JSON Matching Expressions

## JMESPath

JMESPath (pronounced “james path”) allows you to declaratively specify how to extract elements from a JSON document.

For example, given this document:

{"foo": {"bar": "baz"}}

The jmespath expression foo.bar will return “baz”.

JMESPath also supports:

Referencing elements in a list. Given the data:

{"foo": {"bar": ["one", "two"]}}

The expression: foo.bar[0] will return “one”. You can also reference all the items in a list using the * syntax:

{"foo": {"bar": [{"name": "one"}, {"name": "two"}]}}

The expression: foo.bar[*].name will return [“one”, “two”]. Negative indexing is also supported (-1 refers to the last element in the list). Given the data above, the expression foo.bar[-1].name will return “two”.

The * can also be used for hash types:

{"foo": {"bar": {"name": "one"}, "baz": {"name": "two"}}}

The expression: foo.*.name will return [“one”, “two”].

## API

The jmespath.py library has two functions that operate on python data structures. You can use search and give it the jmespath expression and the data:

>>> import jmespath
>>> path = jmespath.search('foo.bar', {'foo': {'bar': 'baz'}})
'baz'

Similar to the re module, you can use the compile function to compile the JMESPath expression and use this parsed expression to perform repeated searches:

>>> import jmespath
>>> expression = jmespath.compile('foo.bar')
>>> expression.search({'foo': {'bar': 'baz'}})
'baz'
>>> expression.search({'foo': {'bar': 'other'}})
'other'

This is useful if you’re going to use the same jmespath expression to search multiple documents. This avoids having to reparse the JMESPath expression each time you search a new document.

### Options

You can provide an instance of jmespath.Options to control how a JMESPath expression is evaluated. The most common scenario for using an Options instance is if you want to have ordered output of your dict keys. To do this you can use either of these options:

>>> import jmespath
>>> jmespath.search('{a: a, b: b},
...                 mydata,
...                 jmespath.Options(dict_cls=collections.OrderedDict))

>>> import jmespath
>>> parsed = jmespath.compile('{a: a, b: b}')
>>> parsed.search('{a: a, b: b},
...               mydata,
...               jmespath.Options(dict_cls=collections.OrderedDict))

## Specification

If you’d like to learn more about the JMESPath language, you can check out the JMESPath tutorial. Also check out the JMESPath examples page for examples of more complex jmespath queries.

The grammar is specified using ABNF, as described in RFC4234. You can find the most up to date grammar for JMESPath here.

You can read the full JMESPath specification here.

## Testing

In addition to the unit tests for the jmespath modules, there is a tests/compliance directory that contains .json files with test cases. This allows other implementations to verify they are producing the correct output. Each json file is grouped by feature.

## Discuss

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