코드 실행 방법에 따라 Python의 정수 캐시 동작이 변경되는 이유는 무엇입니까?

>>> a = 1
>>> b = 1
>>> a is b
True
>>> a = 257
>>> b = 257
>>> a is b
False

>>> a = 257; b = 257; a is b
True

PyRun_FileExFlags()
mod = PyParser_ASTFromFile()
    node *n = PyParser_ParseFileFlagsEx() //source to cst
        parsetoke()
            ps = PyParser_New()
            for (;;)
                PyTokenizer_Get()
                PyParser_AddToken(ps, ...)
mod = PyAST_FromNode(n, ...)  //cst to ast
run_mod(mod, ...)
    co = PyAST_Compile(mod, ...) //ast to CFG
        PyFuture_FromAST()
        PySymtable_Build()
        co = compiler_mod()
    PyEval_EvalCode(co, ...)
        PyEval_EvalCodeEx()

a = 257
b = 257
print "id(a) = %d, id(b) = %d" % (id(a), id(b))

DEBUG: before PyAST_FromNode
name = a
ival = 257, id = 176046536
name = b
ival = 257, id = 176046752
name = a
name = b
DEBUG: after PyAST_FromNode
run_mod
PyAST_Compile ok
id(a) = 176046536, id(b) = 176046536
Eval ok