# Python | Union of Value Lists

The functionality of union has been discussed many times. But sometimes, we can have a more complex container, in which we need to check for the union of lists which are in form of keys of dictionary. Let’s discuss certain ways to solve this type of problem.

Method #1 : Using Loops
Using loops is a naive brute force approach to perform this particular task. In this method, we check for keys present in both list and check for non-repeating values to add to result. We even check for the keys completely not present in other to add its whole list value.

 `# Python3 code to demonstrate ` `# Union of Value Lists ` `# using loops ` ` `  `# initializing dicts ` `test_dict1 ``=` `{ ``"Key1"` `: [``1``, ``3``, ``4``], ``"key2"` `: [``4``, ``5``] } ` `test_dict2 ``=` `{ ``"Key1"` `: [``1``, ``7``, ``8``] } ` ` `  `# printing original dicts ` `print``(``"The original dict 1 : "` `+` `str``(test_dict1)) ` `print``(``"The original dict 2 : "` `+` `str``(test_dict2)) ` ` `  `# using loops ` `# Union of Value Lists ` `for` `key ``in` `test_dict1:  ` `    ``if` `key ``in` `test_dict2:  ` `        ``for` `val ``in` `test_dict1[key]: ` `            ``if` `val ``not` `in` `test_dict2[key]:   ` `                ``test_dict2[key].append(val) ` `    ``else``:  ` `        ``test_dict2[key] ``=` `test_dict1[key][:] ` ` `  `# print result ` `print``(``"The dicts after union is : "` `+` `str``(test_dict2)) `

Output :

```The original dict 1 : {'Key1': [1, 3, 4], 'key2': [4, 5]}
The original dict 2 : {'Key1': [1, 7, 8]}
The dicts after union is : {'Key1': [1, 7, 8, 3, 4], 'key2': [4, 5]}
```

Method #2 : Using dictionary comprehension + set operations
This is the one line approach to solve the similar problem and offers a compact alternative to above method. This solution processes by using the set comprehension to get the necessary elements bound together into lists using dictionary comprehension.

 `# Python3 code to demonstrate ` `# Union of Value Lists ` `# using dictionary comprehension + set operations ` ` `  `# initializing dicts ` `test_dict1 ``=` `{ ``"Key1"` `: [``1``, ``3``, ``4``], ``"key2"` `: [``4``, ``5``] } ` `test_dict2 ``=` `{ ``"Key1"` `: [``1``, ``7``, ``8``] } ` ` `  `# printing original dicts ` `print``(``"The original dict 1 : "` `+` `str``(test_dict1)) ` `print``(``"The original dict 2 : "` `+` `str``(test_dict2)) ` ` `  `# using dictionary comprehension + set operations ` `# Union of Value Lists ` `res ``=` `{key : ``list``(``set``(test_dict1.get(key, []) ``+` `test_dict2.get(key, [])))  ` `      ``for` `key ``in` `set``(test_dict2) | ``set``(test_dict1)} ` ` `  `# print result ` `print``(``"The dicts after union is : "` `+` `str``(res)) `

Output :

```The original dict 1 : {'Key1': [1, 3, 4], 'key2': [4, 5]}
The original dict 2 : {'Key1': [1, 7, 8]}
The dicts after union is : {'Key1': [1, 7, 8, 3, 4], 'key2': [4, 5]}
```

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