Why is が used instead of を in this sentence?

Question

From Shin Kanzen Master JLPT N1 Grammar:

あ、本が出しっぱなしだ。

I thought that 出す always follows を, i.e. you (subject) take out something (object).

Is it because 本 is emphasized in this sentence, similar to how が is used instead of を with the potential form of a verb?

Or is it because 出しっぱなしだ as a whole acts more like an adjective that describes the book?

Answer 1

You could say 出しっぱなし works like an adjective in that sentence, or you could also see 出しっぱなしだ as a shorter way of saying 出しっぱなしになっている. Either way, it describes the state of the book and therefore the book deserves the subject’s spot.

本を出しっぱなしだ is also possible. It means the same as 本を出しっぱなしにしている. This focuses on someone’s earlier act on the book rather than the current state of the book. The book is the object of the act in this case.

Answer 2

ネットの色々な辞書で検索して、「出しっぱなし」は「だす」+ 「しっぱなし」じゃなくて一つの単語として出てくるそうです。 品詞は名詞ですから、「A が　B　だ」という文型に合うと思います。

• 出しっぱなし - Weblio
• 出しっぱなし - Jisho.org