Make nodes.template.Template a bit more dict-like (#252)

* nodes: add a `default` param to Template.get

Similar to dict.get, Template.get with a default param supplied will
return that value instead of raising an exception. If default is unset,
Template.get will keep its previous behavior and raise an exception.

* nodes: Add __getitem__, __setitem__, and __delitem__ to Template

These are just aliases for existing methods, without the ability to
specifiy additional parameters. However, including them makes Template
more dict-like, so it's a good idea to have them.

* nodes: Use def instead of assignment of a lambda in Template

Per PEP8, there is no benefit to using a lambda here, and some
downsides. It's the same number of SLOC either way, so might as well
change it.

mwparserfromhell/nodes/template.py

@@ -29,6 +29,8 @@ from ..utils import parse_anything
 __all__ = ["Template"]
 
 FLAGS = re.DOTALL | re.UNICODE
+# Used to allow None as a valid fallback value
+_UNSET = object()
 
 class Template(Node):
     """Represents a template in wikicode, like ``{{foo}}``."""
@@ -208,23 +210,30 @@ class Template(Node):
                 return True
         return False
 
-    has_param = lambda self, name, ignore_empty=False: \
-                self.has(name, ignore_empty)
-    has_param.__doc__ = "Alias for :meth:`has`."
+    def has_param(self, name, ignore_empty=False):
+        """Alias for :meth:`has`."""
+        return self.has(name, ignore_empty)
 
-    def get(self, name):
+    def get(self, name, default=_UNSET):
         """Get the parameter whose name is *name*.
 
         The returned object is a :class:`.Parameter` instance. Raises
-        :exc:`ValueError` if no parameter has this name. Since multiple
-        parameters can have the same name, we'll return the last match, since
-        the last parameter is the only one read by the MediaWiki parser.
+        :exc:`ValueError` if no parameter has this name. If *default* is set,
+        returns that instead. Since multiple parameters can have the same name,
+        we'll return the last match, since the last parameter is the only one
+        read by the MediaWiki parser.
         """
         name = str(name).strip()
         for param in reversed(self.params):
             if param.name.strip() == name:
                 return param
-        raise ValueError(name)
+        if default is _UNSET:
+            raise ValueError(name)
+        else:
+            return default
+
+    def __getitem__(self, name):
+        return self.get(name)
 
     def add(self, name, value, showkey=None, before=None,
             preserve_spacing=True):
@@ -306,6 +315,9 @@ class Template(Node):
             self.params.append(param)
         return param
 
+    def __setitem__(self, name, value):
+        return self.add(name, value)
+
     def remove(self, param, keep_field=False):
         """Remove a parameter from the template, identified by *param*.
 
@@ -351,3 +363,6 @@ class Template(Node):
             raise ValueError(name)
         for i in reversed(to_remove):
             self.params.pop(i)
+
+    def __delitem__(self, param):
+        return self.remove(param)