#default arguments

FUNCTION in C++

A function in c++ is a group of statement that together performs a particular task. Every C++ program contains a function, that is the main function. Main () is the entry point of the program. At this point, the execution of the program is started.

Syntax:-

void main() { ---------- ---------- }

What is reference?

Reference is an inter pointer. Reference looks like ordinary…

There’s an issue I’ve seen plenty of times before, but caught me off guard again today. Hence I figured I’d write about it as a reminder to myself. The morale of the story is that you should not pass non-static defaults into arguments in a method’s definition in python.

So for example:

class SomeClass(object):

def __init__(self, some_arg=[]): self.some_arg = some_arg

This will cause issues if you create more than one instance of the class with no value passed in for some_arg. It will end up re-using the same list, and different instances of the objects will share the same self.arg.

For example if I were to do:

first_object = SomeClass() first_object.some_arg.append(1) print first_object.some_arg  # prints out [1] second_object = SomeClass() second_object.some_arg.append(2) print second_object.some_arg  # will print out [1,2] even though we never appended 1 to the second object

What’s happening is that both first_object and second_object are referencing the same list, which was instantiated at the definition of the class. A somewhat non-intuitive pitfall, but makes sense if you think about it. This is also true for other mutable objects, so be careful what you set the default to.

A way around this, is to set the default to None and create a new list in such a case within the execution of __init__.

 def __init__(self, some_arg=None):     self.some_arg = some_arg or []

Anyways, just a friendly reminder to be careful about argument defaults!

Also an excerpt about it from Effective Python:

default arguments in constructor

Can I use default arguments in a constructor like this maybe

Soldier(int entyID, int hlth = 100, int exp = 10, string nme) : entityID(entyID = globalID++), health(hlth), experience(exp), name(nme = SelectRandomName(exp)) { }

I want for example exp = 10 by default but be able to override this value if I supply it in the constructor otherwise it should use the default.

How can I do this, I know…