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Hello, @Nasser! As @nbruin pointed out in his comment, this behavior is related to the mpfr_integer_p function from the MPFR library. (is_integer() is a wrapper for that subroutine.)

Basically, Sage uses that library in order to have arbitrary precision real numbers (actually, the precision is limited by the amount of memory). In particular, if you call

type(1.0)

you will see the answer

<class 'sage.rings.real_mpfr.RealLiteral'>

Can you see the "mpfr" in there? That means that the RealLiteral type is internally represented using the MPFR library with a little of Cython magic for the C-Python interface.

Anyway, there are good reasons to return True when you call (1.0).is_integer(). Mathematically speaking, $1.0$ and $1$ are the same number, so both representations are integer. (Remember that Sage is a mathematical tool/programming language.) In essence, you are asking if $1.0\in\mathbb{Z}$ is true ---which is---, i.e., you are asking about the set of integer numbers.

On the other hand, concerning your Sympy example, notice that you are asking something different. The isinstance function is a question from the point of view of software programming, i.e., you are asking what kind of data type is your literal 1.0 or 1. Evidently, 1.0 is an instance of the real data type, while 1 is an instance of the integer data type.

In summary, is_integer is a mathematical question, and isinstance is a software programming question.

I hope this helps!