Openai/6937ffdf-2f14-800e-816d-c3a88af3ea11

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gpt-5-1

User: I'm using jsonpathng.ext for a project that maps a JSON source to a target.

I'm using jsonpath_ng.ext for a project that maps a JSON source to a target. It's largely metadata driven and in many cases a simple jsonpath like "$.email" can map to a target field. In some cases concatenation is required like this; "$.firstName + ' ' + $.lastName". So far, so good. This works.

But; I have an issue whereby source data can have None for an attribute value and this results in the jsonpath parse returning an empty list. I need something similar to this pseudo "($.firstName or ) + ' ' + ($.lastName or )".

Any idea how I can coalesce None to empty string so my concatenation doesn't result in None?

Assistant: Here are three practical, working approaches you can use with jsonpathng.ext to safely coalesce None values to empty st…

Here are three practical, working approaches you can use with jsonpath_ng.ext to safely coalesce None values to empty strings during concatenation.

✅ Problem Recap

• You have expressions like:

"$.firstName + ' ' + $.lastName"

• But if firstName or lastName is None (or missing), jsonpath_ng returns an empty list, causing the expression to fail (or produce None / empty output).
• You want something like:

($.firstName or '') + ' ' + ($.lastName or '')

🔥 Important Detail

jsonpath_ng itself does not support or, boolean logic, or default operators inside expressions.

But you can still safely coalesce values by using Python and jsonpath together.

✅ Solution 1: Use a helper to get a string-safe value

This is the recommended and cleanest approach.

Helper:

from jsonpath_ng.ext import parse

def extract_safe(data, expr):
    match = parse(expr).find(data)
    if not match:
        return ''           # missing
    value = match[0].value
    return value or ''      # None → '', '' stays ''

Usage:

full_name = f"{extract_safe(data, '$.firstName')} {extract_safe(data, '$.lastName')}"

This handles:

• missing values
• None
• empty string
• type safety

✅ Solution 2: Wrap your jsonpath resolution with .get() logic

If your JSON looks like:

data = {
    "firstName": None,
    "lastName": "Ali"
}

You can bypass jsonpath for single-level fields:

full_name = f"{data.get('firstName') or ''} {data.get('lastName') or ''}"

But this only works for simple paths, not nested jsonpath queries.

✅ Solution 3: Pre-clean your source dict recursively

If you want jsonpath to always receive "safe" data, convert every None to "":

def coalesce_none(obj):
    if isinstance(obj, dict):
        return {k: coalesce_none(v) for k, v in obj.items()}
    if isinstance(obj, list):
        return [coalesce_none(v) for v in obj]
    return '' if obj is None else obj

Then:

data = coalesce_none(raw_data)

Now jsonpath always gets strings instead of None.

🔥 Solution 4 (Advanced): Extend jsonpath_ng to add a default() function

If you need to write expressions like this:

default($.firstName, '') + ' ' + default($.lastName, '')

You can extend the grammar—but this requires custom parser work. If you want, I can generate a full working DefaultFunction extension for jsonpath_ng.ext.

🚀 Which approach should you use?

| Approach | Best for | Pros | Cons | | ------------------------ | ------------------------------- | ---------------------- | -------------------------------- | | extract_safe() helper | Concatenation & mapping systems | Simple, explicit, safe | Requires wrapping your paths | | dict.get() | Shallow fields | Very easy | Doesn't support complex jsonpath | | Recursive cleaning | Large JSON transformations | One-time cleanup | Mutates or copies whole JSON | | Custom jsonpath function | Expression-level control | Most flexible | Requires grammar extension |

🎯 Recommended for your mapping engine

Use Solution 1 because your mapping is metadata-driven:

first = extract_safe(data, '$.firstName')
last = extract_safe(data, '$.lastName')

target["fullName"] = f"{first} {last}".strip()

If you want, I can also:

• Add trimming rules
• Add None → handling for lists
• Build a more powerful mapping engine wrapper
• Extend jsonpath to support or or default() syntax

Just tell me!