We stumbled upon an unexpected way default parameters work while writing python code for our current game project.
In Python you can specify a default value for a function parameter, which is used when the function is called with less parameters than the function needs.
>>> def printHello(text="Hello World!"): ... print text >>> >>> printHello() Hello World!
In python strings are immutable, that means you can’t change it after creation. When you for example concatenate two strings a new one is created, so there is no way to change the default parameter string.
But what happens if you use an empty list as the default parameter? I expected to always get an empty list.
So the following function should always print the same.
>>> def append_one(values=): ... values.append("One") ... print values >>> append_one() ['One'] >>> append_one() ['One', 'One']
What happens here?
A list is mutable, that means you can add or change elements in the list. And you get passed the same list instance each time on a call to append_one, as variables are passed by reference in python. So this behaviour is actually not that surprising. The same is the case for each other mutable class you pass as a default argument.
But how can you use a mutable object?
There are several ways, but I like the following.
Instead of passing the object directly, we pass None and check for it in the function.
>>> def append_one(values=None): ... values = values or  ... values.append("One") ... print values ... >>> append_one() ['One'] >>> append_one() ['One'] >>> append_one(['Zero']) ['Zero', 'One']
We also used the compact or notation, see the Python documentation for how that works.
Edit: There is also a good post on stackoverflow answering why it was done this way.