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PYTHON - DICTIONARY

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
1	dict.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

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

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 preparation.
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

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']

68. 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

69. 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']

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PYTHON - DICTIONARY