How do you check for nan in python jmespath?

Question

Assume import jmespath . Let's say I have some data like:

In [3]: data=[{'foo':10}, {'foo':float('nan')}, {'foo':32}]

In [4]: data
Out[4]: [{'foo': 10}, {'foo': nan}, {'foo': 32}]

I want to use jmespath to count the number of nodes that are nan. I can't seem to get it to. I can count the non-nan nodes like so:

jmespath.search('length([?@.foo.abs(@) >= `0`])',data)

I have tried comparing to nan and NaN but that doesn't work, and frankly it shouldn't because nan doesn't equal nan. However, there is no is_nan() function that I can see. I suppose I could convert to string and then compare them? That seems like a last resort. Is there some is_nan I don't see documented?

Answer 1

Context

• python 2.x or python 3.x
• jmespath query

Problem

• how to construct a query that counts the instances of nan (not a number)

Solution

• use Jmespath not-expression ! (exclaimation point) in conjunction with the abs() function
• use jmespath length() function to count the results

Example

jmespath.compile('''[*].foo|[? !(abs(@) >= `0`)]|length([*])''').search(data)

Details

• Jmespath type() function treats nan as number
• Using not-expression in combination with abs(@) greater-than or equal-to zero returns true for nan

Pitfalls

• Jmespath errors out with invalid type for value if an incompatible type is passed into abs() (such as string).