[Python] How to programmatically get a list of all first-level keys stored in a namespace?

In the process of developing a custom Jinja2 extension (a tag, not a filter), I am finding myself in the situation where I need to extract a list of first-level keys that refer to the runtime values saved in a namespace.

Individual first-level values with known keys are easy to extract from the namespace with a simple Getattr(Name('my_ns_name', 'load'), 'my_key', 'load') node. But how can I get the list of all first-level keys when not known in advance?

I inspected Namespace objects as returned at runtime by a Name('my_ns_name', 'load') node and haven't found any relevant information there. I also feel that the NSRef node type could possibly be a key to the solution, but I still fail to understand its purpose, specially considering how little documented it is. I also thought of using a For node and iterate through some collection returned by one of the nodes.iter_* functions, but that doesn't seem to apply to namespace attributes. So I'm a bit stuck, now...

Here's a condensed version of my attempts so far:

import pprint

from jinja2 import nodes
from jinja2.ext import Extension

class InspectExtension(Extension):

tags = {"inspect"}

def __init__(self, environment):
super().__init__(environment)

def parse(self, parser):
lineno = parser.stream.expect("name:inspect").lineno
expr = parser.parse_expression()
context = nodes.DerivedContextReference()

return nodes.Output([
nodes.Const(expr), # Inspect the AST
nodes.TemplateData('\n'), # For a nicer output
self.call_method("_render", [expr, context]) # Runtime inspection
]).set_lineno(lineno)

def _render(self, value, context):
result = {
"class": value.__class__.__name__,
"attrs": dir(value),
"context": list(context.get_all().keys())
}
return pprint.pformat(result, compact=False)

inspect = InspectExtension


With this template:

<!DOCTYPE html>
<html><body><pre>
{% set ns = namespace() %}
{% set ns.test = 'foobar' %}
{{ ns.test }}
{% inspect ns %}
</pre></body></html>


Outputs (rendered HTML):

foobar
Getattr(node=Name(name='ns', ctx='load'), attr='test', ctx='load')
{'attrs': ['__class__',
'__delattr__',
'__dict__',
'__dir__',
'__doc__',
'__eq__',
'__format__',
'__ge__',
'__getattribute__',
'__gt__',
'__hash__',
'__init__',
'__init_subclass__',
'__le__',
'__lt__',
'__module__',
'__ne__',
'__new__',
'__reduce__',
'__reduce_ex__',
'__repr__',
'__setattr__',
'__setitem__',
'__sizeof__',
'__str__',
'__subclasshook__',
'__weakref__'],
'class': 'Namespace',
'context': ['range',
'dict',
'lipsum',
'cycler',
'joiner',
'namespace',
'environ',
'get_context',
'request',
'result',
'ctx',
'ns'],
'value': }


For completeness:

• The extension source code is located in ./j2_ext/inspect.py, along with the customary __init__.py which contains a single statement from .inspect import inspect.
• Jinja2 engine is called through jinja2-cli, passing the extension name to the --extension argument, as such:

jinja2 --format=json --extension=.j2_ext.inspect /path/to/my_tpl.j2 < my_data.json


Some relevant documentation:

• Official Jinja2 documentation: https://jinja.palletsprojects.com/en/stable/extensions/#extension-api
• Examples of Jinja2 extensions: https://github.com/topics/jinja2-extension?o=asc&s=stars

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