Python append two dict

Python | Add dictionary to dictionary without overwriting

In this article, we will discuss how to add the contents of a dictionary to another dictionary without overwriting values of similar keys.

Suppose we have two dictionaries with some similar keys. Like this,

dict_1 and dict_2 have following common keys – ‘test’, ‘at’, ‘Hello’. Now we want to add the contents of a dictionary dict_2 to dictionary dict_1. But instead of overwriting values for common keys, we want to merge the values of common keys while adding. For example, after adding the contents of dict_2 to dict_1, the final content of dict_1 should be like this,

Values of similar keys from both the dictionaries should persist in a list after merging. Let’s see how to do that,

Add dictionary to dictionary without overwriting in Python.

Directly calling the dictionary’s update() function with another dictionary as a argument i.e. dict_1.update(dict_2), will update the existing values of common key. So, instead of it, we will do the following steps,

Frequently Asked:

• Iterate over all key-value pairs of dictionary dict_2 in a loop
• During iteration, for each key-value pair, check if key already exist in dictionary dict_1 or not.
  • If the key already exists in dict_1 and the value for that key is not of list type, create a list and add both values of that key from dict_1 & dict_2 to it. Then replace the current value of that key in dict_1 with this list.
  • If the key already exists in dict_1 and it is has a value of list type. Then append the value of this key from dict_2 to that list value in dict_1.
  • If the key doesn’t exist in dict_1, then add this key-value pair to dict_1.

  Let’s understand by an example,

  dict_1 = < "Hello": 56, "at": 23, "test": 43, "this": 12 >dict_2 = < 'test': 4, 'at': 5, 'why': 6, 'Hello': 20 >for key, value in dict_2.items(): if key in dict_1: if isinstance(dict_1Python append two dict, list): dict_1Python append two dict.append(value) else: temp_list = [dict_1Python append two dict] temp_list.append(value) dict_1Python append two dict = temp_list else: dict_1Python append two dict = value print(dict_1)

  Here, we added all the key-value pairs of dictionary dict_2 to another dictionary dict_1. Both dict_1 and dict_2 had similar keys, and for those keys, the values from dict_2 got added along with the existing values from dict_1. So, nothing got overwritten while adding a dictionary to another dictionary.

  We learned a way to all contents of a dictionary to another dictionary without overwriting the values of similar keys.

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  How to Merge Dictionaries in Python

  

  Ashutosh Krishna

  

  In Python, a dictionary is a collection you use to store data in pairs. It is ordered and mutable, and it cannot store duplicate data.

  We write a dictionary using curly brackets like this:

  Sometimes, we need to merge two or more dictionaries to create a bigger dictionary. For example:

  dict_one = < "id": 1, "name": "Ashutosh", "books": ["Python", "DSA"] >dict_two = < "college": "NSEC", "city": "Kolkata", "country": "India" >merged_dict =

  In the merged_dict we have the key-value pairs of both dict_one and dict_two . This is what we wish to achieve programmatically.

  There are various ways we can do this in Python:

  1. Using a for loop
  2. Using the dict.update() method
  3. Using the ** operator
  4. Using the | (Union) operator (for Python 3.9 and above)

  Let’s explore each way one after another.

  How to Merge Dictionaries in Python Using a For Loop

  We can merge two or more dictionaries using for loop like this:

  >>> dict_one = < . "id": 1, . "name": "Ashutosh", . >>>> dict_two = < . "books": ["Python", "DSA"], . "college": "NSEC", . >>>> dict_three = < . "city": "Kolkata", . "country": "India" . >>>> for key,value in dict_two.items(): . merged_dictPython append two dict = value . >>> merged_dict >>> for key,value in dict_three.items(): . merged_dictPython append two dict = value . >>> merged_dict

  But the problem with this method is that we need to run so many loops to merge the dictionaries.

  How to Merge Dictionaries in Python Using the dict.update() Method

  If you explore the dict class, there are various methods inside it. One such method is the update() method which you can use to merge one dictionary into another.

  >>> dict_one = < . "id": 1, . "name": "Ashutosh", . "books": ["Python", "DSA"] . >>>> dict_two = < . "college": "NSEC", . "city": "Kolkata", . "country": "India" . >>>> dict_one.update(dict_two) >>> dict_one

  But the problem when we use the update() method is that it modifies one of the dictionaries. If we wish to create a third dictionary without modifying any of the other dictionaries, we cannot use this method.

  Also, you can only use this method to merge two dictionaries at a time. If you wish to merge three dictionaries, you first need to merge the first two, and then merge the third one with the modified dictionary.

  >>> dict_one = < . "id": 1, . "name": "Ashutosh", . >>>> dict_two = < . "books": ["Python", "DSA"], . "college": "NSEC", . >>>> dict_three = < . "city": "Kolkata", . "country": "India" . >>>> dict_one.update(dict_two) >>> dict_one >>> dict_one.update(dict_three) >>> dict_one

  Let’s explore some other options.

  How to Merge Dictionaries in Python Using the ** operator

  You can use the double-asterisk (**) method to unpack or expand a dictionary like this:

  >>> dict_one = < . "id": 1, . "name": "Ashutosh", . >>>> dict_two = < . "books": ["Python", "DSA"] . "college": "NSEC", . >>>> dict_three = < . "city": "Kolkata", . "country": "India" . >>>> merged_dict = <**dict_one, **dict_two, **dict_three>>>> merged_dict

  Using the ** operator to merge the dictionaries doesn’t affect any of the dictionaries.

  How to Merge Dictionaries in Python Using the | Operator

  Starting with Python 3.9, we can use the Union ( | ) operator to merge two or more dictionaries.

  >>> dict_one = < . "id": 1, . "name": "Ashutosh", . >>>> dict_two = < . "books": ["Python", "DSA"], . "college": "NSEC", . >>>> dict_three = < . "city": "Kolkata", . "country": "India" . >>>> merged_dict = dict_one | dict_two | dict_three >>> merged_dict

  This is the most convenient method available for merging dictionaries in Python.

  Conclusion

  We have explored several different methods for merging dictionaries. If you have Python 3.9 or above, you should use the | operator. But if you use older versions of Python, you can still use the other methods discussed above.

  

  Ashutosh Krishna

  Application Developer at Thoughtworks India

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