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Welcome back. Nested lists and

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dictionaries are very commonly shared between computer systems.

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A standard format has been defined that makes it easy to share that data.

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It's called JSON, J-S-O-N which stands for JavaScript Object Notation.

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It originated with a different programming language JavaScript.

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But, Python makes it pretty easy to both read in and write out data in JSON format.

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In fact, JSON format looks almost

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exactly like the printout of a Python list or a dictionary.

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In most cases, you wouldn't really notice the difference,

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but there are a couple of small differences;

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like the Python value none being represented as the word null,

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and true and false not being capitalized.

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We'll use the JSON module to read in and write out data in JSON format.

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Little warning, the JSON module that we have in Runestone

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only does a subset of what the full JSON module and a full Python environment would do.

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But we have the two most useful functions.

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Let's take a look at them.

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The first one is called Load S. It takes a string as input,

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and it returns a dictionary or a list.

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So, here we have little code snippet.

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We're importing the JSON module,

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and then we've got a variable called A string whose value is naturally a string,

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and that string has some newline characters in it,

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and then it's got the curly brace which

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would suggest to you that it's going to be a Python dictionary.

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It's really just a string,

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but once we take that string and on line three,

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we pass it as an input to the Load S function,

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which is part of the JSON module.

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The Load S function,

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in the JSON module,

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takes a string as an input and as an output it produces

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either a Python list or

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a Python dictionary depending on what the contents of the string were.

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So, in this case the string has its first non blank space its non white space character,

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the first one is a curly brace.

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So, what's going to be output is a dictionary.

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So on line four,

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we print out the type of D,

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and it is a dictionary.

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Since it's a dictionary,

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we can ask for its keys and it has two keys;

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result count and results.

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We can ask for the value associated with result count and it's 25.

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That's the 25 that you see there in the string.

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One important thing to keep track of here,

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is when you have a string,

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and when you have a dictionary.

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So, we start with something that's a character string,

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and by passing it in to the Load S function,

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we get a dictionary as an output.

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If I lost track of that let's say,

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and I tried to print a_string square bracket result count.

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Because a_string kind of looks like a dictionary,

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and I try to run this, I'm going to get an error.

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It says its string indices must be integers not strings on line seven.

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Let's say on line seven,

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we're trying to treat a_string as if it's a dictionary.

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It isn't a dictionary, it's a string.

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We're allowed to ask for a_string square bracket four to get the fourth character,

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but we can't ask for a_string square bracket result count.

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We can ask for D square bracket result count,

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because D is a dictionary.

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The second useful function from the JSON module is called Dump S.

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It does the opposite of Load S. It takes a Python list or dictionary,

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even a nested one,

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and converts it into a string that's in the JSON format.

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Once we have a string in the JSON format,

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we can write it to a file or do anything else that we would do with a string.

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The Dump S function always takes a list or a dictionary as an input.

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But it also can take a couple of other parameters.

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If you pass sort keys equals true, then,

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whenever you have a dictionary it's going to output those in alphabetic order.

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If you pass the indent parameter,

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it will pretty print the string.

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It'll do some indentation in line breaks.

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You can specify how many characters of indentation you want.

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So let's see what happens when we run this.

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Oops! I got an error.

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It's telling me that on line two,

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JSON is not defined.

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The reason is that I forgot,

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and I need to import the JSON module.

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So, the Dump S function is in the JSON module,

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and this code snippet didn't have the JSON module imported.

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So let me run it again.

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Now you can see, when I pass D to the pretty function on line nine,

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that it is creating a string by calling the json.dumps function and that string,

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because I said indent equals two,

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each time I descend one level into this next to the data structure,

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it's adding two more levels, two more spaces.

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That makes it just a lot easier for a person to read when I print out that string.

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Now, you may have noticed little funny pronunciation that I've done here.

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Instead of calling this Dumps and Loads,

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I've called it Dump S and Load S. That's very deliberate.

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It helps me to remind me that that S is for string.

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The S is not for plural.

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You still have to remember which of Dump S and

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Load S reads from a string and which one writes to a string.

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Here's how I think of it.

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The JSON format is a set of conventions for how a string

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should be structured so that it can be loaded into a Python object.

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Thus, Load S means load from a string or load a string into an object.

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Dump S means dump an object to a string.

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Dumping from a string wouldn't really make sense,

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so that's how I can keep track of it.

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Dump S, dump an object to a string.

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Load S, load an object from a string.

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That's the JSON format folks.

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We'll be seeing a lot more of it when we look at

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rest APIs that let us fetch data from servers on the internet,

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like from iTunes or OMDb.

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JSON dot Load S,

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loads from a string,

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into a Python object,

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either a list or a dictionary,

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and JSON dot dump S dumps from a Python list or dictionary into a string.

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JSON dot Dump S,

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with the indent parameter,

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yields a string with indentation,

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that makes it easier for people to read and understand. See you next time.