# JMESPath Tutorial¶

This is a tutorial of the JMESPath language. JMESPath is a query language for JSON. You can extract and transform elements from a JSON document. The examples below are interactive. You can change the JMESPath expressions and see the results update automatically.

For each of these examples, the JMESPath expression is applied to the input JSON on the left, and the result of evaluting the JMESPath expression is shown in the JSON document on the right hand side.

## Basic Expressions¶

The simplest JMESPath expression is an identifier, which selects a key in an JSON object:

##### Result

Try changing the expression above to b, and c and note the updated result. Also note that if you refer to a key that does not exist, a value of null (or the language equivalent of null) is returned.

You can use a subexpression to return to nested values in a JSON object:

##### Result

If you refer to a key that does not exist, a value of null is returned. Attempting to subsequently access identifiers will continue to return a value of null. Try changing the expression to b.c.d.e above.

Index Expressions allow you to select a specific element in a list. It should look similar to array access in common programming languages. Indexing is 0 based.

##### Result

If you specify an index that’s larger than the list, a value of null is returned. You can also use negative indexing to index from the end of the list. [-1] refers to the last element in the list, [-2] refers to the penultimate element. Try it out in the example above.

You can combine identifiers, sub expressions, and index expressions to access JSON elements.

## Slicing¶

Slices allow you to select a contiguous subset of an array. If you’ve ever used slicing in python, then you already know how to use JMESPath slices. In its simplest form, you can specify the starting index and the ending index. The ending index is the first index which you do not want included in the slice. Let’s take a look at some examples. First, given an array of integers from 0 to 9, let’s select the first half of the array:

##### Result

This slice result contains the elements 0, 1, 2, 3, and 4. The element at index 5 is not included. If we want to select the second half of the array, we can use this expression:

##### Result

The two example above can be shortened. If the start or step value is omitted it is assumed to be the start or the end of the array. For example:

##### Result

Try modifying changing the example above to only include the last half of the array elements without specifying the end value of 10.

The general form of a slice is [start:stop:step]. So far we’ve looked at the [start:stop] form. By default, the step value is 1, which means to include every element in the range specified by the start and stop value. However, we can use the step value to skip over elements. For example, to select only the even elements from the array.

##### Result

Also note in this example we’re omitting the start as well as the stop value, which means to use 0 for the start value, and 10 for the stop value. In this example, the expression [::2] is equivalent to [0:10:2].

The last thing to know about slices is that just like indexing a single value, all the values can be negative. If the step value is negative, then the slice is created in reverse order. For example:

##### Result

The above expression creates a slice but in reverse order.

If you want all the details about how slices work, check out the section in the JMESPath specification.

## Projections¶

Projections are one of the key features of JMESPath. It allows you to apply an expression to a collection of elements. There are five kinds of projections:

• List Projections
• Slice Projections
• Object Projections
• Flatten Projections
• Filter Projections

### List and Slice Projections¶

A wildcard expression creates a list projection, which is a projection over a JSON array. This is best illustrated with an example. Let’s say we have a JSON document describing a people, and each array element is a JSON object that has a first, last, and age key. Suppose we wanted a list of all the first names in our list.

##### Result

In the example above, the first expression, which is just an identifier, is applied to each element in the people array. The results are collected into a JSON array and returned as the result of the expression. The expression can be more complex than a basic identifier. For example, the expression foo[*].bar.baz would project the bar.baz expression to each element in the foo array.

There’s a few things to keep in mind when working with projections. These are discussed in more detail in the wildcard expressions section of the spec, but the main points are:

• Projections are evaluated as two steps. The left hand side (LHS) creates a JSON array of initial values. The right hand side (RHS) of a projection is the expression to project for each element in the JSON array created by the left hand side. Each projection type has slightly different semantics when evaluating either the left hand side and/or the right hand side.
• If the result of the expression projected onto an individual array element is null, then that value is omitted from the collected set of results.
• You can stop a projection with a Pipe Expression (discussed later).
• A list projection is only valid for a JSON array. If the value is not a list, then the result of the expression is null.

You can try this out in the demo above. Notice how people[*].first only included three elements, even though the people array has four elements. This is because the last element, {"missing": "different"} evalues to null when the expression first is applied, and null values are not added to the collected result array. If you try the expression foo[*].bar you’ll see a result of null, because the value associated with the foo key is a JSON object, not an array, and a list projection is only defined for JSON arrays.

Slice projections are almost identical to a list projection, with the exception that the left hand side is the result of evaluating the slice, which may not include all the elements in the original list:

### Object Projections¶

Whereas a list projection is defined for a JSON array, an object projection is defined for a JSON object. You can create an object projection using the * syntax. This will create a list of the values of the JSON object, and project the right hand side of the projection onto the list of values.

##### Result

In the example above the * creates a JSON array of the values associated with the ops JSON object. The RHS of the projection, numArgs, is then applied to the JSON array, resulting in the final array of [2, 3]. Below is a sample walkthrough of how an implementation could potentially implement evaluating an object projection. First, the object projection can be broken down into its two components, the left hand side (LHS) and its right hand side (RHS):

• LHS: ops
• RHS: numArgs

First, the LHS is evaluated to create the initial array to be projected:

```evaluate(ops, inputData) -> [{"numArgs": 2}, {"numArgs": 3},
```

Then the RHS is applied to each element in the array:

```evaluate(numArgs, {numArgs: 2}) -> 2
evaluate(numArgs, {numArgs: 3}) -> 3
```

Any null values are not included in the final result, so the result of the entire expression is therefore [2, 3].

