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