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Add a read-only visitor pattern. #96

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Merged
merged 1 commit into from
Feb 7, 2019
Merged

Add a read-only visitor pattern. #96

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30 changes: 30 additions & 0 deletions fluent.syntax/fluent/syntax/ast.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,36 @@
import json


class Visitor(object):
'''Read-only visitor pattern.

Subclass this to gather information from an AST.
To generally define which nodes not to descend in to, overload
`generic_visit`.
To handle specific node types, add methods like `visit_Pattern`.
The boolean value of the returned value determines if the visitor
descends into the children of the given AST node.
'''
def visit(self, value):
if isinstance(value, BaseNode):
self.visit_node(value)
if isinstance(value, list):
for node in value:
self.visit(node)

def visit_node(self, node):
nodename = type(node).__name__
visit = getattr(self, 'visit_{}'.format(nodename), self.generic_visit)
should_descend = visit(node)
if not should_descend:
return
for propname, propvalue in vars(node).items():
self.visit(propvalue)
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@stasm stasm Feb 7, 2019 •
edited
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I'd like to suggest a slightly more verbose way of writing this method. It's a nit, so feel free to ignore. As you know, I've been trying to promote the idea of self-documenting code in my reviews :) Here, the value I think this brings is that it gives an explicit meaning to the return value of the process_node method.

def visit_node(self, node):
    nodename = type(node).__name__
    process_node = getattr(self, 'visit_{}'.format(nodename), self.generic_visit)
    should_continue = process_node(node)
    if should_continue:
        for propname, propvalue in vars(node).items():
            self.visit(propvalue)


def generic_visit(self, node):
return True


def to_json(value, fn=None):
if isinstance(value, BaseNode):
return value.to_json(fn)
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