What is the best way to make a shallow copy of a Python dictionary?

Question

Assume we have a simple Python dictionary:

dict_ = {'foo': 1, 'bar': 2}

Which is the better way to copy this dictionary?

copy1 = dict(dict_)
copy2 = dict_.copy()

Is there a compelling reason to favour one approach over the other?

Answer 1

I always use the dict constructor: it makes it obvious that you are creating a new dict whereas calling the copy method on an object could be copying anything. Similarly for list I prefer calling the constructor over copying by slicing.

Note that if you use subclasses of dict using the copy method can get confusing:

>>> from collections import defaultdict
>>> d = defaultdict(int)
>>> d['a']
0
>>> d.copy()
defaultdict(<class 'int'>, {'a': 0})
>>> dict(d)
{'a': 0}
>>> 

The copy method of defaultdict gives you another defaultdict, but unless you override copy in a subclass the default action is just to give you a dict:

>>> class MyDict(dict): pass

>>> d = MyDict(a=1)
>>> d
{'a': 1}
>>> type(d)
<class '__main__.MyDict'>
>>> type(d.copy())
<class 'dict'>

That means you have to know about the internal details of a subclass to know what type the copy method will return.