How to copy a dict and modify it in one line of code

Question

Very often I need to create dicts that differ one from another by an item or two. Here is what I usually do:

setup1 = {\'param1\': val1, 
            \'param         


        

Answer 1

You can write your own class using UserDict wrapper, and simply add dicts like

# setup1 is of Dict type (see below)
setup2 = setup1 + {'param1': val10}

All you have to do is

• Define a new class using UserDict as base class
• Implement __add__ method for it.

---

Something like :

class Dict(dict):
    def __add__(self, _dict):
        if isinstance(_dict, dict):
            tmpdict = Dict(self)
            tmpdict.update(_dict)
            return tmpdict

        else:
            raise TypeError

    def __radd__(self, _dict):
         return Dict.__add__(self, _dict)

Answer 2

The simplest way in my opinion is something like this:

new_dict = {**old_dict, 'changed_val': value, **other_new_vals_as_dict}

Answer 3

I like this line (after from itertools import chain):

d3 = dict(chain(d1.items(), d2.items()))

(Thanks for juanpa.arrivillaga for the improvement!)

Answer 4

You could use keyword arguments in the dictionary constructor for your updates

new = dict(old, a=1, b=2, c=3)

# You can also unpack your modifications
new = dict(old, **mods)

This is equivalent to:

new = old.copy()
new.update({"a": 1, "b": 2, "c": 3})

Source

Notes

• dict.copy() creates a shallow copy.
• All keys need to be strings since they are passed as keyword arguments.

Answer 5

This is an extension to the nice answer posted by Adam Matan:

def copy_dict(d, diffs={}, **kwargs):
    res = dict(d)
    res.update(diffs)
    res.update(kwargs)
    return res

The only difference is the addition of kwargs. Now one can write

setup2 = copy_dict(setup1, {'param1': val10, 'param2': val20})

or

setup2 = copy_dict(setup1, param1=val10, param2=val20)

Answer 6

setup2 = dict((k, {'param1': val10, 'param2': val20}.get(k, v))
              for k, v in setup1.iteritems())

This only works if all keys of the update dictionary are already contained in setup1.

If all your keys are strings, you can also do

setup2 = dict(setup1, param1=val10, param2=val20)