Each key is separated from its value by a colon (:), the items are separated by commas, and the whole thing is enclosed in curly braces. An empty dictionary without any items is written with just two curly braces, like this: {}.

Keys are unique within a dictionary while values may not be. The values of a dictionary can be of any type, but the keys must be of an immutable data type such as strings, numbers, or tuples.

Accessing Values in Dictionary

To access dictionary elements, you can use the familiar square brackets along with the key to obtain its value. Following is a simple example:

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};

print "dict['Name']: ", dict['Name']; print "dict['Age']: ", dict['Age'];

When the above code is executed, it produces the following result:

dict['Name']:     Zara

dict['Age']:     7

If we attempt to access a data item with a key, which is not part of the dictionary, we get an error as follows:

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};

print "dict['Alice']: ", dict['Alice'];

When the above code is executed, it produces the following result:

dict['Zara']:

Traceback (most recent call last):

File "test.py", line 4, in <module>

print "dict['Alice']: ", dict['Alice']; KeyError: 'Alice'

Updating Dictionary

You can update a dictionary by adding a new entry or a key-value pair, modifying an existing entry, or deleting an existing entry as shown below in the simple example:

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};

dict['Age'] = 8; # update existing entry dict['School'] = "DPS School"; # Add new entry

print "dict['Age']: ", dict['Age']; print "dict['School']: ", dict['School'];

When the above code is executed, it produces the following result:

dict['Age']:     8

dict['School']:     DPS School

Delete Dictionary Elements

You can either remove individual dictionary elements or clear the entire contents of a dictionary. You can also delete entire dictionary in a single operation.

To explicitly remove an entire dictionary, just use the del statement. For example:

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};

del dict['Name']; # remove entry with key 'Name' dict.clear(); # remove all entries in dict del dict ; # delete entire dictionary

print "dict['Age']: ", dict['Age']; print "dict['School']: ", dict['School'];

This produces the following result. Note that an exception is raised because after del dict, dictionary does not exist anymore:

dict['Age']:

Traceback (most recent call last):

File "test.py", line 8, in <module>

print "dict['Age']: ", dict['Age']; TypeError: 'type' object is unsubscriptable

Note: del() method is discussed in subsequent section.

Properties of Dictionary Keys

Dictionary values have no restrictions. They can be any arbitrary Python object, either standard objects or user-defined objects. However, same is not true for the keys.

There are two important points to remember about dictionary keys:

(a) More than one entry per key not allowed. Which means no duplicate key is allowed. When duplicate keys encountered during assignment, the last assignment wins. For example:

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7, 'Name': 'Manni'};

print "dict['Name']: ", dict['Name'];

When the above code is executed, it produces the following result:

dict['Name']:     Manni

(b) Keys must be immutable. Which means you can use strings, numbers or tuples as dictionary keys but something like ['key'] is not allowed. Following is a simple example:

#!/usr/bin/python

dict = {['Name']: 'Zara', 'Age': 7};

print "dict['Name']: ", dict['Name'];

When the above code is executed, it produces the following result:

Traceback (most recent call last):

File "test.py", line 3, in <module>

dict = {['Name']: 'Zara', 'Age': 7}; TypeError: list objects are unhashable

Built-in Dictionary Functions and Methods

Python includes the following dictionary functions:

Sr. No.	Function with Description
1	cmp(dict1, dict2)
	Compares elements of both dict.
2	len(dict)
	Gives the total length of the dictionary. This would be equal to the number
	of items in the dictionary.
3	str(dict)
	Produces a printable string representation of a dictionary
4	type(variable)
	Returns the type of the passed variable. If passed variable is dictionary,
	then it would return a dictionary type.

Let us go through these briefly:

Cmp(dict1, dict2)

Description

The method cmp() compares two dictionaries based on key and values.

Syntax

Following is the syntax for cmp() method:

cmp(dict1, dict2)

Parameters

• dict1 -- This is the first dictionary to be compared with dict2.
• dict2 -- This is the second dictionary to be compared with dict1.

Return Value

This method returns 0 if both dictionaries are equal, -1 if dict1 < dict2, and 1 if dict1 > dic2.

Example

The following example shows the usage of cmp() method.

#!/usr/bin/python

dict1 = {'Name': 'Zara', 'Age': 7}; dict2 = {'Name': 'Mahnaz', 'Age': 27}; dict3 = {'Name': 'Abid', 'Age': 27}; dict4 = {'Name': 'Zara', 'Age': 7};

print "Return Value : %d" % cmp (dict1, dict2) print "Return Value : %d" % cmp (dict2, dict3) print "Return Value : %d" % cmp (dict1, dict4)

When we run above program, it produces following result:

Return Value : -1

Return Value : 1

Return Value : 0

58. len(dict)

Description

The method len() gives the total length of the dictionary. This would be equal to the number of items in the dictionary.

