I've been stuck on this question for quite sometime and just can't figure it out. I just want to be able to understand what I'm missing and why it's needed. What I need to do is make a function which adds each given key/value pair to the dictionary. The argument key_value_pairs will be a list of tuples in the form (key, value).
def add_to_dict(d, key_value_pairs):
newinputs = [] #creates new list
for key, value in key_value_pairs:
d[key] = value #updates element of key with value
if key in key_value_pairs:
newinputs.append((d[key], value)) #adds d[key and value to list
return newinputs
I can't figure out how to update the "value" variable when d and key_value_pairs have different keys.
The first three of these scenarios work but the rest fail
>>> d = {}
>>> add_to_dict(d, [])
[]
>>> d
{}
>>> d = {}
>>> add_to_dict(d, [('a', 2])
[]
>>> d
{'a': 2}
>>> d = {'b': 4}
>>> add_to_dict(d, [('a', 2)])
[]
>>> d
{'a':2, 'b':4}
>>> d = {'a': 0}
>>> add_to_dict(d, [('a', 2)])
[('a', 0)]
>>> d
{'a':2}
>>> d = {'a', 0, 'b': 1}
>>> add_to_dict(d, [('a', 2), ('b': 4)])
[('a', 2), ('b': 1)]
>>> d
{'a': 2, 'b': 4}
>>> d = {'a': 0}
>>> add_to_dict(d, [('a', 1), ('a': 2)])
[('a', 0), ('a':1)]
>>> d
{'a': 2}
Thanks
Edited.
7 Answers
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 ;)
