contract/contract_variable_quantity/views/contract_view.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>

    <record id="account_analytic_account_recurring_form_form" model="ir.ui.view">
        <field name="model">account.analytic.account</field>
        <field name="inherit_id" ref="contract.account_analytic_account_recurring_form_form"/>
        <field name="arch" type="xml">
            <xpath expr="//field[@name='recurring_invoice_line_ids']//field[@name='quantity']" position="before">
                <field name="qty_type"/>
            </xpath>
            <xpath expr="//field[@name='recurring_invoice_line_ids']//field[@name='quantity']" position="after">
                <field name="qty_formula_id"
                       attrs="{'required': [('qty_type', '=', 'variable')], 'invisible': [('qty_type', '!=', 'variable')]}"
                />
            </xpath>
            <xpath expr="//field[@name='recurring_invoice_line_ids']//field[@name='quantity']" position="attributes">
                <attribute name="attrs">{'required': [('qty_type', '=', 'fixed')], 'invisible': [('qty_type', '!=', 'fixed')]}</attribute>
            </xpath>
        </field>
    </record>

    <record id="view_contract_line_qty_formula_tree" model="ir.ui.view">
        <field name="model">contract.line.qty.formula</field>
        <field name="arch" type="xml">
            <tree>
                <field name="name"/>
            </tree>
        </field>
    </record>

    <record id="view_contract_line_qty_formula_form" model="ir.ui.view">
        <field name="model">contract.line.qty.formula</field>
        <field name="arch" type="xml">
            <form >
                <sheet>
                    <div class="oe_title">
                        <h1>
                            <field name="name" placeholder="Name"/>
                        </h1>
                    </div>
                    <group string="Code">
                        <div style="margin-top: 4px;">
                            <field name="code" nolabel="1"/>
                            <h3>Help with Python expressions.</h3>
                            <p>You have to insert valid Python code block that stores at some moment a float/integer value of the quantity to invoice in the variable 'result'.</p>
                            <p>You can use these variables to compute your formula:</p>
                            <ul>
                                <li><i>env</i>: Environment variable for getting other models.</li>
                                <li><i>context</i>: Current context dictionary.</li>
                                <li><i>user</i>: Current user.</li>
                                <li><i>line</i>: Contract recurring invoice line that triggers this formula.</li>
                                <li><i>contract</i>: Contract whose line belongs to.</li>
                                <li><i>invoice</i>: Invoice (header) being created.</li>
                            </ul>
                            <div>
                                <p>Example of Python code</p>
                                <code>
                                    result = env['product.product'].search_count([('sale_ok', '=', True)])
                                </code>
                            </div>
                        </div>
                    </group>
                </sheet>
            </form>
        </field>
    </record>

    <record id="action_contract_quantity_formula" model="ir.actions.act_window">
        <field name="name">Formulas (quantity)</field>
        <field name="res_model">contract.line.qty.formula</field>
        <field name="view_type">form</field>
        <field name="view_mode">tree,form</field>
        <field name="help" type="html">
            <p class="oe_view_nocontent_create">
                Click to create a new formula for variable quantities.
            </p>
        </field>
    </record>

    <menuitem id="menu_contract_quantity_formula"
              action="action_contract_quantity_formula"
              parent="contract.menu_config_contract"
    />

</odoo>