[jira] [Commented] (THRIFT-2642) Recursive structs don't work in python

["Lars Schöning (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/THRIFT-2642?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15856364#comment-15856364 ] 

Lars Schöning commented on THRIFT-2642:
---------------------------------------

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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