python中浮点数使用int()的问题

Question

int()描述如下：

int(x=0) -> int or long
int(x, base=10) -> int or long

Convert a number or string to an integer, or return 0 if no arguments
are given. If x is floating point, the conversion truncates towards
zero. If x is outside the integer range, the function returns a long
instead.

If x is not a number or if base is given, then x must be a string or
Unicode object representing an integer literal in the given base. The
literal can be preceded by '+' or '-' and be surrounded by whitespace.
The base defaults to 10. Valid bases are 0 and 2-36. Base 0 means to
interpret the base from the string as an integer literal.

上述提出如果是浮点数，就截去小数。为什么我截图的这个例子结果跟我想象的不一样呢？
谢谢大家帮我解答！

Answer 1

Cpython中的Float底层是靠double (64 bit)来实现的.

根据IEEE 754的标准，double就52位的mantissa，所以你那N多9超过了能无损代表的整数范围了。

下面是临街点，正整数里比2 ** 53小的都没问题

>>> 2 ** 53
9007199254740992
>>> int(9007199254740992.1)
9007199254740992
>>> int(9007199254740993.1)
9007199254740994