How can I modify/merge Jinja2 dictionaries?

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.