Move from Excel to Python with Pandas Transcripts
Chapter: Appendix: Python language concepts
Lecture: Concept: Dictionaries

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0:01 Dictionaries are essential in Python.
0:03 A dictionary is a data structure that very efficiently stores
0:07 and can rapidly look up and retrieve items by some kind of key.
0:11 You can think of this as kind of a primary key in a database
0:14 or some other unique element representing the thing that you want to look up.
0:18 Dictionaries come in a couple of forms, the form you see on the screen here
0:22 we put multiple related pieces of information together that we can lookup,
0:27 so here maybe we have the age of a person and their current location.
0:31 Other types of dictionaries are maybe long lists of homogeneous data
0:35 maybe a list of a hundred thousand customers
0:37 and you can look them up by key which is say their email address,
0:41 which is unique in your system.
0:43 Whichever type you are working with, the way they function is the same.
0:45 We can create dictionaries in many ways, three of them here are on the screen;
0:49 the first block we initialize a dictionary by name and then we set
0:53 the value for age to 42, we set the location to Italy.
0:56 We can do this in one line by calling the dict initializer
0:59 and pass the key value argument, we can say dict age and location
1:04 or we can use the language syntax version, if you will,
1:07 with curly braces and then key colon value,
1:10 and it turns out all three of these are equivalent,
1:13 and you can use whichever one makes the most sense for your situation,
1:15 so here the created and then populated,
1:18 here created via the name and keyword arguments
1:21 or here created via the language structures.
1:24 The fact that this is so built-in to the language to tell you dictionaries are pretty important.
1:28 Now, if we want to access an item, from the dictionary,
1:31 we just use this index [ ] and then we pass the key whatever the key is.
1:36 In this case, we are using the location or the name of the property
1:40 we are trying to look up so we are asking for the location.
1:43 My other example if we had a dictionary
1:45 populated with a hundred thousand customer objects,
1:47 and the keyword is the email address, you would put in the email
1:51 for the specific customer you are looking for.
1:53 Now, if we ask for something that doesn't exist, this will crash with a KeyError exception,
1:57 so for example if I said "info['height']", there is no height, so it will crash.
2:01 there is a wide range of ways in which we can get the value out
2:05 or check for the existence of a value,
2:07 but the most straightforward is to use it in this "in" operator,
2:10 so here we can test whether age is in this info object
2:14 we can say "if age in info" and then it's safe to use info of age.
2:18 So this is just scratching the surface of dictionaries,
2:22 you'll see that they appear in many places and they play a central role
2:25 to many of the internal implementations in Python,
2:28 so be sure to get familiar with them.