New template function: field_exists()

manual/template_lang.rst

@@ -390,6 +390,7 @@ parameters can be statements (sequences of expressions). Note that the definitiv
       `[[` for the `{` character and `]]` for the '}' character; they are converted automatically. Note also that prefixes and suffixes
       (the `|prefix|suffix` syntax) cannot be used in the argument to this function when using Template Program Mode.
     * ``field(name)`` -- returns the metadata field named by ``name``.
+    * ``field_exists(field_name)`` -- checks if a field (column) named ``field_name`` exists, returning '1' if so and '' if not.
     * ``finish_formatting(val, fmt, prefix, suffix)`` -- apply the format,
       prefix, and suffix to a value in the same way as done in a template like
       ``{series_index:05.2f| - |- }``. This function is provided to ease

src/calibre/utils/formatter_functions.py

@@ -1894,6 +1894,19 @@ class BuiltinGlobals(BuiltinFormatterFunction):
         # The globals function is implemented in-line in the formatter
         raise NotImplementedError()
 
+class BuiltinFieldExists(BuiltinFormatterFunction):
+    name = 'field_exists'
+    arg_count = 1
+    category = 'If-then-else'
+    __doc__ = doc = _('field_exists(field_name) -- checks if a field '
+                      '(column) named field_name exists, returning '
+                      "'1' if so and '' if not.")
+
+    def evaluate(self, formatter, kwargs, mi, locals, field_name):
+        if field_name.lower() in mi.all_field_keys():
+            return '1'
+        return ''
+
 
 _formatter_builtins = [
     BuiltinAdd(), BuiltinAnd(), BuiltinApproximateFormats(), BuiltinArguments(),
@@ -1903,7 +1916,8 @@ _formatter_builtins = [
     BuiltinCmp(), BuiltinConnectedDeviceName(), BuiltinConnectedDeviceUUID(), BuiltinContains(),
     BuiltinCount(), BuiltinCurrentLibraryName(), BuiltinCurrentLibraryPath(),
     BuiltinDaysBetween(), BuiltinDivide(), BuiltinEval(), BuiltinFirstNonEmpty(),
-    BuiltinField(), BuiltinFinishFormatting(), BuiltinFirstMatchingCmp(), BuiltinFloor(),
+    BuiltinField(), BuiltinFieldExists(),
+    BuiltinFinishFormatting(), BuiltinFirstMatchingCmp(), BuiltinFloor(),
     BuiltinFormatDate(), BuiltinFormatNumber(), BuiltinFormatsModtimes(),
     BuiltinFormatsPaths(), BuiltinFormatsSizes(), BuiltinFractionalPart(),
     BuiltinGlobals(),