Python pitfall with mutable default parameters

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()
>>> 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()
>>> append_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.

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