Never use a [] or {} as a default argument.

I've done this a couple times.

When you make [] a default argument, it will be the same list on every subsequent invocation, with the same id(). If you append to it, what you've appended will be still be there for later invocations of the function.

Example:

def functionx(mylist = []):

    mylist.append("F")

    print mylist

for i in range(4): functionx()

The output of this program should be:

F

F, F

F, F, F

F, F, F, F

Needless to say, this is really, really stupid. The reason is because the default arguments get defined at definition time, and any mutable type will persist. This is also why somedict.setdefault({},...) will also mess up, any why defaultdict instead uses a callable.

So, you should only use constant values as default parameters, and never a mutable type. I've made this mistake a couple times.

I've seen some obtuse lunkheads actually saying it's better this way. It seems to be a rule that all language designers eventually start rationalizing their mistakes and the corners they cut (another example: see this interview with Håkon Lie , as he is unable to admit there could be any flaw in the design of CSS) and pointlessly defend bad decisions in the work they've invested so much time that no one who approached the language with fresh eyes would ever, ever defend.

I've also seen some people saying that you can use this to simulate static variables. To which I say: so, give us proper static variables then, and don't plant your language full of landmines sure to screw people up when they're not expecting it.