我想你可以查看NA type promotions：

When introducing NAs into an existing Series or DataFrame via reindex or some other means,boolean and integer types will be promoted to a different dtype in order to store the NAs. These are summarized by this table:

Typeclass   Promotion dtype for storing NAs
floating    no change
object      no change
integer     cast to float64
boolean     cast to object

While this may seem like a heavy trade-off,in practice I have found very few cases where this is an issue in practice. Some explanation for the motivation here in the next section.