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Python From Scratch
Python Dictionaries
Lesson 9 Content
- Python Dictionaries
- Dictionary
- Dictionary Items
- Ordered or Unordered?
- Changeable
- Duplicates Not Allowed
- Dictionary Length
- Dictionary Items - Data Types
- type()
- The dict() Constructor
- Python Collections (Arrays)
- Access Items
- Accessing Items
- Get Keys
- Get Values
- Get Items
- Check if Key Exists
- Change Items
- Change Values
- Update Dictionary
- Add Items
- Adding Items
- Update Dictionary
- Remove Items
- Loop Dictionaries
- Copy Dictionaries
- Nested Dictionaries
- Python - Dictionaries Methods
- Python - Dictionaries Exercises
Python Dictionaries
Dictionary Items
Dictionaries are used to store data values in key:value pairs.
A dictionary is a collection which is ordered*, changeable and do not allow duplicates.
As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.
Dictionaries are written with curly brackets, and have keys and values:
Example
Create and print a dictionary:
thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } print(thisdict)
Dictionary Items
Dictionary items are ordered, changeable, and does not allow duplicates.
Dictionary items are presented in key:value pairs, and can be referred to by using the key name.
Example
Print the "brand" value of the dictionary:
thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } print(thisdict["brand"])
Ordered or Unordered?
As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.
When we say that dictionaries are ordered, it means that the items have a defined order, and that order
will not change.
Unordered means that the items does not have a defined order, you cannot refer to an item by using an
index.
Changeable
Dictionaries are changeable, meaning that we can change, add or remove items after the dictionary has
been created.
Duplicates Not Allowed
Dictionaries cannot have two items with the same key:
Example
Duplicate values will overwrite existing values:
thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 , "year": 2020 } print(thisdict)
Get Items
The items() method will return each item in a dictionary, as tuples in a list.
Example
Get a list of the key:value pairs
x = thisdict.items()
The returned list is a view of the items of the dictionary, meaning that any changes done to the
dictionary will be reflected in the items list.
Example
Make a change in the original dictionary, and see that the items list gets updated as well:
car = { "brand": "Ford", "model": "Mustang", "year": 1964 } x = car.items() print(x) #before the change car["year"] = 2020 print(x) #after the change
Example
Add a new item to the original dictionary, and see that the items list gets updated as well:
car = { "brand": "Ford", "model": "Mustang", "year": 1964 } x = car.items() print(x) #before the change car["color"] = "red" print(x) #after the change
Check if Key Exists
To determine if a specified key is present in a dictionary use the in keyword:
Example
Check if "model" is present in the dictionary:
thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } if "model" in thisdict: print("Yes, 'model' is one of the keys in the thisdict dictionary")
Python - Change Dictionary Items
Change Values
You can change the value of a specific item by referring to its key name:
Example
Change the "year" to 2018:
thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } thisdict["year"] = 2018
Update Dictionary
The update() method will update the dictionary with the items from the given argument.
The argument must be a dictionary, or an iterable object with key:value pairs.
Example
Update the "year" of the car by using the update() method:
thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } thisdict.update({"year": 2020 })
Python - Add Dictionary Items
Adding Items
Adding an item to the dictionary is done by using a new index key and assigning a value to it:
Example
thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } thisdict["color"] = "red" print(thisdict)
Update Dictionary
The update() method will update the dictionary with the items from a given argument. If the item does
not exist, the item will be added.
The argument must be a dictionary, or an iterable object with key:value pairs.
Example
Add a color item to the dictionary by using the update() method:
thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } thisdict.update({"color": "red"})
Python - Loop Dictionaries
Loop Through a Dictionary
You can loop through a dictionary by using a for loop.
When looping through a dictionary, the return value are the keys of the dictionary, but there are methods
to return the values as well.
Example
Print all key names in the dictionary, one by one:
for x in thisdict: print(x)
Example
You can also use the values() method to return values of a dictionary:
for x in thisdict.values(): print(x)
Example
You can use the keys() method to return the keys of a dictionary:
for x in thisdict.keys(): print(x)
Example
Loop through both keys and values, by using the items() method:
for x, y in thisdict.items(): print(x, y)
Python - Copy Dictionaries
Copy a Dictionary
You cannot copy a dictionary simply by typing dict2 = dict1, because: dict2 will only be a reference to
dict1, and changes made in dict1 will automatically also be made in dict2.
There are ways to make a copy, one way is to use the built-in Dictionary method copy().
Example
Make a copy of a dictionary with the copy() method:
thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } mydict = thisdict.copy() print(mydict)
Another way to make a copy is to use the built-in function dict().
Example
Make a copy of a dictionary with the dict() function:
thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964 } mydict = dict(thisdict) print(mydict)
Example
Print all values in the dictionary, one by one:
for x in thisdict: print(thisdict[x])
Python - Nested Dictionaries
Nested Dictionaries
A dictionary can contain dictionaries, this is called nested dictionaries.
Example
Create a dictionary that contain three dictionaries:
myfamily = { "child1" : { "name" : "Emil", "year" : 2004 }, "child2" : { "name" : "Tobias", "year" : 2007 }, "child3" : { "name" : "Linus", "year" : 2011 } }
Or, if you want to add three dictionaries into a new dictionary:
Example
Create three dictionaries, then create one dictionary that will contain the other three dictionaries:
child1 = { "name" : "Emil", "year" : 2004 } child2 = { "name" : "Tobias", "year" : 2007 } child3 = { "name" : "Linus", "year" : 2011 } myfamily = { "child1" : child1, "child2" : child2, "child3" : child }
