code: def test_mean_agg_constructor_1():
      agg = MeanAggregator(output_dim=4, bias=True, act=lambda x: x + 1)
      assert agg.output_dim == 4
      assert agg.has_bias
      assert agg.act(2) == 3
  
imports: from tensorflow import keras
  from tensorflow.keras import initializers, regularizers
  import tensorflow
  import numpy
  import pytest
  from stellargraph.mapper import GraphSAGENodeGenerator
  from stellargraph.layer.graphsage import GraphSAGE, MeanAggregator, MaxPoolingAggregator, MeanPoolingAggregator, AttentionalAggregator
  from ..test_utils.graphs import example_graph
  from ..None import test_utils
label_desc: Use image node_id 2 for calling a global function with example usage: test_mean_agg_constructor_1() without return types