[Discuss-gnuradio] About gr.buffer

[Zhonghua]

Hi every one,
    Now I am trying to use gr.buffer. I wrote some very simple codes to 
test how to use gr.buffer correctly. The main codes are as below:
     self.u=gr.sig_source_c(1e4,gr.GR_SIN_WAVE,1e3,1)           #to 
create a sin source
     self.buf=gr.buffer(1000,gr.sizeof_gr_complex,self.u)         
#create a buffer by applying gr.buffer
     self.csink=gr.vector_sink_c()
     
self.connect(self.buf,self.csink)                                           
#send buffer volume to the sink

But unfortunately, I got a despondent error reply says: in method 
'buffer', argument 3 of type 'gr_block_sptr'. I looked up the gr_buffer 
class and found that the so called argument 3 is a link to a block which 
writes data to this buffer. As my understanding, the sig_source block 
should write data to the buffer so I write self.buf as above way. From 
the error hint, I ever thought perhaps the error just comes from the 
incorrect type. Because the sin_source object is pointed by the pointer 
type of "gr_sin_source_c_sptr". But how can I get an object that merely 
inherited from gr.block class? I tried to construct a class inherits 
from gr.block but there exists another error says that "'module' object 
has no attribute 'block' ".
I checked the internet but found nothing helpful. A surprising fact is 
that looks no one ever used gr.buffer in gnuradio by adopting python 
way. Is there anybody know how to use it correctly?
Your instruction is fairly appreciated.

Br,
Zhonghua