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1# -*- coding: utf-8 -*-
2"""
3    jinja2.visitor
4    ~~~~~~~~~~~~~~
5
6    This module implements a visitor for the nodes.
7
8    :copyright: (c) 2017 by the Jinja Team.
9    :license: BSD.
10"""
11from jinja2.nodes import Node
12
13
14class NodeVisitor(object):
15    """Walks the abstract syntax tree and call visitor functions for every
16    node found.  The visitor functions may return values which will be
17    forwarded by the `visit` method.
18
19    Per default the visitor functions for the nodes are ``'visit_'`` +
20    class name of the node.  So a `TryFinally` node visit function would
21    be `visit_TryFinally`.  This behavior can be changed by overriding
22    the `get_visitor` function.  If no visitor function exists for a node
23    (return value `None`) the `generic_visit` visitor is used instead.
24    """
25
26    def get_visitor(self, node):
27        """Return the visitor function for this node or `None` if no visitor
28        exists for this node.  In that case the generic visit function is
29        used instead.
30        """
31        method = 'visit_' + node.__class__.__name__
32        return getattr(self, method, None)
33
34    def visit(self, node, *args, **kwargs):
35        """Visit a node."""
36        f = self.get_visitor(node)
37        if f is not None:
38            return f(node, *args, **kwargs)
39        return self.generic_visit(node, *args, **kwargs)
40
41    def generic_visit(self, node, *args, **kwargs):
42        """Called if no explicit visitor function exists for a node."""
43        for node in node.iter_child_nodes():
44            self.visit(node, *args, **kwargs)
45
46
47class NodeTransformer(NodeVisitor):
48    """Walks the abstract syntax tree and allows modifications of nodes.
49
50    The `NodeTransformer` will walk the AST and use the return value of the
51    visitor functions to replace or remove the old node.  If the return
52    value of the visitor function is `None` the node will be removed
53    from the previous location otherwise it's replaced with the return
54    value.  The return value may be the original node in which case no
55    replacement takes place.
56    """
57
58    def generic_visit(self, node, *args, **kwargs):
59        for field, old_value in node.iter_fields():
60            if isinstance(old_value, list):
61                new_values = []
62                for value in old_value:
63                    if isinstance(value, Node):
64                        value = self.visit(value, *args, **kwargs)
65                        if value is None:
66                            continue
67                        elif not isinstance(value, Node):
68                            new_values.extend(value)
69                            continue
70                    new_values.append(value)
71                old_value[:] = new_values
72            elif isinstance(old_value, Node):
73                new_node = self.visit(old_value, *args, **kwargs)
74                if new_node is None:
75                    delattr(node, field)
76                else:
77                    setattr(node, field, new_node)
78        return node
79
80    def visit_list(self, node, *args, **kwargs):
81        """As transformers may return lists in some places this method
82        can be used to enforce a list as return value.
83        """
84        rv = self.visit(node, *args, **kwargs)
85        if not isinstance(rv, list):
86            rv = [rv]
87        return rv
88