Dictionary
A dictionary is an unordered, mutable, and indexed collection. In Python, dictionaries are written in braces and have keys and values.
an actual example
Creates and prints a dictionary:
thisdict = {
"brand": "Porsche",
"model": "911",
"year": 1963
}
print(thisdict)
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Visiting projects
You can access a dictionary item by referring to its key name in square brackets:
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Gets the value of the "model" key:
x = thisdict["model"]
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There is also a method called get() that will give you the same result:
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Gets the value of the "model" key:
x = ("model")running example
value change
You can change the value of a specific item by referencing its key name:
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Change "year" to 2019:
thisdict = {
"brand": "Porsche",
"model": "911",
"year": 1963
}
thisdict["year"] = 2019
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Iterative dictionary
You can use a for loop to traverse the dictionary.
When looping through a dictionary, the return value is the key of the dictionary, but there are methods that return values.
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Prints all key names in the dictionary one by one:
for x in thisdict: print(x)
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Prints all values in the dictionary one by one:
for x in thisdict: print(thisdict[x])
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You can also use the values() function to return dictionary values:
for x in (): print(x)
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Iterate over the keys and values by using the items() function:
for x, y in (): print(x, y)
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Check if the key exists
To determine if the specified key exists in the dictionary, use the in keyword:
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Checks for the presence of "model" in the dictionary:
thisdict = {
"brand": "Porsche",
"model": "911",
"year": 1963
}
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) the dictionary has, use the len() method.
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Prints the number of items in the dictionary:
print(len(thisdict))
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Add Project
Items can be added to the dictionary by using a new index key and assigning a value to it:
an actual example
thisdict = {
"brand": "Porsche",
"model": "911",
"year": 1963
}
thisdict["color"] = "red"
print(thisdict)
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Deleting items
There are several ways to remove items from the dictionary:
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The pop() method deletes the item with the specified key name:
thisdict = {
"brand": "Porsche",
"model": "911",
"year": 1963
}
("model")
print(thisdict)
running example
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The popitem() method deletes the last inserted item (in versions prior to 3.7, it deletes random items):
thisdict = {
"brand": "Porsche",
"model": "911",
"year": 1963
}
()
print(thisdict)
running example
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The del keyword deletes the item with the specified key name:
thisdict = {
"brand": "Porsche",
"model": "911",
"year": 1963
}
del thisdict["model"]
print(thisdict)
running example
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The del keyword also removes the dictionary entirely:
thisdict = {
"brand": "Porsche",
"model": "911",
"year": 1963
}
del thisdict
print(thisdict) #this causes an error because "thisdict" no longer exists.
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clear() keyword clears the dictionary:
thisdict = {
"brand": "Porsche",
"model": "911",
"year": 1963
}
()
print(thisdict)
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Copy Dictionary
You can't copy the dictionary by typing dict2 = dict1 because: dict2 is just a reference to dict1 and changes in dict1 will automatically be made in dict2 as well.
There are a number of ways to make a copy, one way is to use the built-in dictionary method copy().
an actual example
Use the copy() method to copy the dictionary:
thisdict = {
"brand": "Porsche",
"model": "911",
"year": 1963
}
mydict = ()
print(mydict)
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Another way to make a copy is to use the built-in method dict().
an actual example
Use the dict() method to create a copy of the dictionary:
thisdict = {
"brand": "Porsche",
"model": "911",
"year": 1963
}
mydict = dict(thisdict)
print(mydict)
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nested dictionary
Dictionaries can also contain many dictionaries, which are called nested dictionaries.
an actual example
Creates a dictionary containing three dictionaries:
myfamily = {
"child1" : {
"name" : "Phoebe Adele",
"year" : 2002
},
"child2" : {
"name" : "Jennifer Katharine",
"year" : 1996
},
"child3" : {
"name" : "Rory John",
"year" : 1999
}
}
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{'child1': {'name': 'Phoebe Adele', 'year': 2002}, 'child2': {'name': 'Jennifer Katharine', 'year': 1996}, 'child3': {'name': 'Rory John', 'year': 1999}}
Or, if you want to nest three dictionaries that already exist as dictionaries:
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Create three dictionaries and then create a dictionary that contains the other three dictionaries:
child1 = {
"name" : "Phoebe Adele",
"year" : 2002
}
child2 = {
"name" : "Jennifer Katharine",
"year" : 1996
}
child3 = {
"name" : "Rory John",
"year" : 1999
}
myfamily = {
"child1" : child1,
"child2" : child2,
"child3" : child3
}
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{'child1': {'name': 'Phoebe Adele', 'year': 2002}, 'child2': {'name': 'Jennifer Katharine', 'year': 1996}, 'child3': {'name': 'Rory John', 'year': 1999}}
dict() constructor
A new dictionary can also be created using the dict() constructor:
an actual example
thisdict = dict(brand="Porsche", model="911", year=1963) # Note that the keyword is not a string literal # Note the use of an equal sign instead of a colon to assign values print(thisdict)
running example
dictionary method
Python provides a set of built-in methods that can be used on dictionaries.
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