Merging Dictionaries in Python

Question

I am intrigued by the following python expression:

d3 = dict(d1, **d2)

The task is to merge 2 dictionaries into a third one, and the above expression accomplishes the task just fine. I am interested in the ** operator and what exactly is it doing to the expression. I thought that ** was the power operator and haven't seen it used in the context above yet.

The full snippet of code is this:

>>> d1 = {'a': 1, 'b': 2}
>>> d2 = {'c': 3, 'd': 4}
>>> d3 = dict(d1, **d2)
>>> print d3
{'a': 1, 'c': 3, 'b': 2, 'd': 4}

Answer 1

In Python, any function can accept multiple arguments with *;
or multiple keyword arguments with **.

Receiving-side example:

>>> def fn(**kwargs):
...   for kwarg in kwargs:
...     print kwarg
... 
>>> fn(a=1,b=2,c=3)
a
c
b

Calling-side example (thanks Thomas):

>>> mydict = dict(a=1,b=2,c=3)
>>> fn(**mydict)
a
c
b

Answer 2

** in argument lists has a special meaning, as covered in section 4.7 of the tutorial. The dictionary (or dictionary-like) object passed with **kwargs is expanded into keyword arguments to the callable, much like *args is expanded into separate positional arguments.

Answer 3

The ** turns the dictionary into keyword parameters:

>>> d1 = {'a': 1, 'b': 2}
>>> d2 = {'c': 3, 'd': 4}
>>> d3 = dict(d1, **d2)

Becomes:

>>> d3 = dict(d1, c=3, d=4)

Answer 4

That operator is used to unpack argument list: http://docs.python.org/tutorial/controlflow.html#unpacking-argument-lists

Answer 5

you have got your answer of the ** operator. here's another way to add dictionaries

>>> d1 = {'a': 1, 'b': 2}
>>> d2 = {'c': 3, 'd': 4}
>>> d3=d1.copy()
>>> d3.update(d2)
>>> d3
{'a': 1, 'c': 3, 'b': 2, 'd': 4}

Answer 6

It's also worth mentioning the mechanics of the dict constructor. It takes an initial dictionary as its first argument and can also take keyword arguments, each representing a new member to add to the newly created dictionary.