Python Cookbook

Making a Fast Copy of an Object

Credit: Alex Martelli

Problem

You need to implement the special method _ _copy_ _ so your class can cooperate with the copy.copy function. If the _ _init_ _ method of your class is slow, you need to bypass it and get an empty object of the class.

Solution

Here’s a solution that works for both new-style and classic classes:

def empty_copy(obj):
    class Empty(obj._ _class_ _):
        def _ _init_ _(self): pass
    newcopy = Empty(  )
    newcopy._ _class_ _ = obj._ _class_ _
    return newcopy

Your classes can use this function to implement _ _copy_ _ as follows:

class YourClass:
    def _ _init_ _(self):
        print "assume there's a lot of work here"
    def _ _copy_ _(self):
        newcopy = empty_copy(self)
        print "now copy some relevant subset of self's attributes to newcopy"
        return newcopy

Here’s a usage example:

if _ _name_ _ == '_ _main_ _':
    import copy
    y = YourClass(  )    # This, of course, does run _ _init_ _
    print y
    z = copy.copy(y)     # ...but this doesn't
    print z

Discussion

Python doesn’t implicitly copy your objects when you assign them. This is a great thing, because it gives fast, flexible, and uniform semantics. When you need a copy, you explicitly ask for it, ideally with the copy.copy function, which knows how to copy built-in types, has reasonable defaults for your own objects, and lets you customize the copying process by defining a special method _ _copy_ _ in your own classes. If you want instances of a class to be noncopyable, you can define _ _copy_ _ and raise a TypeError there. In ...