I added this code for adding digits in price unit 

<xpath expr="//field[@name='invoice_line_ids']//field[@name='price_unit']" position="replace">
    <field name="price_unit" digits="[12,12]" column_invisible="parent.cover_sheet_count == 0"/>
    <field name="price_unit" digits="[12,2]" column_invisible="parent.cover_sheet_count != 0"/>
</xpath>

it is working perfectly in using replace , when coversheet count != 0 the price unit with 12 decimal precision . 

when coversheet count == 0 price unit with 2 decimal precision 

When i use attribute like this 

<xpath expr="//field[@name='invoice_line_ids']//field[@name='price_unit']" position="attributes">
    <attribute name="digits">[12,12]</attribute>
    <attribute name="column_invisible">parent.cover_sheet_count == 0</attribute>
</xpath>

<xpath expr="//field[@name='invoice_line_ids']//field[@name='price_unit']" position="attributes">
    <attribute name="digits">[12,2]</attribute>
    <attribute name="column_invisible">parent.cover_sheet_count != 0</attribute>
</xpath>

when coversheet count == 0 price unit with 2 decimal precision  , only one scenario is working , 
coversheet count !=0 case the price unit field is invisible not decimal precision with 12 digits 

What is the reason for this behaviour ??