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Merging dictionaries

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Consider the following dictionaries: 

>>> fish = {'name': "Nemo", 'hands': "fins", 'special': "gills"}
>>> dog = {'name': "Clifford", 'hands': "paws", 'color': "red"}

Python 3.5+ 

>>> fishdog = {**fish, **dog}
>>> fishdog
{'hands': 'paws', 'color': 'red', 'name': 'Clifford', 'special': 'gills'}

As this example demonstrates, duplicate keys map to their lattermost value (for example “Clifford” overrides “Nemo”).

Python 3.3+ 

>>> from collections import ChainMap
>>> dict(ChainMap(fish, dog))
{'hands': 'fins', 'color': 'red', 'special': 'gills', 'name': 'Nemo'}

With this technique the foremost value takes precedence for a given key rather than the last (“Clifford” is thrown out in favor of “Nemo”).

Python 2.x, 3.x 

>>> from itertools import chain
>>> dict(chain(fish.items(), dog.items()))
{'hands': 'paws', 'color': 'red', 'name': 'Clifford', 'special': 'gills'}

This uses the lattermost value, as with the **-based technique for merging (“Clifford” overrides “Nemo”). 

>>> fish.update(dog)
>>> fish
{'color': 'red', 'hands': 'paws', 'name': 'Clifford', 'special': 'gills'}

dict.update uses the latter dict to overwrite the previous one.

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Dictionary:
* Dictionary
* Introduction to Dictionary
* Avoiding KeyError Exceptions
* Iterating Over a Dictionary
* Dictionary with default values
* Merging dictionaries
* Unpacking dictionaries using the ** operator
* Creating an ordered dictionary
* Creating a dictionary
* Accessing values of a dictionary
* Accessing keys and values
* All combinations of dictionary values
* The trailing comma
* The dict constructor
* Dictionaries Example
Table Of Contents
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165 hashlib
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189 Raise Custom Errors Exceptions
190 Using loops within functions
191 Contributors