Add API docs for the Metadata object to the template tutorial

src/calibre/ebooks/metadata/book/base.py

@@ -68,7 +68,19 @@ composite_formatter = SafeFormat()
 class Metadata(object):
 
     '''
-    A class representing all the metadata for a book.
+    A class representing all the metadata for a book. The various standard metadata
+    fields are available as attributes of this object. You can also stick
+    arbitrary attributes onto this object.
+
+    Metadata from custom columns should be accessed via the get() method,
+    passing in the lookup name for the column, for example: "#mytags".
+
+    Use the :meth:`is_null` method to test if a filed is null.
+
+    This object also has functions to format fields into strings.
+
+    The list of standard metadata fields grows with time is in
+    :data:`STANDARD_METADATA_FIELDS`.
 
     Please keep the method based API of this class to a minimum. Every method
     becomes a reserved field name.
@@ -88,11 +100,19 @@ class Metadata(object):
             if title:
                 self.title = title
             if authors:
-                #: List of strings or []
+                # List of strings or []
                 self.author = list(authors) if authors else []# Needed for backward compatibility
                 self.authors = list(authors) if authors else []
 
     def is_null(self, field):
+        '''
+        Return True if the value of filed is null in this object.
+        'null' means it is unknown or evaluates to False. So a title of
+        _('Unknown') is null or a language of 'und' is null.
+
+        Be careful with numeric fields since this will return True for zero as
+        well as None.
+        '''
         null_val = NULL_VALUES.get(field, None)
         val = getattr(self, field, None)
         return not val or val == null_val
@@ -547,13 +567,16 @@ class Metadata(object):
         return unicode(self.rating)
 
     def format_field(self, key, series_with_index=True):
+        '''
+        Returns the tuple (display_name, formatted_value)
+        '''
         name, val, ign, ign = self.format_field_extended(key, series_with_index)
         return (name, val)
 
     def format_field_extended(self, key, series_with_index=True):
         from calibre.ebooks.metadata import authors_to_string
         '''
-        returns the tuple (field_name, formatted_value, original_value,
+        returns the tuple (display_name, formatted_value, original_value,
         field_metadata)
         '''
 
@@ -637,6 +660,10 @@ class Metadata(object):
         return (None, None, None, None)
 
     def __unicode__(self):
+        '''
+        A string representation of this object, suitable for printing to
+        console
+        '''
         from calibre.ebooks.metadata import authors_to_string
         ans = []
         def fmt(x, y):
@@ -680,6 +707,9 @@ class Metadata(object):
         return u'\n'.join(ans)
 
     def to_html(self):
+        '''
+        A HTML representation of this object.
+        '''
         from calibre.ebooks.metadata import authors_to_string
         ans = [(_('Title'), unicode(self.title))]
         ans += [(_('Author(s)'), (authors_to_string(self.authors) if self.authors else _('Unknown')))]

src/calibre/manual/template_lang.rst

@@ -396,3 +396,19 @@ You might find the following tips useful.
     * In a plugboard, you can set a field to empty (or whatever is equivalent to empty) by using the special template ``{null}``. This template will always evaluate to an empty string.
     * The technique described above to show numbers even if they have a zero value works with the standard field series_index.
     
+API of the Metadata objects
+----------------------------
+
+.. module:: calibre.ebooks.metadata.book.base
+
+.. autoclass:: Metadata
+   :members:
+   :member-order: bysource
+
+.. data:: STANDARD_METADATA_FIELDS
+
+    The set of standard metadata fields.
+
+.. literalinclude:: ../ebooks/metadata/book/__init__.py
+   :lines: 7-
+