### Flatten Projections¶

More than one projection can be used in a JMESPath expression. In the case of a List/Object projection, the structure of the original document is preserved when creating projection within a projection. For example, let’s take the expression reservations[*].instances[*].state. This expression is saying that the top level key reservations has an array as a value. For each of those array elements, project the instances[*].state expression. Within each list element, there’s an instances key which itself is a value, and we create a sub projection for each each list element in the list. Here’s an example of that:

##### Result

The result of this expression is [["running", "stopped"], ["terminated", "running"]], which is a list of lists. The outer list is from the projection of reservations[*], and the inner list is a projection of state created from instances[*]:

```1st       r0                         r1
2nd i0          i1             i0            i1
[["running", "stopped"], ["terminated", "running"]]
```

What if we just want a list of all the states of our instances? We’d ideally like a result ["running", "stopped", "terminated", "running"]. In this situation, we don’t care which reservation the instance belonged to, we just want a list of states.

This is the problem that a Flatten Projection solves. To get the desired result, you can use [] instead of [*] to flatten a list: reservations[].instances[].state. Try changing [*] to [] in the expression above and see how the result changes.

While the spec goes into more detail, a simple rule of thumb to use for the flatten operator, [], is that:

• It flattens sublists into the parent list (not recursively, just one level).
• It creates a projection, so anything on the RHS of the flatten projection is projected onto the newly created flattened list.

You can also just use [] on its own to flatten a list:

##### Result

If you flattened the result of the expression again, [][], you’d then get a result of [0, 1, 2, 3, 4, 5, 6, 7]. Try it out in the example above.

### Filter Projections¶

Up to this point we’ve looked at:

• List/Slice projections
• Object projections
• Flatten projections

Evaluating the RHS of a projection is a basic type of filter. If the result of the expression evaluated against an individual element results in null, then the element is excluded from the final result.

A filter projection allows you to filter the LHS of the projection before evaluating the RHS of a projection.

For example, let’s say we have a list of machines, each has a name and a state. We’d like the name of all machines that are running. In pseudocode, this would be:

```result = []
foreach machine in inputData['machines']
if machine['state'] == 'running'
result.insert_at_end(machine['name'])
return result
```

A filter projection can be used to accomplish this:

##### Result

Try changing running to stopped in the example above. You can also remove the .name at the end of the expression if you just want the entire JSON object of each machine that has the specified state.

A filter expression is defined for an array and has the general form LHS [? <expression> <comparator> <expression>] RHS. The filter expression spec details exactly what comparators are available and how they work, but the standard comparators are supported, i.e ==, !=, <, <=, >, >=.

## Pipe Expressions¶

Projections are an important concept in JMESPath. However, there are times when projection semantics are not what you want. A common scenario is when you want to operate of the result of a projection rather than projecting an expression onto each element in the array. For example, the expression people[*].first will give you an array containing the first names of everyone in the people array. What if you wanted the first element in that list? If you tried people[*].first that you just evaluate first for each element in the people array, and because indexing is not defined for strings, the final result would be an empty array, []. To accomplish the desired result, you can use a pipe expression, <expression> | <expression>, to indicate that a projection must stop. This is shown in the example below:

##### Result

In the example above, the RHS of the list projection is first. When a pipe is encountered, the result up to that point is passed to the RHS of the pipe expression. The pipe expression is evaluated as:

```evaluate(people[*].first, inputData) -> ["James", "Jacob", "Jayden"]
evaluate(, ["James", "Jacob", "Jayden"]) -> "James"
```

## MultiSelect¶

Up to this point, we’ve looked at JMESPath expressions that help to pare down a JSON document into just the elements you’re interested in. This next concept, multiselect lists and multiselect hashes allow you to create JSON elements. This allows you to create elements that don’t exist in a JSON document. A multiselect list creates a list and a multiselect hash creates a JSON object.

This is an example of a multiselect list:

##### Result

In the expression above, the [name, state.name] portion is a multiselect list. It says to create a list of two element, the first element is the result of evaluating the name expression against the list element, and the second element is the result of evaluating state.name. Each list element will therefore create a two element list, and the final result of the entire expression is a list of two element lists.

Unlike a projection, the result of the expression in always included, even if the result is a null. If you change the above expression to people[].[foo, bar] each two element list will be [null, null].

A multiselect has the same basic idea of a multiselect list, except it instead creates a hash instead of an array. Using the same example above, if we instead wanted to create a two element hash that had two keys, Name and State, we could use this:

## Functions¶

JMESPath supports function expressions, for example:

##### Result

Functions can be used to transform and filter data in powerful ways. The full list of functions can be found here, and the function expression spec has the complete details.

Below are a few examples of functions.

This example prints the name of the oldest person in the people array:

##### Result

Functions can also be combined with filter expressions. In the example below, the JMESPath expressions finds all elements in myarray that contains the string foo.

##### Result

The @ character in the example above refers to the current element being evaluated in myarray. The expression contains(@, `foo`) will return true if the current element in the myarray array contains the string foo.

While the function expression spec has all the details, there are a few things to keep in mind when working with functions:

• Function arguments have types. If an argument for a function has the wrong type, an invalid-type error will occur. There are functions that can do type conversions (to_string, to_number) to help get arguments converted to their proper type.
• If a function is called with the wrong number of arguments, an invalid-arity will occur.

## Next Steps¶

We’ve now seen an overview of the JMESPath language. The next things to do are:

• See the JMESPath Examples. You’ll see common JMESPath expressions that go beyond the tutorial. You’ll also see you how to combine multiple features together in order to best leverage JMESPath expressions.
• To actually start using JMESPath, pick the language of your choice, and check out the JMESPath Libraries page for more information on using JMESPath in the language of your choice.
• Read the JMESPath Spec, which has the official ABNF grammar and full details of the semantics of the language.
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