I am trying to make a yaml sequence in python that creates a custom python object. The object needs to be constructed with dicts and lists that are deconstructed after __init__
. However, it seems that the construct_mapping function does not construct the entire tree of embedded sequences (lists) and dicts.
Consider the following:
import yaml
class Foo(object):
def __init__(self, s, l=None, d=None):
self.s = s
self.l = l
self.d = d
def foo_constructor(loader, node):
values = loader.construct_mapping(node)
s = values["s"]
d = values["d"]
l = values["l"]
return Foo(s, d, l)
yaml.add_constructor(u'!Foo', foo_constructor)
f = yaml.load('''
--- !Foo
s: 1
l: [1, 2]
d: {try: this}''')
print(f)
# prints: 'Foo(1, {'try': 'this'}, [1, 2])'
This works fine because f
holds the references to the l
and d
objects, which are actually filled with data after the Foo
object is created.
Now, let's do something a smidgen more complicated:
class Foo(object):
def __init__(self, s, l=None, d=None):
self.s = s
# assume two-value list for l
self.l1, self.l2 = l
self.d = d
Now we get the following error
Traceback (most recent call last):
File "test.py", line 27, in <module>
d: {try: this}''')
File "/opt/homebrew/lib/python2.7/site-packages/yaml/__init__.py", line 71, in load
return loader.get_single_data()
File "/opt/homebrew/lib/python2.7/site-packages/yaml/constructor.py", line 39, in get_single_data
return self.construct_document(node)
File "/opt/homebrew/lib/python2.7/site-packages/yaml/constructor.py", line 43, in construct_document
data = self.construct_object(node)
File "/opt/homebrew/lib/python2.7/site-packages/yaml/constructor.py", line 88, in construct_object
data = constructor(self, node)
File "test.py", line 19, in foo_constructor
return Foo(s, d, l)
File "test.py", line 7, in __init__
self.l1, self.l2 = l
ValueError: need more than 0 values to unpack
This is because the yaml constructor is starting at the outer layer of nesting before and constructing the object before all nodes are finished. Is there a way to reverse the order and start with deeply embedded (e.g. nested) objects first? Alternatively, is there a way to get construction to happen at least after the node's objects have been loaded?
See Question&Answers more detail:
os