Note that there's an important difference between the value attribute and the value property.
The simplified explanation is that the value attribute is what's found in the HTML tag and the value property is what you see on the page.
Basically, the value attribute sets the element's initial value, while the value property contains the current value.
You can read more about that here and see an example of the difference here.
If you want the value
attribute, then you should use get_attribute:
input.get_attribute('value')
If you want the value
property, then you should use get_property
input.get_property("value")
Though, according to the docs, get_attribute actually returns the property rather than the attribute, unless the property doesn't exist. get_property will always return the property.