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Python Dictionary

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Python Dictionary

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  1. Python Dictionary

  2. Python Dictionary : A Python dictionary is a sequence of key/value or item pairs separated by commas. A Python dictionary is created by using curly braces ( { } ). Syntax to create empty dictionary. < variable-name > = {} Or < variable-name > = dict() Syntax to create Python dictionary with items <variable-name> = { key : value , key : value } The pair of key and value is called item, key and value are separated by colon ( : ). The Each item is separated by comma ( , ), whole thing is enclosed in curly braces ( { } ). Copyright @ 2018 Learntek. All Rights Reserved.

  3. Features of Python dictionary The key of Python dictionary can not be changed. The value of key can be changed. Ordering is not significant, the order in which you have entered the items in dictionary, may not get the items in same order. Features of Python dictionary’s key Key are unique in the Python dictionary. We can not assign same key again. Let us see the example. >>> dict1 = {“Mohit” : 1, “Rahul” : 2, “Mohit” : 3} >>> dict1 {‘Mohit’: 3, ‘Rahul’: 2} >>> Copyright @ 2018 Learntek. All Rights Reserved.

  4. Key are unique in the Python dictionary. We can not assign same key again. In the above example, we used the same key “Mohit” two times although the program is not giving any error. But You can see the most recent key replaced the old one. The int, float, and string can be used as a key. Tuple which does not contain any list can be used as a key. Let us see the example. >>> tuple = () >>> dict2 = {tuple : “a”} >>> dict2 {(): ‘a’} >>> tuple1 = ( [],9 ) >>> dict2 = { tuple1 : “a” } Copyright @ 2015 Learntek. All Rights Reserved.

  5. Traceback (most recent call last): File “<stdin>”, line 1, in <module> Type Error: unshushable type: ‘list’ >>> In the example, it is very clear if a tuple with a list is used then the interpreter will give the error. Copyright @ 2018 Learntek. All Rights Reserved.

  6. Features of Python Dictionary’s value. The value can be repeated. You can store any thing in the value. Adding and updating the value from dictionary Let us create an empty dictionary and add some items. >>> AV = {} >>> AV[ ‘IRON-MAN’ ] = “Tony Stark” >>> AV[ ‘Thor’ ] = “Chris” >>> AV { ‘IRON-MAN’ : ‘Tony Stark’, ‘Thor’ : ‘Chris’ } >>> Copyright @ 2018 Learntek. All Rights Reserved.

  7. Let us update the values ‘Chris’. >>> AV[ ‘Thor’ ] = “Chris Hemsworth” >>> AV {‘IRON-MAN’ : ‘Tony Stark’, ‘Thor’ : ‘Chris Hemsworth’} >>> You have seen we can update the value by using the dictionary’s key. Copyright @ 2018 Learntek. All Rights Reserved.

  8. Accessing the values of Python dictionary Like Python lists there is not indexing exists in Python dictionary.   The values can only be accessed by its key. Let us take the example of computer network ports. >>> port1 = { 22: “SSH”, 23 : ‘Telnet’, 80: “HTTP”, 53: “DNS”, 443: “HTTPS” } >>> port1[22] ‘SSH’ >>> port1[53] ‘DNS’ >>> port1[67] Copyright @ 2015 Learntek. All Rights Reserved.

  9. Traceback (most recent call last): File “<stdin>”, line 1, in <module> Key Error: 67 You can see if a key does not exist then the interpreter shows error. In order to tackle this problem then get() method is used. See the example below. >>> port1.get(53, “not found” ) ‘DNS’ >>> port1.get(67, “not found” ) ‘not found’ >>> Instead of error a customize message get printed. Copyright @ 2018 Learntek. All Rights Reserved.

  10. Different type of ways to check the existence of a key In this section, we’ll see how to check whether a particular key exists or not. You have already see the get() method. >>> port1.get( 22, “Key not found”) ‘SSH’ >>> port1.get( 24, “Key not found”) ‘Key not found’ >>> Copyright @ 2018 Learntek. All Rights Reserved.

