Updating a Dictionary in Python

Python has this feature built-in:

>>> d = {'b': 4}
>>> d.update({'a': 2})
>>> d
{'a': 2, 'b': 4}

Or given you're not allowed to use dict.update:

>>> d = dict(d.items() + {'a': 2}.items())   # doesn't work in python 3

With python 3.9 you can use an |= update operator:

>>> d = {'b': 4}
>>> d |= {'a': 2}
>>> d
{'a': 2, 'b': 4}

Here's a more elegant solution, compared to Eric's 2nd snippet

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

It works both in Python 2 and 3

And of course, the update method

>>> a
{'a': 1, 'b': 2}
>>> b
{'a': 2, 'c': 3}
>>> a.update(b)
>>> a
{'a': 2, 'b': 2, 'c': 3}

However, be careful with the latter, as might cause you issues in case of misuse like here

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

The new version of Python3.9 introduces two new operators for dictionaries: union (|) and in-place union (|=). You can use | to merge two dictionaries, while |= will update a dictionary in place. Let's consider 2 dictionaries d1 and d2

d1 = {"name": "Arun", "height": 170}
d2 = {"age": 21, "height": 170}

d3 = d1 | d2 # d3 is the union of d1 and d2
print(d3)

Output:

{'name': 'Arun', 'height': 170, 'age': 21}

Update d1 with d2

d1 |= d2
print(d1)

Output:

{'name': 'Arun', 'height': 170, 'age': 21}

You can update d1 with a new key weight as

d1 |= {"weight": 80}
print(d1)

Output:

{'name': 'Arun', 'height': 170, 'age': 21, 'weight': 80}

So if I understand you correctly you want to return a list of of tuples with (key, old_value) for the keys that were replaced.

You have to save the old value before you replace it:

def add_to_dict(d, key_value_pairs):

    newinputs = [] #creates new list
    for key, value in key_value_pairs:
        if key in d:
            newinputs.append((key, d[key]))
        d[key] = value #updates element of key with value
    return newinputs

Each key in a python dict corresponds to exactly one value. The cases where d and key_value_pairs have different keys are not the same elements.

Is newinputs supposed to contain the key/value pairs that were previously not present in d? If so:

def add_to_dict(d, key_value_pairs):
  newinputs = []
  for key, value in key_value_pairs:
    if key not in d:
      newinputs.append((key, value))
    d[key] = value
  return newinputs

Is newinputs supposed to contain the key/value pairs where the key was present in d and then changed? If so:

def add_to_dict(d, key_value_pairs):
  newinputs = []
  for key, value in key_value_pairs:
    if key in d:
      newinputs.append((key, value))
    d[key] = value
  return newinputs

If I understand you correctly, you only want to add the keys that do not exist in the dictionary. Here is the code:

def add_to_dict(d, key_value_pairs):
    newinputs = [];
    for key, value in key_value_pairs:
        if key not in d.keys():
            d[key] = value
            newinputs.append((key, value));
    return newinputs

For each key in new key,value pairs list you have to check if the key is new to the dictionary and add it only then. Hope it helps ;)