Dictionaries in Python are used to store data values in key-value pairs. A dictionary is created using curly braces {} with keys and their corresponding values separated by colons. Values can be accessed using their keys and dictionaries can be iterated over to access both keys and values. Common uses of dictionaries include storing contact information with name, phone, email as keys and values. Keys must be immutable objects like strings or numbers.

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Dictionaries
Pavan Verma
@YinYangPavan
Founder, P3 InfoTech Solutions Pvt. Ltd.
Python Programming Essentials
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What is a dictionary
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• Dictionaries are collections of items that have
a “key” and a “value”
• Like a list is indexed by a number from 0 to len-
1, dictionaries are indexed by the keys

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Sample use cases of dictionary
 Storing a record of information as key-
value pairs
 Example: Contact list
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Creating a dictionary
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mydict = {
'one': 1,
'two': 2,
}
print mydict
Output:
{'two': 2, 'one': 1}
Syntax
mydict = {
key1: value1,
key2: value2,
}

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Accessing elements of a dictionary
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mydict = {
'one': 1,
'two': 2,
}
print mydict['one']
print mydict['two']
Output:
1
2
dict[key]

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Accessing elements of a dictionary (2)
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mydict = {
'one': 1,
'two': 2,
}
print mydict['three']
Output:
KeyError: 'three' Accessing a key that does
not exist throws
KeyError

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Key types
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Keys can be of any immutable type
1. Strings
2. Numbers
3. Tuples

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Key types (2)
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mydict = {
'one': 1,
'two': 2,
}
Strings as keys

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Key types (3)
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points = {
(1, 2): 10,
(3, 4): 20,
}
Tuples as keys
mydict = {
1: 'one',
2: 'two',
}
Numbers as keys

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Tuples as keys
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• Tuples can be used as keys if they contain only other
immutable types – strings, numbers, or tuples
• If a tuple contains any mutable type, it cannot be used
as a key
mydict = {
([1, 2, 3], 4): 'one',
}
TypeError: unhashable type: 'list'

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Accessing keys
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contact = {
"first": "John",
"last": "Doe",
"age": 39,
"sex": "M",
"salary": 70000,
"registered": True,
}
print contact.keys()
['salary', 'last', 'age', 'sex', 'registered',
'first']
dict.keys() gives an
unordered list of keys in a
dictionary

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Accessing values
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contact = {
"first": "John",
"last": "Doe",
"age": 39,
"sex": "M",
"salary": 70000,
"registered": True,
}
print contact.values()
[70000, 'Doe', 39, 'M', True, 'John']
dict.values() gives an
unordered list of values in
dictionary

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Updating an element
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contact = {
"first": "John",
"last": "Doe",
"age": 39,
"sex": "M",
"salary": 70000,
"registered": True,
}
contact['salary'] = 90000
print contact
Output:
{'salary': 90000, 'last': 'Doe', 'age': 39, 'sex': 'M',
'registered': True, 'first': 'John'}
dict[key] = newvalue

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Deleting an element
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contact = {
"first": "John",
"last": "Doe",
"age": 39,
"sex": "M",
"salary": 90000,
"registered": True,
}
del contact['last']
print contact
Output:
{'salary': 90000, 'age': 39, 'sex': 'M', 'registered': True,
'first': 'John'}
del dict[key]

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Test if a key exists in a dictionary
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contact = {
"first": "John",
"last": "Doe",
"age": 39,
"sex": "M",
"salary": 90000,
"registered": True,
}
print 'first' in contact
Output: True
print 'second' in contact
Output: False
Use in operator to test weather
a key exists or not

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Loop over a dictionary
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for key in contact:
print key
Output:
salary
age
sex
registered
first
By default looping over a
dictionary, gives access to all the
keys

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Loop over a dictionary (2)
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for key in contact.iterkeys():
print key
Output:
salary
age
sex
registered
first
dict.iterkeys():use this
to iterate over keys of a
dictionary

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Loop over a dictionary (3)
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for key in contact.itervalues():
print key
Output:
90000
39
M
True
John
dict.itervalues(): use
this to iterate over values of a
dictionary

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Loop over a dictionary (4)
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for key, value in contact.iteritems():
print key, value
Output:
salary 90000
age 39
sex M
registered True
first John
dict.iteritems(): use
this to iterate over both keys and
values

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Example: Contact information
contacts = [
{'name': 'Pavan Verma',
'mobile': '9999900000',
'email': 'pavan@p3infotech.in', }
{'name': 'Sahil Chug',
'mobile': '9999900001',
'email': 'sahil@p3infotech.in', }
{'name': 'Abijith MG',
'mobile': '9999900007',
'email': 'abijith@p3infotech.in', }
]
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Example: Contact information (2)
contacts = {
1: {'name': 'Pavan Verma',
'mobile': '9999900000',
'email': 'pavan@p3infotech.in', }
2: {'name': 'Sahil Chug',
'mobile': '9999900001',
'email': 'sahil@p3infotech.in', }
8: {'name': 'Abijith MG',
'mobile': '9999900007',
'email': 'abijith@p3infotech.in', }
}
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Summary
 What is a dictionary
 Creating a dictionary
 Accessing elements of a dictionary
 Key types
 Tuples as keys
 Iterating over dictionary
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References
 Dictionaries tutorial
http://www.tutorialspoint.com/python/python_dictionary.htm
 Dictionaries in detail in python docs
http://docs.python.org/2/library/stdtypes.html#mapping-
types-dict
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