Syntax

Following is the syntax for len() method:

len(dict)

Parameters

• dict -- This is the dictionary, whose length needs to be calculated.

Return Value

This method returns the length.

Example

The following example shows the usage of len() method.

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7}; print "Length : %d" % len (dict)

When we run above program, it produces following result:

Length : 2

59. str(dict)

Description

The method str() produces a printable string representation of a dictionary.

Syntax

Following is the syntax for str() method:

str(dict)

Parameters

• dict -- This is the dictionary.

Return Value

This method returns string representation.

Example

The following example shows the usage of str() method.

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7};

print "Equivalent String : %s" % str (dict)

When we run above program, it produces following result:

Equivalent String : {'Age': 7, 'Name': 'Zara'}

60. type()

Description

The method type() returns the type of the passed variable. If passed variable is dictionary then it would return a dictionary type.

Syntax

Following is the syntax for type() method:

type(dict)

Parameters

• dict -- This is the dictionary.

Return Value

This method returns the type of the passed variable.

Example

The following example shows the usage of type() method.

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7};

print "Variable Type : %s" %     type (dict)

When we run above program, it produces following result:

Variable Type : <type 'dict'>

Python includes following dictionary methods:

Sr. No.       Methods with Description

• clear()

Removes all elements of dictionary dict

2	dict.copy()
	Returns a shallow copy of dictionary dict
3	dict.fromkeys()
	Create a new dictionary with keys from seq and values set to value.
4	dict.get(key, default=None)
	For key key, returns value or default if key not in dictionary
5	dict.has_key(key)
	Returns true if key in dictionary dict, false otherwise
6	dict.items()
	Returns a list of dict's (key, value) tuple pairs
7	dict.keys()
	Returns list of dictionary dict's keys
8	dict.setdefault(key, default=None)
	Similar to get(), but will set dict[key]=default if key is not already in dict
9	dict.update(dict2)
	Adds dictionary dict2's key-values pairs to dict
10	dict.values()
	Returns list of dictionary dict's values

Let us go through them briefly:

61. dict.clear()

Description

The method clear() removes all items from the dictionary.

Syntax

Following is the syntax for clear() method:

dict.clear()

Parameters

• NA

Return Value

This method does not return any value.

Example

The following example shows the usage of clear() method.

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7};

print "Start Len : %d" % len(dict) dict.clear()

print "End Len : %d" %     len(dict)

When we run above program, it produces following result:

Start Len : 2

End Len : 0

62. Dict.copy()

Description

The method copy() returns a shallow copy of the dictionary.

Syntax

Following is the syntax for copy() method:

dict.copy()

Parameters

• NA

Return Value

This method returns a shallow copy of the dictionary.

Example

The following example shows the usage of copy() method.

#!/usr/bin/python

dict1 = {'Name': 'Zara', 'Age': 7};

dict2 = dict1.copy()

print "New Dictinary : %s" %     str(dict2)

When we run above program, it produces following result:

New Dictinary : {'Age': 7, 'Name': 'Zara'}

63. Dict.fromkeys()

Description

The method fromkeys() creates a new dictionary with keys from seq and values set to value.

Syntax

Following is the syntax for fromkeys() method:

dict.fromkeys(seq[, value]))

Parameters

• seq -- This is the list of values which would be used for dictionary keys
• value -- This is optional, if provided then value would be set to this value

Return Value

This method returns the list.

Example

The following example shows the usage of fromkeys() method.

#!/usr/bin/python

seq = ('name', 'age', 'sex')

dict = dict.fromkeys(seq)

print "New Dictionary : %s" %     str(dict)

dict = dict.fromkeys(seq, 10)

print "New Dictionary : %s" %     str(dict)

When we run above program, it produces following result:

New Dictionary : {'age': None, 'name': None, 'sex': None}

New Dictionary : {'age': 10, 'name': 10, 'sex': 10}

64. Dict.get(key,default=none)

Description

The method get() returns a value for the given key. If key is not available then returns default value None.

Syntax

Following is the syntax for get() method:

dict.get(key, default=None)

Parameters

• key -- This is the Key to be searched in the dictionary.
• default -- This is the Value to be returned in case key does not exist.

Return Value

This method return a value for the given key. If key is not available, then returns default value None.

Example

The following example shows the usage of get() method.

#!/usr/bin/python

dict = {'Name': 'Zabra', 'Age': 7}

print "Value : %s" %     dict.get('Age')

print "Value : %s" %     dict.get('Education', "Never")

When we run above program, it produces the following result:

Value : 7

Value : Never

65. Dict.has_key(key)

Description

The method has_key() returns true if a given key is available in the dictionary, otherwise it returns a false.

Syntax

Following is the syntax for has_key() method:

dict.has_key(key)

Parameters

• key -- This is the Key to be searched in the dictionary.

Return Value

This method return true if a given key is available in the dictionary, otherwise it returns a false.

