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class InstallData implements InstallDataInterface
{
  public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
  {
    /** @var CustomerSetup $customerSetup */
    $customerSetup = $this->customerSetupFactory->create(['setup' => $setup]);
  
    $customerEntity = $customerSetup->getEavConfig()->getEntityType('customer');
    $attributeSetId = $customerEntity->getDefaultAttributeSetId();
  
    /** @var $attributeSet AttributeSet */
    $attributeSet = $this->attributeSetFactory->create();
    $attributeGroupId = $attributeSet->getDefaultGroupId($attributeSetId);
    
    $customerSetup->addAttribute(
    \Magento\Customer\Model\Customer::ENTITY,
    'attribute_name',
    [
        'type' => 'int',
        'label' => 'Attribute Name',
        'input' => 'select',
        'source' => 'Magento\Eav\Model\Entity\Attribute\Source\Boolean',
        'required' => false,
        'default' => '0',
        'sort_order' => 100,
        'system' => false,
        'position' => 100
    ]
    );
    
    $attribute = $customerSetup->getEavConfig()->getAttribute(Customer::ENTITY, 'attribute_name')
      ->addData([
          'attribute_set_id' => $attributeSetId,
          'attribute_group_id' => $attributeGroupId,
          'used_in_forms' => ['adminhtml_customer','customer_account_edit'],
      ]);
    
    $attribute->save();
  }
}