Magento 2: Default Values for `system.xml` Configuration

yes, Magento 2 still allows you to define the default values inside a configuration file. namespace\modulename\etc\config.xml

<?xml version="1.0"?>
     <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Store:etc/config.xsd">
        <default>
            <sectionname>
                <groupname>
                    <fieldid>1</fieldid>
                </groupname>
            </sectionname>
        </default>
    </config>

System configuration system.xml

<?xml version="1.0"?>

    <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
        <system>
            <tab id="namespace_tab" translate="label" sortOrder="1">
                <label>COnfig Title</label>
            </tab>
            <section id="sectionname" translate="label" sortOrder="1" showInDefault="1" 
    showInWebsite="1" showInStore="1">
                <label>Some Title</label>
                <tab>namespace_tab</tab>
                <resource>Namespace_Modulename::system_config</resource>
                <group id="groupname" translate="label" type="text" sortOrder="1" showInDefault="1" 
    showInWebsite="1" showInStore="1">
                    <label>Some Configuration</label>
                    <field id="fieldid" translate="label" type="select" sortOrder="1" 
    showInDefault="1" showInWebsite="1" showInStore="1">
                        <label>Enable in frontend</label>
                        <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
                    </field>
                 </group>   
            </section>
        </system>
    </config>

Magento2 allow you to set default value same like Magento1. When you look at the contact module from Magento2,

system.xml for contact module

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
    <system>
        <section id="contact" translate="label" type="text" sortOrder="100" showInDefault="1" showInWebsite="1" showInStore="1">
            <label>Contacts</label>
            <tab>general</tab>
            <resource>Magento_Contact::contact</resource>
            <group id="contact" translate="label" type="text" sortOrder="10" showInDefault="1" showInWebsite="1" showInStore="1">
                <label>Contact Us</label>
                <field id="enabled" translate="label" type="select" sortOrder="10" showInDefault="1" showInWebsite="1" showInStore="1">
                    <label>Enable Contact Us</label>
                    <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
                    <backend_model>Magento\Contact\Model\System\Config\Backend\Links</backend_model>
                </field>
            </group>
            <group id="email" translate="label" type="text" sortOrder="50" showInDefault="1" showInWebsite="1" showInStore="1">
                <label>Email Options</label>
                <field id="recipient_email" translate="label" type="text" sortOrder="10" showInDefault="1" showInWebsite="1" showInStore="1">
                    <label>Send Emails To</label>
                    <validate>validate-email</validate>
                </field>
                <field id="sender_email_identity" translate="label" type="select" sortOrder="20" showInDefault="1" showInWebsite="1" showInStore="1">
                    <label>Email Sender</label>
                    <source_model>Magento\Config\Model\Config\Source\Email\Identity</source_model>
                </field>
                <field id="email_template" translate="label comment" type="select" sortOrder="30" showInDefault="1" showInWebsite="1" showInStore="1">
                    <label>Email Template</label>
                    <comment>Email template chosen based on theme fallback when "Default" option is selected.</comment>
                    <source_model>Magento\Config\Model\Config\Source\Email\Template</source_model>
                </field>
            </group>
        </section>
    </system>
</config>

and config.xml for the contact module

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Store:etc/config.xsd">
    <default>
            <contact>
                <enabled>1</enabled>
            </contact>
            <email>
                <recipient_email>
                    <![CDATA[[email protected]]]>
                </recipient_email>
                <sender_email_identity>custom2</sender_email_identity>
                <email_template>contact_email_email_template</email_template>
            </email>
    </default>
</config>

---

If we want to set default value, then we have to match its id like

<section id="contact"> <group id="contact"> <field id="enabled">

then it becomes

 <default>
     <contact>
           <contact>
                  <enabled>1</enabled>
           </contact>
     </contact>
 </default>