While I was trying to implement allowing filter expressions as
arguments to some built-in template tags (see #5756), I found
inconsistencies in the handling of literal strings in the template
system.
Collin Grady's patch [1] for {% ifequal %} looks good, but using the
same logic for other tags causes unit tests to fail. The reason is
that the FilterExpression class [2] (called by compile_filter()) only
accepts double quoted string literals, whereas tests generally use
single quoted strings. The current implementations of several template
tags resolve their arguments using the Variable class [3], which turns
out to handle both single and double quoted strings.
After studying the code for filter expression and variable resolution
I came up with a patch suggestion [4] for allowing single quotes in
filter expressions. I also made sure that escaping of the chosen quote
character and the backslash character works correctly. I also re-
factored the FilterExpression and Variable classes a bit so they use
the same code to unquote and un-escape string literals.
In FilterExpression this change required a different approach to the
filter_re regular expression. Single and double quoted translated and
un-translated strings are now all caught with a single group and
parsed further in the Python code. I have not yet looked into
performance effects or possible optimizations of this implementation.
I couldn't find any design desicions wrt single and double quotes. But
if there is a decision against single quotes, of course an entirely
different approach has to be taken.
All current unit tests pass with the suggested patch. The patch is
attached to #7295.
#5756 http://code.djangoproject.com/ticket/5756
#7295 http://code.djangoproject.com/ticket/7295
[1] http://code.djangoproject.com/attachment/ticket/5756/5756.patch
[2]
http://code.djangoproject.com/browser/django/trunk/django/template/__init__.py#L459
[3]
http://code.djangoproject.com/browser/django/trunk/django/template/__init__.py#L595
[4] http://code.djangoproject.com/attachment/ticket/7295/7295.1.diff
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