that any code that purposefully exploits that behavior to populate a structure that persists across calls will run a very real risk of being misunderstood.

We all know what happens when the next programmer to touch a program misunderstands it.

"Python's Mutable Default Problem" by JustRanting

Fri, 20 Jun 2008 14:37:03 +0000

I wonder how Python and professional fit together ;-)

"Python's Mutable Default Problem" by yachris2112

Tue, 27 May 2008 07:31:37 +0000

Once again, I have to point out that Lisp had this first :-). I remember (way) back in the day having this explained to me by none other than David Moon. For pythonistas, this is like seeing a noob post responded to by Guido.

Anyway, Ruby has a similar idiom:

somevar ||= default_val

although the ‘or’ syntax would work as well. I think the ’||’ version is a bit more noticeable.

matthew: almost certainly not.

"Python's Mutable Default Problem" by matthew

Fri, 23 May 2008 11:34:29 +0000

I wonder if this was a deliberate decision, to aid with recursion?

"Python's Mutable Default Problem" by John Roth

Fri, 23 May 2008 11:04:56 +0000

Two comments. First, the or isn’t a hack. It’s a standard Python idiom, although it looks a bit strange to someone who doesn’t have a lot of Python experience.

Second, having a mutable object as a parameter is definitely a gotcha for most Python programmers. The interesting thing is that it could be used as a way of getting a persistent variable into a method (local in some languages). However, I’ve never seen it used that way, and I’d be a bit leery of doing it myself except in programs that are part of a project where all the developers are comfortable with the idiom, and where the code isn’t expected to leave the project.

John Roth

"Python's Mutable Default Problem" by Tim

Fri, 23 May 2008 07:33:35 +0000

I’ve not used it on purpose. So far as I know, I’ve not seen it used on purpose. I’ve accidentally used it a few times. The tinker in me wants to try using it on purpose, and the professional refuses. Yet another case of ambivalence.

"Python's Mutable Default Problem" by Kevin

Fri, 23 May 2008 03:33:54 +0000

So is that link up at the top supposed to point back to this article? Just curious :-)

Kevin

Ooops. Fixed now. Thanks. —tim

"Python's Mutable Default Problem" by paddy3118

Fri, 23 May 2008 01:10:34 +0000

You get bit once, but have you seen anyone leave it in on purpose?

- Paddy.