Example

The following example shows the usage of has_key() method.

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7}

print "Value : %s" % dict.has_key('Age') print "Value : %s" % dict.has_key('Sex')

When we run above program, it produces following result:

Value : True

Value : False

66. Dict.items()

Description

The method items() returns a list of dict's (key, value) tuple pairs

Syntax

Following is the syntax for items() method:

dict.items()

Parameters

• NA

Return Value

This method returns a list of tuple pairs.

Example

The following example shows the usage of items() method.

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7}

print "Value : %s" %     dict.items()

When we run above program, it produces following result:

Value : [('Age', 7), ('Name', 'Zara')]

67. Dict.keys()

Description

The method keys() returns a list of all the available keys in the dictionary.

Syntax

Following is the syntax for keys() method:

dict.keys()

Parameters

NA

Return Value

This method returns a list of all the available keys in the dictionary.

Example

The following example shows the usage of keys() method.

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7}

print "Value : %s" %     dict.keys()

When we run above program, it produces following result:

Value : ['Age', 'Name']

dict.setdefault(key, default=None)

Description

The method setdefault() is similar to get(), but will set dict[key]=default if key is not already in dict.

Syntax

Following is the syntax for setdefault() method:

dict.setdefault(key, default=None)

Parameters

• key -- This is the key to be searched.
• default -- This is the Value to be returned in case key is not found.

Return Value

This method returns the key value available in the dictionary and if given key is not available then it will return provided default value.

Example

The following example shows the usage of setdefault() method.

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7}

print "Value : %s" % dict.setdefault('Age', None) print "Value : %s" % dict.setdefault('Sex', None)

When we run above program, it produces following result:

Value : 7

Value : None

dict.update(dict2)

Description

The method update() adds dictionary dict2's key-values pairs in to dict. This function does not return anything.

Syntax

Following is the syntax for update() method:

dict.update(dict2)

Parameters

• dict2 -- This is the dictionary to be added into dict.

Return Value

This method does not return any value.

Example

The following example shows the usage of update() method.

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7} dict2 = {'Sex': 'female' }

dict.update(dict2)

print "Value : %s" %     dict

When we run above program, it produces following result:

Value : {'Age': 7, 'Name': 'Zara', 'Sex': 'female'}

70. dict.values()

Description

The method values() returns a list of all the values available in a given dictionary.

Syntax

Following is the syntax for values() method:

dict.values()

Parameters

NA

Return Value 

This method returns a list of all the values available in a given dictionary.

Example

The following example shows the usage of values() method.

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7}

print "Value : %s" %     dict.values()

When we run above program, it produces following result:

Value : [7, 'Zara']

 

#############################################

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.

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:

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.

Check if Key Exists

To determine if a specified key is present in a dictionary use the in keyword:

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:

Removing Items

There are several methods to remove items from a dictionary:

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().

Another way to make a copy is to use the built-in method dict().

The dict() Constructor

It is also possible to use the dict() constructor to make a new dictionary:

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

---

#########################

 

 

#############################################

p1={"name":"a","age":23,"city":"hyd","frnds":["c","b","e"]}
p2={"name":"b","age":22,"city":"Bng","frnds":["c","f","e","h"]}

print(p1)
print("key=","name", "value=", p1["name"])

#Update
p1["city"]="bng"

print("total items",len(p1))

if("city" in p1):
print("key is there")

# Delete
del p1["city"]

#iterator Access
for key in p1:
print("key=",key,"value=",p1[key])

 

#########

p1={"name":"a","age":23,"city":"hyd","friends":["b","c","d","e"]}
p2={"name":"b","age":23,"city":"hyd","friends":["a","c","d","g"]}

p1_frnds=p1["friends"]
p2_frnds=p2["friends"]

print("p1 frnds:",p1_frnds)
print("p2 frnds:",p2_frnds)

print("mutual friends")

for f1 in p1_frnds:
for f2 in p2_frnds:
if(f1==f2):
print(f1)

#print(d.items())
#print(len(d))
#print('name' in d.keys())

#for key in d:
# print(key,":",d[key])

 

@@@

p1={"name":"a","age":23,"city":"hyd","frnds":["c","b","e"]}
p2={"name":"b","age":22,"city":"Bng","frnds":["c","f","e","h"]}

s1=set(p1["frnds"])
print(s1)
s2=set(p2["frnds"])
print(s2)
print("common friends:",s1&s2)

 

##########################################

d={"name":"abc","age":20}

print(d["name"])

d["name"]="xyz"

print(d["name"])

for k in d:
print("key:",k)
print("value:",d[k])

print("age1" in d)

d["clg"]="xyz"

print(d)

d.pop("clg")
print(d)

d2=d.copy()
print(d2)

d3=dict(d)
print(d3)

print(d.keys())
l=d.keys()
print(list(l))
print(d.values())