Re: [Py4j-users] How to access attributes ?

[Barthelemy D. <bar...@in...>]

Hi,

So I made a small mistake when replying, but the documentation is actually
correct.

"get_field" is a function declared in the py4j.java_gateway module and not
a method of the JavaGateway object.

So you need to call:
from py4j.java_gateway import get_field
get_field(obj, "field_name")

Also, this only works for publicly accessible fields.
Private/Package/Protected fields won't be accessible. For instance,
py4j.examples.Stack has no public fields.

When you do "stack.test3", py4j cannot resolve "test3" to a field, so
instead, it creates a JavaMember in an attempt to resolve it later (ex:
maybe it's a method, an internal class, etc.).

Hope this helps,
Barthelemy

On Thu Dec 04 2014 at 9:09:53 PM Wilfried Fauvel <wil...@gm...>
wrote:

> Hi,
>
> thank you for your fast answer !
>
> I added : c = gateway.get_field(stack, 'test3')
> but then it raises :
>
> Traceback (most recent call last):
>   File "test.py", line 22, in <module>
>     main()
>   File "test.py", line 18, in main
>     c = gateway.get_field(stack, 'test3')
>   File "/usr/lib/python2.7/site-packages/py4j/java_gateway.py", line
> 538, in __call__
>     self.target_id, self.name)
>   File "/usr/lib/python2.7/site-packages/py4j/protocol.py", line 304,
> in get_return_value
>     format(target_id, '.', name, value))
> py4j.protocol.Py4JError: An error occurred while calling t.get_field.
> Trace:
> py4j.Py4JException: Method get_field([class py4j.examples.Stack, class
> java.lang.String]) does not exist
> at py4j.reflection.ReflectionEngine.getMethod(ReflectionEngine.java:333)
> at py4j.reflection.ReflectionEngine.getMethod(ReflectionEngine.java:342)
> at py4j.Gateway.invoke(Gateway.java:252)
> 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)
>
> and auto_field=True returns <py4j.java_gateway.JavaMember object at
> 0x7f2d7ce7aa10>
>
> I am using : pip2.7 show py4j
> ---
> Name: py4j
> Version: 0.8.2.1
> Location: /usr/lib/python2.7/site-packages
> Requires:
>
> and
>
> java version "1.7.0_71"
> OpenJDK Runtime Environment (IcedTea 2.5.3) (Arch Linux build
> 7.u71_2.5.3-1-x86_64)
> OpenJDK 64-Bit Server VM (build 24.65-b04, mixed mode)
>
> Thank you.
>
> On Thu, Dec 4, 2014 at 5:23 AM, Barthelemy Dagenais
> <bar...@in...> wrote:
> > Hi!
> >
> > Thanks for using Py4J!
> >
> > To access fields, you can either use gateway.get_field(object, "test1")
> or
> > set auto_field=True when initializing the gateway to access object.test1.
> >
> > Reference: http://py4j.sourceforge.net/faq.html#how-to-access-a-field
> >
> > auto_field is False by default because it costs an extra lookup.
> >
> > Barthelemy
> >
> > On Wed Dec 03 2014 at 10:30:12 PM Wilfried Fauvel
> > <wil...@gm...> wrote:
> >>
> >> Hi, I am testing py4j and I would like to know if it is possible to
> >> access class attributes.
> >> In the Stack class from the getting started page, I added :
> >>
> >>     public static final String test1 = "TEST";
> >>     public static final int test2 = 12;
> >>     public int test3 = 12;
> >>
> >> and with python 2.7 :
> >>
> >> gateway = JavaGateway()
> >> stack = gateway.entry_point.getStack()
> >> print(gateway.help(stack))
> >> >>> field section is empty
> >>
> >> print stack.test1
> >> print stack.test2
> >> print stack.test3
> >> >>> <py4j.java_gateway.JavaMember object at 0x7fd7b1647250>
> >> >>> <py4j.java_gateway.JavaMember object at 0x7fd7b1647a10>
> >> >>> <py4j.java_gateway.JavaMember object at 0x7fd7b16479d0>
> >>
> >> Thank you,
> >> Wilfried.
> >>
> >>
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