How can I modify/merge Jinja2 dictionaries? - merge

Question

I have a Jinja2 dictionary and I want a single expression that modifies it - either by changing its content, or merging with another dictionary.
>>> import jinja2
>>> e = jinja2.Environment()
Modify a dict: Fails.
>>> e.from_string("{{ x[4]=5 }}").render({'x':{1:2,2:3}})
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "jinja2/environment.py", line 743, in from_string
return cls.from_code(self, self.compile(source), globals, None)
File "jinja2/environment.py", line 469, in compile
self.handle_exception(exc_info, source_hint=source)
File "<unknown>", line 1, in template
jinja2.exceptions.TemplateSyntaxError: expected token
'end of print statement', got '='
Two-stage update: Prints superfluous "None".
>>> e.from_string("{{ x.update({4:5}) }} {{ x }}").render({'x':{1:2,2:3}})
u'None {1: 2, 2: 3, 4: 5}'
>>> e.from_string("{{ dict(x.items()+ {3:4}.items()) }}").render({'x':{1:2,2:3}})
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "jinja2/environment.py", line 868, in render
return self.environment.handle_exception(exc_info, True)
File "<template>", line 1, in top-level template code
TypeError: <lambda>() takes exactly 0 arguments (1 given)
Use dict(x,**y): Fails.
>>> e.from_string("{{ dict((3,4), **x) }}").render({'x':{1:2,2:3}})
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "jinja2/environment.py", line 868, in render
return self.environment.handle_exception(exc_info, True)
File "<template>", line 1, in top-level template code
TypeError: call() keywords must be strings
So how does one modify the dictionary x in Jinja2 by changing an attribute or merging with another dictionary?
This question is similar to: How can I merge two Python dictionaries as a single expression? -- insofar as Jinja2 and Python are analogous.

Answer 1

I found another solution without any extension.
{% set _dummy = x.update({4:5}) %}
It makes x updated. Don't use _dummy.

Answer 2

Sounds like the Jinja2 "do" statement extension may help. Enabling this extension would allow you to rewrite:
{{ x.update({4:5}) }} {{ x }}
as
{% do x.update({4:5}) %} {{ x }}
Example:
>>> import jinja2
>>> e = jinja2.Environment(extensions=["jinja2.ext.do",])
>>> e.from_string("{% do x.update({4:5}) %} {{ x }}").render({'x':{1:2,2:3}})
u' {1: 2, 2: 3, 4: 5}'
>>>

Answer 3

I added a filter to merge dictionaries, namely:
>>> def add_to_dict(x,y): return dict(x, **y)
>>> e.filters['add_to_dict'] = add_to_dict
>>> e.from_string("{{ x|add_to_dict({4:5}) }}").render({'x':{1:2,2:3}})
u'{1: 2, 2: 3, 4: 5}'