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JSON Matching Expressions

Project description


JMESPath (pronounced “jaymz 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 will return “baz”.

JMESPath also supports:

Referencing elements in a list. Given the data:

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

The expression:[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:[*].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[-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”].

NOTE: jmespath is being actively developed. There are a number of features it does not currently support that may be added in the future.


The grammar is specified using ABNF, as described in RFC4234

expression        = sub-expression / index-expression / or-expression / identifier / "*"
sub-expression    = expression "." expression
or-expression     = expression "||" expression
index-expression  = expression bracket-specifier / bracket-specifier
bracket-specifier = "[" (number / "*") "]"
number            = [-]1*digit
digit             = "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9" / "0"
identifier        = 1*char
char              = unescaped /
                    escape (
                        %x20-2F /    ; Space,!,",#,$,%,&,',(,),*,+,comma,-,.,/
                        %x3A-40 /    ; :,;,<,=,>,?,@
                        %x5B    /    ; Left bracket: [
                        %x5C    /    ; Back slash: \
                        %x5D    /    ; Right bracket: ]
                        %x5E    /    ; Caret: ^
                        %x60    /    ; Backtick: `
                        %x7B-7E /    ; {,|,},~
                        b       /    ; backspace
                        n       /    ; new line
                        f       /    ; form feed
                        r       /    ; carriage return
                        t       )    ; tab
escape            = %x5C   ; Back slash: \
unescaped         = %x30-39 / %x41-5A / %x5F / %x61-7A / %x7F-10FFFF


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.

Python Library

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

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

Similar to the re module, you can store the compiled expressions and reuse them to perform repeated searches:

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

You can also use the jmespath.parser.Parser class directly if you want more control.

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