# -*- coding: utf-8 -*-
"""API for traversing the AST nodes. Implemented by the compiler and
meta introspection.
"""
from .nodes import Node


class NodeVisitor(object):
 """Walks the abstract syntax tree and call visitor functions for every
 node found. The visitor functions may return values which will be
 forwarded by the `visit` method.

 Per default the visitor functions for the nodes are ``'visit_'`` +
 class name of the node. So a `TryFinally` node visit function would
 be `visit_TryFinally`. This behavior can be changed by overriding
 the `get_visitor` function. If no visitor function exists for a node
 (return value `None`) the `generic_visit` visitor is used instead.
 """

 def get_visitor(self, node):
 """Return the visitor function for this node or `None` if no visitor
 exists for this node. In that case the generic visit function is
 used instead.
 """
 method = "visit_" + node.__class__.__name__
 return getattr(self, method, None)

 def visit(self, node, *args, **kwargs):
 """Visit a node."""
 f = self.get_visitor(node)
 if f is not None:
 return f(node, *args, **kwargs)
 return self.generic_visit(node, *args, **kwargs)

 def generic_visit(self, node, *args, **kwargs):
 """Called if no explicit visitor function exists for a node."""
 for node in node.iter_child_nodes():
 self.visit(node, *args, **kwargs)


class NodeTransformer(NodeVisitor):
 """Walks the abstract syntax tree and allows modifications of nodes.

 The `NodeTransformer` will walk the AST and use the return value of the
 visitor functions to replace or remove the old node. If the return
 value of the visitor function is `None` the node will be removed
 from the previous location otherwise it's replaced with the return
 value. The return value may be the original node in which case no
 replacement takes place.
 """

 def generic_visit(self, node, *args, **kwargs):
 for field, old_value in node.iter_fields():
 if isinstance(old_value, list):
 new_values = []
 for value in old_value:
 if isinstance(value, Node):
 value = self.visit(value, *args, **kwargs)
 if value is None:
 continue
 elif not isinstance(value, Node):
 new_values.extend(value)
 continue
 new_values.append(value)
 old_value[:] = new_values
 elif isinstance(old_value, Node):
 new_node = self.visit(old_value, *args, **kwargs)
 if new_node is None:
 delattr(node, field)
 else:
 setattr(node, field, new_node)
 return node

 def visit_list(self, node, *args, **kwargs):
 """As transformers may return lists in some places this method
 can be used to enforce a list as return value.
 """
 rv = self.visit(node, *args, **kwargs)
 if not isinstance(rv, list):
 rv = [rv]
 return rv