Python optional parameters

You need to understand how default values work in order to use them effectively.

Functions are objects. As such, they have attributes. So, if I create this function:

>>> def f(x, y=[]):        y.append(x)        return y

I've created an object. Here are its attributes:

>>> dir(f)['__call__', '__class__', '__closure__', '__code__', '__defaults__', '__delattr__',   '__dict__', '__doc__', '__format__', '__get__', '__getattribute__', '__globals__',    '__hash__', '__init__', '__module__', '__name__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'func_closure', 'func_code', 'func_defaults', 'func_dict', 'func_doc', 'func_globals', 'func_name']

One of them is func_defaults. That sounds promising, what's in there?

>>> f.func_defaults([],)

That's a tuple that contains the function's default values. If a default value is an object, the tuple contains an instance of that object.

This leads to some fairly counterintuitive behavior if you're thinking that f adds an item to a list, returning a list containing only that item if no list is provided:

>>> f(1)[1]>>> f(2)[1, 2]

But if you know that the default value is an object instance that's stored in one of the function's attributes, it's much less counterintuitive:

>>> x = f(3)>>> y = f(4)>>> x == yTrue>>> x[1, 2, 3, 4]>>> x.append(5)>>> f(6)[1, 2, 3, 4, 5, 6]

Knowing this, it's clear that if you want a default value of a function's parameter to be a new list (or any new object), you can't simply stash an instance of the object in func_defaults. You have to create a new one every time the function is called:

>>>def g(x, y=None):       if y==None:           y = []       y.append(x)       return y

you need to do the following:

class A:    def __init__(self, builds=None):        if builds is None:            builds = B()        self.builds = builds

it's a very wide-spread error, using mutable parameters as a default arguments. there are plenty of dupes probably on SO.

Yes; default parameters are evaluated only at the time when the function is defined.

One possible solution would be to have the parameter be a class rather than an instance, a la

def foo(blah, klass = B):    b = klass()    # etc