In Odoo templates, the t-set attribute is used to set a variable to a given value. However, t-set doesn't support modifying a list in-place, such as appending elements to it.

To accomplish this, you can use the t-operation attribute instead, which allows you to perform operations on the value of a variable. In your case, you can use the t-operation attribute to append elements to the foo list as follows:

t-set="foo" t-value="[]"/>

t-foreach="[1,2,3]" t-as="i"> 
  t-operation="foo.append(10)"/>
  
t-esc="i"/>

Alternatively, you can use the t-call attribute to call a Python function that modifies the list in-place. Here is an example:

!-- Define a Python function that appends 10 to the given list -->
t-call="website.append_to_list" t-args="foo"/>


t-foreach="[1,2,3]" t-as="i">
  t-call="website.append_to_list" t-args="foo"/>
  
t-esc="i"/>



foo = t-esc="foo"/>

In the Python code of your website, you would need to define the append_to_list function as follows:

def append_to_list(foo):
  foo.append(10)

Hope this helps!

Hi,

Please try like as follows:-

<t t-set="foo" t-value="[]"/>

<t t-foreach="[1,2,3]" t-as="i">

    <t t-set="foo" t-value="foo + [10]"/>

</t>

<p>

    <t t-esc="foo"/>

</p>

Regards