reproduction Flowgraph for https://github.com/gnuradio/gnuradio/issues/6723

repro6723.grc

options:
  parameters:
    author: marcus
    catch_exceptions: 'True'
    category: '[GRC Hier Blocks]'
    cmake_opt: ''
    comment: ''
    copyright: ''
    description: ''
    gen_cmake: 'On'
    gen_linking: dynamic
    generate_options: qt_gui
    hier_block_src_path: '.:'
    id: repro6723
    max_nouts: '0'
    output_language: python
    placement: (0,0)
    qt_qss_theme: ''
    realtime_scheduling: ''
    run: 'True'
    run_command: '{python} -u {filename}'
    run_options: prompt
    sizing_mode: fixed
    thread_safe_setters: ''
    title: Return -1
    window_size: (1000,1000)
  states:
    bus_sink: false
    bus_source: false
    bus_structure: null
    coordinate: [8, 8]
    rotation: 0
    state: enabled

blocks:
- name: samp_rate
  id: variable
  parameters:
    comment: ''
    value: '32000'
  states:
    bus_sink: false
    bus_source: false
    bus_structure: null
    coordinate: [184, 12]
    rotation: 0
    state: enabled
- name: blocks_null_source_0
  id: blocks_null_source
  parameters:
    affinity: ''
    alias: ''
    bus_structure_source: '[[0,],]'
    comment: ''
    maxoutbuf: '0'
    minoutbuf: '0'
    num_outputs: '1'
    type: complex
    vlen: '1'
  states:
    bus_sink: false
    bus_source: false
    bus_structure: null
    coordinate: [312, 112.0]
    rotation: 0
    state: enabled
- name: epy_block_0
  id: epy_block
  parameters:
    _source_code: "import numpy as np\nfrom gnuradio import gr\n\nclass blk(gr.sync_block):\
      \  # other base classes are basic_block, decim_block, interp_block\n    def\
      \ __init__(self):  # only default arguments here\n        gr.sync_block.__init__(\n\
      \            self,\n            name='instant return -1 block',   # will show\
      \ up in GRC\n            in_sig=[np.complex64],\n            out_sig=[]\n  \
      \      )\n\n    def work(self, input_items, output_items):\n        return -1\n"
    affinity: ''
    alias: ''
    comment: ''
    maxoutbuf: '0'
    minoutbuf: '0'
  states:
    _io_cache: ('instant return -1 block', 'blk', [], [('0', 'complex', 1)], [], '',
      [])
    bus_sink: false
    bus_source: false
    bus_structure: null
    coordinate: [512, 112.0]
    rotation: 0
    state: enabled

connections:
- [blocks_null_source_0, '0', epy_block_0, '0']

metadata:
  file_format: 1
  grc_version: v3.11.0.0git-655-g7d61746e

repro6723.svg