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https://issues.apache.org/jira/browse/THRIFT-2642?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15856364#comment-15856364
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Lars Schöning commented on THRIFT-2642:
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I also just came across this issue. Fbthrift has a straightforward enough fix
for this. I don't fully understand the IP situation with fbthrift, so maybe
using their approach is out of the question.
In either case, there are two parts to this problem:
(1) to solve recursive and cyclic dependencies between structs themselves, (2)
solve cycles of {{<Struct>.thrift_spec}}.
The first part is adequately solved by setting {{<Struct>.thrift_spec}} at the
bottom of the file.
{code}
class A(object):
thrift_spec = None
class B(object):
thrift_spec = None
A.thrift_spec = # (...)
B.thrift_spec = # (...)
{code}
However, since the thrift_spec tuple has the format {{(<Struct>,
<Struct>.thrift_spec)}}, this won't work. There are three approaches now. The
fbthrift approach is to change {{(<Struct>, <Struct>.thrift_spec)}} to a list
{{\[<Struct>, None\]}} and fill in the missing bit with a function call.
https://github.com/facebook/fbthrift/blob/3ef868c969464e935b39e336807af1486b2739c3/thrift/lib/py/util/Recursive.py
A simpler alternative is to make {{thrift_spec}} itself a list and update the
code like so:
{code}
class Cyclic(object):
thrift_spec = []
Cyclic.thrift_spec += # [...]
{code}
I don't know what the performance implications are. Iterating over a tuple
should be slightly faster than iterating over a list, however the fbthrift
solution also requires accessing lists. Both solutions make {{thrift_spec}}
mutable.
The third alternative is to change {{(<Struct>, <Struct>.thrift_spec)}} to
just {{<Struct>}}. As far as I can see {{thrift_spec}} is only used by
{{TProtocol.readStruct()}}. So the signature of {{TProtocol.readStruct(self,
obj, thrift_spec, is_immutable=False)}} should be changed to
{{TProtocol.readStruct(self, obj, is_immutable=False)}} and the access to
{{thrift_spec}} should be changed to {{obj.thrift_spec}}. As far as I can see,
{{readStruct()}} is only called from {{TBase.read()}} and
{{TFrozenBase.read()}}, so this would be a simple change. The method is also
undocumented. But I don't know how you feel about signature changes.
> Recursive structs don't work in python
> --------------------------------------
>
> Key: THRIFT-2642
> URL: https://issues.apache.org/jira/browse/THRIFT-2642
> Project: Thrift
> Issue Type: Bug
> Components: Python - Compiler, Python - Library
> Affects Versions: 0.9.2
> Reporter: Igor Kostenko
>
> Recursive structs in 0.9.2 work fine in c++ & c#, but not in python, because
> generated code trying to use objects which not constructed yet.
> Struct:
> {code}
> struct Recursive {
> 1: list<Recursive> Children
> }
> {code}
> Python code:
> {code}
> class Recursive:
> thrift_spec = (
> None, # 0
> (1, TType.LIST, 'Children', (TType.STRUCT,(Recursive,
> Recursive.thrift_spec)), None, ), # 1
> )
> {code}
> Error message:
> {code}
> Traceback (most recent call last):
> File "ttypes.py", line 20, in <module>
> class Recursive:
> File "ttypes.py", line 28, in Recursive
> (1, TType.LIST, 'Children', (TType.STRUCT,(Recursive,
> Recursive.thrift_spec)), None, ), # 1
> NameError: name 'Recursive' is not defined
> {code}
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