Dictionary
Dictionary in Python is an unordered collection of data values, used to store data values like a map, which unlike other Data Types that hold only single value as an element, Dictionary holds key: value pair. May 27, 2019 As a Python coder, you’ll often be in situations where you’ll need to iterate through a dictionary in Python, while you perform some actions on its key-value pairs. When it comes to iterating through a dictionary in Python, the language provides you with some great tools that we’ll cover in this article. Iterating Through Keys Directly. Python: How to create a list of all the Values in a dictionary? Python: Filter a dictionary by conditions on keys or values; Python: How to add / append key value pairs in dictionary using dict.update Python: How to find keys by value in dictionary? What is a dictionary in python and why do we need it? Python: How to check if a key.
A dictionary is a collection which is unordered, changeable and indexed. In Python dictionaries are written with curly brackets, and they have keys and values.
Example
Create and print a dictionary:
'brand': 'Ford',
'model': 'Mustang',
'year': 1964
}
print(thisdict)
Accessing Items
You can access the items of a dictionary by referring to its key name, inside square brackets:
Example
Get the value of the 'model' key:
Try it Yourself »
There is also a method called get() that will give you the same result:
Example
Get the value of the 'model' key:
Try it Yourself »Change Values
You can change the value of a specific item by referring to its key name:
Loop Through Keys Python
Example
Change the 'year' to 2018:
'brand': 'Ford',
'model': 'Mustang',
'year': 1964
}
thisdict['year'] = 2018
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:
Try it Yourself »Example
Print all values in the dictionary, one by one:
Try it Yourself »Example
You can also use the values() function to return values of a dictionary:
Example
Loop through both keys and values, by using the items() function:
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:
'brand': 'Ford',
'model': 'Mustang',
'year': 1964
}
if 'model' in thisdict:
print('Yes, 'model' is one of the keys in the thisdict dictionary')
Dictionary Length
To determine how many items (key-value pairs) a dictionary has, use the len() method.
Example
Print the number of items in the dictionary:
Try it Yourself »Adding Items
Adding an item to the dictionary is done by using a new index key and assigning a value to it:
Example
'brand': 'Ford',
'model': 'Mustang',
'year': 1964
}
thisdict['color'] = 'red'
print(thisdict)
Python Dictionary For Loop Generate Keys Free
Try it Yourself »Removing Items
There are several methods to remove items from a dictionary:
Example
The pop() method removes the item with the specified key name:
'brand': 'Ford',
'model': 'Mustang',
'year': 1964
}
thisdict.pop('model')
print(thisdict)
Example
The popitem() method removes the last inserted item (in versions before 3.7, a random item is removed instead):
'brand': 'Ford',
'model': 'Mustang',
'year': 1964
}
thisdict.popitem()
print(thisdict)
Example
The del keyword removes the item with the specified key name:
'brand': 'Ford',
'model': 'Mustang',
'year': 1964
}
del thisdict['model']
print(thisdict)
Example
The del keyword can also delete the dictionary completely:
'brand': 'Ford',
'model': 'Mustang',
'year': 1964
}
del thisdict
print(thisdict) #this will cause an error because 'thisdict' no longer exists.
Example
The clear() method empties the dictionary:
'brand': 'Ford',
'model': 'Mustang',
'year': 1964
}
thisdict.clear()
print(thisdict)
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:
'brand': 'Ford',
'model': 'Mustang',
'year': 1964
}
mydict = thisdict.copy()
print(mydict)
Another way to make a copy is to use the built-in method dict().
Example
Make a copy of a dictionary with the dict() method:
'brand': 'Ford',
'model': 'Mustang',
'year': 1964
}
mydict = dict(thisdict)
print(mydict)
Nested Dictionaries
A dictionary can also contain many dictionaries, this is called nested dictionaries.
Example
Create a dictionary that contain three dictionaries:
'child1' : {
'name' : 'Emil',
'year' : 2004
},
'child2' : {
'name' : 'Tobias',
'year' : 2007
},
'child3' : {
'name' : 'Linus',
'year' : 2011
}
}
Or, if you want to nest three dictionaries that already exists as dictionaries:
Example
Create three dictionaries, than create one dictionary that will contain the other three dictionaries:
'name' : 'Emil',
'year' : 2004
}
child2 = {
'name' : 'Tobias',
'year' : 2007
}
child3 = {
'name' : 'Linus',
'year' : 2011
}
myfamily = {
'child1' : child1,
'child2' : child2,
'child3' : child3
}
The dict() Constructor
It is also possible to use the dict() constructor to make a new dictionary:
Example
# note that keywords are not string literals
# note the use of equals rather than colon for the assignment
print(thisdict)
Dictionary Methods
Python has a set of built-in methods that you can use on dictionaries.
| Method | Description |
|---|---|
| clear() | Removes all the elements from the dictionary |
| copy() | Returns a copy of the dictionary |
| fromkeys() | Returns a dictionary with the specified keys and value |
| get() | Returns the value of the specified key |
| items() | Returns a list containing a tuple for each key value pair |
| keys() | Returns a list containing the dictionary's keys |
| pop() | Removes the element with the specified key |
| popitem() | Removes the last inserted key-value pair |
| setdefault() | Returns the value of the specified key. If the key does not exist: insert the key, with the specified value |
| update() | Updates the dictionary with the specified key-value pairs |
| values() | Returns a list of all the values in the dictionary |