Add the 'template' function to the formatter.

src/calibre/manual/template_lang.rst

@@ -208,11 +208,12 @@ The following functions are available in addition to those described in single-f
     * ``cmp(x, y, lt, eq, gt)`` -- compares x and y after converting both to numbers. Returns ``lt`` if x < y. Returns ``eq`` if x == y. Otherwise returns ``gt``.
     * ``divide(x, y)`` -- returns x / y. Throws an exception if either x or y are not numbers.
     * ``field(name)`` -- returns the metadata field named by ``name``.
-    * ``multiply``	-- returns x * y. Throws an exception if either x or y are not numbers.
+    * ``multiply(x, y)`` -- returns x * y. Throws an exception if either x or y are not numbers.
     * ``strcat(a, b, ...)`` -- can take any number of arguments. Returns a string formed by concatenating all the arguments.
     * ``strcmp(x, y, lt, eq, gt)`` -- does a case-insensitive comparison x and y as strings. Returns ``lt`` if x < y. Returns ``eq`` if x == y. Otherwise returns ``gt``.
     * ``substr(str, start, end)`` -- returns the ``start``'th through the ``end``'th characters of ``str``. The first character in ``str`` is the zero'th character. If end is negative, then it indicates that many characters counting from the right. If end is zero, then it indicates the last character. For example, ``substr('12345', 1, 0)`` returns ``'2345'``, and ``substr('12345', 1, -1)`` returns ``'234'``.
-    * ``subtract``	-- returns x - y. Throws an exception if either x or y are not numbers.
+    * ``subtract(x, y)`` -- returns x - y. Throws an exception if either x or y are not numbers.
+    * ``template(x)`` -- evaluates x as a template. The evaluation is done in its own context, meaning that variables are not shared between the caller and the template evaluation. Because the `{` and `}` characters are special, you must use `[[` for the `{` character and `]]` for the '}' character; they are converted automatically. For example, ``template('[[title_sort]]') will evaluate the template ``{title_sort}`` and return its value.
     
 Special notes for save/send templates
 -------------------------------------

src/calibre/utils/formatter.py

@@ -57,6 +57,11 @@ class _Parser(object):
         y = float(y if y else 0)
         return ops[op](x, y)
 
+    def _template(self, template):
+        template = template.replace('[[', '{').replace(']]', '}')
+        return self.parent.safe_format(template, self.parent.kwargs, 'TEMPLATE',
+                                       self.parent.book)
+
     local_functions = {
             'add'      : (2, partial(_math, op='+')),
             'assign'   : (2, _assign),
@@ -68,6 +73,7 @@ class _Parser(object):
             'strcmp'   : (5, _strcmp),
             'substr'   : (3, lambda s, x, y, z: x[int(y): len(x) if int(z) == 0 else int(z)]),
             'subtract' : (2, partial(_math, op='-')),
+            'template' : (1, _template)
     }
 
     def __init__(self, val, prog, parent):