exec tag added to enable treating evaluated string as a part of the t…

[sprksh]

…emplate, added tests

Description of the change

Idea is to provide users a functionality so that by using {% exec %} templatetag, they can evaluate a string based template (say passed as an argument) as a part of the original template.

For example user can define a macro in string and use that in the same execution flow.

Our use case was that user can define macros in yaml and then call it from the yaml itself and we should be able to treat the macro as one of the predefined macros. Earlier we were doing two step evaluation but that limited the capability of the system.

We could not define this templateTag from outside as it has to use few variables private to the package.

func test() {
	tpl, err := pongo2.FromFile("some_file.template")
	if err != nil {
		log.Fatal(err)
	}

	c1 := `{% macro double(arg) %}{{2*arg}}{% endmacro %}`
	c2 := "{{ double(10) }}"
	res, err := tpl.Execute(pongo2.Context{"c1": c1, "c2": c2})
	if err != nil {
		log.Fatal(err)
	}

}

some_file.template

{% exec %}{{c1}}{% endexec %}
{% exec %}{{c2}}{% endexec %}

so effectively this file will be evaluated as

{% macro double(arg) %}{{2*arg}}{% endmacro %}
{{ double(10) }}

and the result will be

20

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Related issues

Fix #1

Checklists

Development

• Lint rules pass locally
• The code changed/added as part of this pull request has been covered with tests
• All tests related to the changed code pass in development

Code review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• "Ready for review" label attached to the PR and reviewers mentioned in a comment
• Changes have been reviewed by at least one other engineer
• Issue from task tracker has a link to this pull request

[gane5hvarma]

There is a typo in description. instead of c2 its c1

{% exec %}{{c1}}{% endexec %}
{% exec %}{{c2}}{% endexec %}

[gane5hvarma]

Not need to have differrent name for err. you can use as err instead of err2

[sprksh]

issue is that err and err2 are of different types.. err is basically pongo2.Error. thats why it is causing type error

[gane5hvarma]

Suggested change

	currentTemplate, err2 := templateSet.FromString(s)
	tempBytes := temp.Bytes()
	currentTemplate, err := templateSet.FromBytes(tempBytes)

Change is because FromString converts string to bytes. Using FromBytes will not do conversion from temp to tempString(temp.String()) and tempString to bytes again(inside FromString)

[sprksh]

done.