Python Dictionaries
Dictionary
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:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
print(thisdict)
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Accessing Items
You can access the items of a dictionary by referring to its key name, inside square brackets:
There is also a method called get() that will give you the same result:
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
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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)
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Example
Print all values in the dictionary, one by one:
for x in thisdict:
print(thisdict[x])
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Example
You can also use the values() function to
return values of a dictionary:
for x in thisdict.values():
print(x)
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Example
Loop through both keys and values, by using the
items() function:
for x, y in thisdict.items():
print(x, y)
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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")
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Dictionary Length
To determine how many items (key-value pairs) a dictionary has, use the len() method.
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)
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Removing Items
There are several methods to remove items from a dictionary:
Example
The pop() method removes the item with the specified key name:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
thisdict.pop("model")
print(thisdict)
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Example
The popitem() method removes the last
inserted item (in versions before 3.7, a random item is removed instead):
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
thisdict.popitem()
print(thisdict)
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Example
The del keyword removes the item with the specified
key name:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
del thisdict["model"]
print(thisdict)
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Example
The del keyword can also delete the
dictionary completely:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
del thisdict
print(thisdict) #this will cause an error because "thisdict"
no longer exists.
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Example
The clear() keyword empties the
dictionary:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
thisdict.clear()
print(thisdict)
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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)
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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:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
mydict
= dict(thisdict)
print(mydict)
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The dict() Constructor
It is also possible to use the dict() constructor to make a new dictionary:
Example
thisdict = dict(brand="Ford", model="Mustang", year=1964)
# note that keywords are not string literals
# note the use of equals rather than colon for the assignment
print(thisdict)
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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 values |
| get() | Returns the value of the specified key |
| items() | Returns a list containing the 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 |