  11. Has key() The has key() method return the True and False statement. >>> port1.has_key(22) True >>> port1.has_key(24) False >>> By using in operator >>> if 22 in port1: …     print “Key found” … Key found >>> Copyright @ 2018 Learntek. All Rights Reserved.

  12. Delete Elements of Python dictionary you can delete a single element of the dictionary as well entire dictionary. Example >>> port1 = {22: “SSH”, 23 : ‘Telnet’,80: “HTTP”, 53: “DNS”, 443: “HTTPS”} >>> del port1[22] >>> port1 {80: ‘HTTP’, 443: ‘HTTPS’, 53: ‘DNS’, 23: ‘Telnet’} >>> del port1 >>> port1 Copyright @ 2018 Learntek. All Rights Reserved.

  13. Traceback (most recent call last): File “<stdin>”, line 1, in <module> Name Error: name ‘port1’ is not defined You can see the error because dictionary named port does exist after command del port1. Copyright @ 2018 Learntek. All Rights Reserved.

  14. Iterating the Python dictionary using for loop In order to iterate over the Python dictionary, we used items() or iteritems() method. The items() method return a list of tuple pairs. The iteritems() method is the iterator which saves the memory. In Python 3 the iteritems() method has been removed and items() method works as iteritems(). port1 = {22: “SSH”,23:’Telnet’,80: “HTTP”, 53: “DNS”, 443: “HTTPS” } for k, v in port1.items(): print k, ” : ” ,v Copyright @ 2018 Learntek. All Rights Reserved.

  15. Output : Copyright @ 2018 Learntek. All Rights Reserved.

  16. Consider you want to update the dictionary with another dictionary See the example below. port1 = { 22: “SSH”, 23 : ‘Telnet’,80: “HTTP” } port2 = { 53: “DNS”, 443: “HTTPS” } port1.update( port2 ) print port1 Copyright @ 2018 Learntek. All Rights Reserved.

  17. Output: { 80: ‘HTTP’, 443: ‘HTTPS’, 53: ‘DNS’, 22: ‘SSH’, 23: ‘Telnet’ } So far you learned Python dictionary creation, deleting an item, adding an item, updating items. Let us create a copy of an existing dictionary.  In order to make a copy of a dictionary, we will use the copy() method. >>> port1 = {80: ‘HTTP’, 443: ‘HTTPS’, 53: ‘DNS’ } >>> port2 = port1.copy() >>> port2 {80: ‘HTTP’, 443: ‘HTTPS’, 53: ‘DNS’} >>> id(port1) 47341080L >>> id(port2) 47374680L >>> Copyright @ 2018 Learntek. All Rights Reserved.

  18. In the above example, a new dictionary,  port2, which is the copy of port1 has been created. The memory addresses of both the dictionary are different. Practical example Let us create a dictionary by using 2 lists list1 = [1 ,2, 3, 4] list2 = [ ‘a’, ‘b’, ‘c’, ‘d’ ] dict1 = { } for i in xrange( len(list1) ): dict1[list1[i]] = list2[i] print dict1 Copyright @ 2018 Learntek. All Rights Reserved.

  19. Output : {1: ‘a’, 2: ‘b’, 3: ‘c’, 4: ‘d’} Same above result can be achieved in one line.  See the example below. list1 = [1 ,2, 3, 4] list2 = [ ‘a’, ‘b’, ‘c’, ‘d’ ] print dict(zip(list1,list2)) Output: {1: ‘a’, 2: ‘b’, 3: ‘c’, 4: ‘d’} Copyright @ 2018 Learntek. All Rights Reserved.

  20. For more Training Information , Contact Us Email : info@learntek.org USA : +1734 418 2465 INDIA : +40 4018 1306 +7799713624 Copyright @ 2018 Learntek. All Rights Reserved.

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