Python optional parameters

Question

Guys, I just started python recently and get confused with the optional parameters, say I have the program like this:

class B:    pass  class A:     def __init__(self, builds = B()):         self.builds = builds 

If I create A twice

b = A() c = A() 

and print their builds

print b.builds print c.builds 

I found they are using the exactly same object,

<__main__.B instance at 0x68ee0> <__main__.B instance at 0x68ee0> 

But it is not what I want, since if b changed some internal state of builds, the one in c object will also be changed.

Is it possible to recreate this optional parameters each time by using this optional parameters syntax?

Answer 1

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 == y True >>> 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