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In the example JSON at the bottom of this question, how can I count the number of key/value pairs in the array "Tags" using JMESPath?

According to the JMESPath documentation, I can do this using the count() function -

For example, the following expression creates an array containing the total number of elements in the foo object followed by the value of foo["bar"].

However, it seems that the documentation is incorrect. Using the JMESPath website, the query Reservations.Instances.[count(@), Tags] yeilds the result [ [ null ] ]. I then tested via the AWS command line and an error was returned -

Unknown function: count()

Is there actually a way of doing this using JMESPath?

Example JSON -

 "Reservations": [
 
 "Instances": [
 
 "InstanceId": "i-asdf1234",
 "InstanceName": "My Instance",
 "Tags": [
 
 "Value": "Value1",
 "Key": "Key1"
 ,
 
 "Value": "Value2",
 "Key": "Key2"
 ,
 
 "Value": "Value3",
 "Key": "Key3"
 ,
 
 "Value": "Value4",
 "Key": "Key4"
 
 ]
 
 ]
 
 ]

The answer here is that the JMESPath documentation is shocking, and for some reason I was seeing out of date documentation (check the bottom right corner of the screen to see what version you are viewing.

I can do what I need to do using the length() function -

Reservations.Instances.Tags | length(@)

I managed to incorporate this usage of length length(Tags[*]) within a larger statement I think is useful and wanted to share:

aws ec2 describe-instances --region us-west-2 --query 'Reservations[*].Instances[*].id: InstanceId, ami_id: ImageId, type: InstanceType, tag_count: length(Tags[*])' --profile prod --output table;

--------------------------------------------------------------------
| DescribeInstances |
+--------------+-----------------------+------------+--------------+
| ami_id | id | tag_count | type |
+--------------+-----------------------+------------+--------------+
| ami-abc123 | i-redacted1 | 1 | m3.medium |
| ami-abc456 | i-redacted2 | 7 | m3.xlarge |
| ami-abc789 | i-redacted3 | 12 | t2.2xlarge |
+--------------+-----------------------+------------+--------------+