code: def test_constraints_are_applied(self):
      v = backend.Variable(np.random.random((2, 2)) - 1.0)
      v.constraint = constraints.NonNeg()
      optimizer = optimizers.SGD(learning_rate=0.0001)
      grad = backend.numpy.zeros((2, 2))
      optimizer.apply_gradients([(grad, v)])
      self.assertAlmostEqual(np.min(v), 0.0)
  
file_path: keras/keras/optimizers/optimizer_test.py
imports: import numpy
  import pytest
  from keras import backend
  from keras import constraints
  from keras import layers
  from keras import models
  from keras import optimizers
  from keras import testing
label_desc: Use image node_id 4 for calling the OptimizerTest obj's underlying member method code with example usage: obj.test_constraints_are_applied() without return types