[Py4j-users] 回覆: 回覆: Exception when printing a returned List<String> object

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Hi,

(A) exception from accessing list

The versions are here: 
py4j 0.8.2.1
python 2.7.8
Java 1.8.0_20

I open two cygwin consoles. One for running the Gateway Server, and the other is to run python script.

When I run the python script to print an ArrayList as your advice, the same exception raised in the server side. Part of the trace stack is as below:
...
Caused by: java.util.NoSuchElementException
        at java.util.ArrayList$Itr.next(ArrayList.java:854)

It's OK to access the list individually by indices, but it causes exception when directly typing the "a_list" or "print(a_ list)". 

When the list is accessed as a whole, what would the gateway server do the list? It seems the server tries to iterate to elements that don't exist.

(B) exception from setting logging in python script

I follow your faq page of how to turn logging on-off and get the following exception:

>>> logging = gw.jvm.java.util.logging
>>> logger = logging.Logger.getLogger("py4j")
>>> logger.setLevel(logging.DEBUG)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/site-packages/py4j/java_gateway.py", line 529, in __call__
    [get_command_part(arg, self.pool) for arg in new_args])
  File "/usr/lib/python2.7/site-packages/py4j/protocol.py", line 242, in get_command_part
    if parameter == None:
  File "/usr/lib/python2.7/site-packages/py4j/java_gateway.py", line 726, in __getattr__
    raise Py4JError('Trying to call a package.')
py4j.protocol.Py4JError: Trying to call a package.

Maybe faq page needs to add more information on this situation.

Frank Yang

________________________________________
從: Barthelemy Dagenais <bar...@in...>
寄件日期: 2014年12月28日 04:42
至: Support and Comments about Py4J
主旨: Re: [Py4j-users]    回覆: Exception when printing a returned List<String> object

Hi,

apologies for the delays :-)

The behavior you are seeing is strange.

Here are a few pointers:

a) Error Logging can be turned on and off on both sides:
http://py4j.sourceforge.net/faq.html#how-to-turn-logging-on-off

b) Which version of py4j, python, and Java are you using?

c) Are you able to print a standard list, e.g., ArrayList? e.g.,

a_list = gateway.jvm.java.util.ArrayList()
a_list.append("a")
a_list.append("b")
print(a_list)

d) The exceptions you shown seems to come from the JVM. Do you see any
exception on the Python side?

Regards,
Barthelemy

On Tue, Dec 23, 2014 at 9:56 PM,  <hg...@it...> wrote:
> Hi,
>
> Yes, it prints <class 'py4j.java_collections.JavaIterator'>.
> So couldn't I directly assign the return List<String> to a python list like the following and then print it?
>
>>>>ret = []
>>>>ret = gateway.entry_point.getMyStringList(inputStr)
>>>>print ret
>
> If that's the case, for now my workaround for this is below:
>
>>>>ans = [ret[i] for i in range(len(ret))]
>
> and then use "ans" instead.
>
> Could you have any suggestion on this otherwise?
>
> Besides, because the exception message only appears partially, how can I log full error messages and debug when the server stops? I think it's especially necessary when the error behavior is in the server side.
>
> Thanks,
> Frank Yang
> ________________________________________
> 從: Barthelemy Dagenais <bar...@in...>
> 寄件日期: 2014年12月23日 21:14
> 至: Support and Comments about Py4J
> 主旨: Re: [Py4j-users] Exception when printing a returned List<String>    object
>
> Hi,
>
> When printing the list, Py4J retrieves an Iterator and wraps it with
> py4j.java_collections.JavaIterator. It then cycles through the list
> with the iterator. JavaIterator should catch the
> NoSuchElementException and raise instead a StopIteration. Because it
> does not, I suspect something is messing with the automated wrapping
> of collections.
>
> could you do:
>
> iter = ret.iterator()
> print(type(iter))
>
> This should print something like <class 'py4j.java_collections.JavaIterator'>
>
> Thanks,
> Barthelemy
>
> On Tue, Dec 23, 2014 at 6:10 AM,  <hg...@it...> wrote:
>> Hi,
>>
>>
>> The method in my gateway server will return a List<String> object. When I
>> access it in my python client code, the server has exception and then
>> closed. The following is part of the exceptions
>>
>>
>>
>>
>> ▒Q▒G▒▒ 23, 2014 5:18:14 ▒U▒▒ py4j.reflection.MethodInvoker invoke
>> ĵ▒i: Exception occurred in client code.
>> java.lang.reflect.InvocationTargetException
>>         at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
>>         at
>> sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
>>         at
>> sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
>>         at java.lang.reflect.Method.invoke(Method.java:483)
>>         at py4j.reflection.MethodInvoker.invoke(MethodInvoker.java:231)
>>         at
>> py4j.reflection.ReflectionEngine.invoke(ReflectionEngine.java:379)
>>         at py4j.Gateway.invoke(Gateway.java:259)
>>         at
>> py4j.commands.AbstractCommand.invokeMethod(AbstractCommand.java:133)
>>         at py4j.commands.CallCommand.execute(CallCommand.java:79)
>>         at py4j.GatewayConnection.run(GatewayConnection.java:207)
>>         at java.lang.Thread.run(Thread.java:745)
>> Caused by: java.util.NoSuchElementException
>>         at java.util.AbstractList$Itr.next(AbstractList.java:364)
>>         ... 11 more
>>
>> What I do in client code is the following:
>>
>>>>>ret = gateway.entry_point.getMyStringList(inputStr)    # it's ok to run
>>>>> this line
>>>>>print ret       # when running to this line, the server raised exception
>>
>>
>> Below is the method in server side:
>>
>> public List<String> getMyStringList(String input) throws IOException {
>>     List<String> strList = new ArrayList<String>();
>>     strList = segmenter.segmentString(input);    //  this is some 3rd party
>> API for string tokenization and has been tested OK in console, the return
>> object is List<String>
>>
>>     return strList;
>> }
>>
>> It's OK to access "ret" using indices and len(), but when I just type "ret"
>> or "print ret", then it would display the content of "ret", but then the
>> server poped Exception and stopped. What would be the problemt? Thanks for
>> any help!
>>
>>
>>
>> Frank Yang
>>
>>
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>
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> 本信件可能包含工研院機密資訊，非指定之收件者，請勿使用或揭露本信件內容，並請銷毀此信件。
> This email may contain confidential information. Please do not use or disclose it in any way and delete it if you are not the intended recipient.
> ------------------------------------------------------------------------------
> Dive into the World of Parallel Programming! The Go Parallel Website,
> sponsored by Intel and developed in partnership with Slashdot Media, is your
> hub for all things parallel software development, from weekly thought
> leadership blogs to news, videos, case studies, tutorials and more. Take a
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Dive into the World of Parallel Programming! The Go Parallel Website,
sponsored by Intel and developed in partnership with Slashdot Media, is your
hub for all things parallel software development, from weekly thought
leadership blogs to news, videos, case studies, tutorials and more. Take a
look and join the conversation now. http://goparallel.sourceforge.net
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This email may contain confidential information. Please do not use or disclose it in any way and delete it if you are not the intended recipient.