Object Mentor Blog: Python's Mutable Default Problem

Python's Mutable Default Problem

tottinger — Thu, 22 May 2008 22:16:00 -0500

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 mobile software development

Mon, 06 Feb 2012 07:06:18 -0600

This is the perfect blog for anyone who wants to know about this topic. The article is nice and it is pleasant to read. I have known very important things over here.

"Python's Mutable Default Problem" by Career coaching

Sat, 04 Feb 2012 00:20:41 -0600

This is the perfect blog for anyone who wants to know about this topic. The article is nice and it is pleasant to read. I have known very important things over here.

"Python's Mutable Default Problem" by fantakuchen

Fri, 03 Feb 2012 06:48:27 -0600

“Totally agree with you dude… And the best part is all the comments placed by the participants are really interesting and informative… Thanks!!

"Python's Mutable Default Problem" by ralph lauren

Thu, 02 Feb 2012 02:57:28 -0600

Hiver n’est pas terminée, mais les grandes marques ont commencé le début de ce printemps, Ralph Lauren est l’un d’entre eux. Marque sous le soleil printanier et séries allé à los angeles plage en Californie, Etats-Unis, avec le thème de los angeles côte de los angeles Californie, los angeles série de femmes répartis en Californie du Nord avec not layout bohème et setting rétro, comme theme flower, robe en mousseline de soie corps mince et ainsi de range.http://www.vetementpoloralphlauren2012.net

"Python's Mutable Default Problem" by campervan in australia

Wed, 01 Feb 2012 06:51:49 -0600

Thank you very much for providing outstanding information, your site is just what i was looking for…

"Python's Mutable Default Problem" by airlie beach accommodation

Mon, 30 Jan 2012 22:29:26 -0600

You gave nice ideas here. I done a research on the issue and learn most peoples will agree with your blog. Certainly, these practices are unfair; but they say that most of their rules are only to apply to people who overdraw.

"Python's Mutable Default Problem" by Binary Options Questions

Mon, 30 Jan 2012 05:42:57 -0600

I will always give a nice thrust look in to you from my bookmark feed. Thanks for sharing. It is interesting.

"Python's Mutable Default Problem" by Aeronaves

Sun, 29 Jan 2012 19:10:45 -0600

A source of valuable information in an accessible form.

"Python's Mutable Default Problem" by technical writing services

Fri, 27 Jan 2012 11:56:15 -0600

I am in fact happy to glance at this blog posts which contains plenty of useful facts, thanks for providing these kinds of statistics.

"Python's Mutable Default Problem" by coolum accommodation

Wed, 25 Jan 2012 19:05:54 -0600

Howdy. Very cool blog!! Guy .. Excellent .. Superb .. I’ll bookmark your web site and take the feedadditionally…I’m happy to locate so much helpful info here within the post. Thank you for sharing.

"Python's Mutable Default Problem" by maroochydore accommodation

Wed, 25 Jan 2012 17:41:04 -0600

I must we appreciate you your time and efforts you earn in publishing this website post. I’m hoping the same best article by you later on also. Actually your creative writing expertise has encouraged me to start out my own blog site now. Actually the blogging is spreading its wings quickly. Your article can be a fine style of it.

"Python's Mutable Default Problem" by Port Douglas Accommodation

Wed, 25 Jan 2012 10:28:03 -0600

I beleive this research helped to understand why one may lose weight faster or slower than another. All this computer simulations are very good way of research.

"Python's Mutable Default Problem" by coolangatta accommodation

Wed, 25 Jan 2012 01:21:05 -0600

This is the first time i am reading your post and admire that you posted article which gives users lot of information regarding particular topic thnaks for this share.

"Python's Mutable Default Problem" by Port Douglas Accommodation

Tue, 24 Jan 2012 16:06:11 -0600

Yes, I have found that my husband can lose weight easier than I can. I would consider myself an average weight for my age and height but it still becomes discouraging that it is easier for others to lose weight. It is a lifetime change and I that is an interesting point about permanently cutting out 10 calories a day. Thanks for posting.

"Python's Mutable Default Problem" by airlie beach accommodation

Tue, 24 Jan 2012 02:43:56 -0600

First of all let me tell you, you have got a great blog .I am interested in looking for more of such topics and would like to have further information. Hope to see the next blog soon.

"Python's Mutable Default Problem" by maroochydore accommodation

Mon, 23 Jan 2012 14:01:19 -0600

Its my great pleasure to visit your site and to enjoy your awesome post here. I like it very much. I can feel that you put much attention for these articles, as all of them make sense and are very useful.

"Python's Mutable Default Problem" by maroochydore accommodation

Mon, 23 Jan 2012 12:03:21 -0600

am delighted today for the first time you have.A source of valuable information in an accessible form.

"Python's Mutable Default Problem" by sunshine beach accommodation

Thu, 19 Jan 2012 01:05:17 -0600

I like the valuable info you provide in your articles. I’ll bookmark your blog and check again here often. I am quite positive I will learn plenty of new stuff right here!

"Python's Mutable Default Problem" by I will surely share this with my friends. Your are really done great job, You style of working is amazing. keep it up dear.

Sat, 14 Jan 2012 05:42:56 -0600

I will surely share this with my friends. Your are really done great job, You style of working is amazing. keep it up dear.

"Python's Mutable Default Problem" by law school personal statement

Sat, 07 Jan 2012 02:38:17 -0600

Very good work on such topic. When I started to read article I was thinking about the way you present such great topic. By the way could you please post more.

"Python's Mutable Default Problem" by NH Wedding Reception

Fri, 06 Jan 2012 21:51:41 -0600

I actually do not face such kind of problem. i am keeping it in my dairy for further use actually. I hope this will help me a lot.

"Python's Mutable Default Problem" by Games For Young Children

Thu, 05 Jan 2012 11:46:17 -0600

Robert Martin’s keynote at Agile2008 and your previous post on pairing brought the topic to the front of my mind recently and it’s nice to see some real data on the subject. It’s nice to see a trend of unreduced productivity with better defect rates across most of the studies. Definitely a lot of food for thought and further reading here.

"Python's Mutable Default Problem" by http://www.thebettercleanse.com

Thu, 29 Dec 2011 05:20:38 -0600

The blog was absolutely fantastic. Lots of great information which can be helpful in some or the other way.

"Python's Mutable Default Problem" by Laser tattoo removal Gold Coast

Wed, 28 Dec 2011 04:07:20 -0600

All the contents you mentioned in post are too good and can be very useful. This one is great and is really a good post. I think it will help me a lot in the related stuff and is very much useful for me. I will keep it in mind, thanks for sharing the information keep updating, looking forward for more posts.

"Python's Mutable Default Problem" by gucci scarf

Fri, 23 Dec 2011 20:51:59 -0600

speaking bad languages