Undefined index: shipping in classes/order.php

Php 7.1.8
this is causing me an undefiend index notice (but not in normal use).

'shipping_module_code' => (isset($_session['shipping']['id']) && strpos($session['shipping']['id'], '') > 0 ? $_session['shipping']['id'] : $_session['shipping']),

If the clause is false why would shipping_module_code be set to $_session['shipping']?

in addition, i have never understood the use of the underscore ( _ ) in the code for the shipping module, and the searching for it as demonstrated here.

it seems that there is some sort of intelligence built into the code that makes it that more difficult to read/debug.

carlwhat:

in addition, i have never understood the use of the underscore ( _ ) in the code for the shipping module, and the searching for it as demonstrated here.

it seems that there is some sort of intelligence built into the code that makes it that more difficult to read/debug.
Shipping modules can have multiple shipping-methods; the selected method is appended (separated by an underscore) to the end of the shipping_module selection.

See also this bit of documentation on shipping-module innards.