How can I add new keys to a dictionary?

Question

Is it possible to add a key to a Python dictionary after it has been created?

It doesn\'t seem to have an .add() method.

Answer 1

Let's pretend you want to live in the immutable world and do NOT want to modify the original but want to create a new dict that is the result of adding a new key to the original.

In Python 3.5+ you can do:

params = {'a': 1, 'b': 2}
new_params = {**params, **{'c': 3}}

The Python 2 equivalent is:

params = {'a': 1, 'b': 2}
new_params = dict(params, **{'c': 3})

After either of these:

params is still equal to {'a': 1, 'b': 2}

and

new_params is equal to {'a': 1, 'b': 2, 'c': 3}

There will be times when you don't want to modify the original (you only want the result of adding to the original). I find this a refreshing alternative to the following:

params = {'a': 1, 'b': 2}
new_params = params.copy()
new_params['c'] = 3

or

params = {'a': 1, 'b': 2}
new_params = params.copy()
new_params.update({'c': 3})

Reference: https://stackoverflow.com/a/2255892/514866