先来看几个使用os,path.isabs()的例子：

很显然，第一个路径是没有问题，而后面都是我随便输入的一串字符，为什么有的返回True有的返回False呢？

我查看了Python3.5的文档，发现了这样一句话：

os.path.isabs(path)

Return True if path is an absolute pathname. On Unix, that means it begins with a slash, on Windows that it begins with a (back)slash after chopping off a potential drive letter.

也就是说在WIndow系统下，如果输入的字符串以” / “开头，os.path.isabs()就会返回True，那么第四个例子就可以理解了。

而第二个和第三个有什么区别呢？不是都应该返回False吗？

查阅资料后我找到了下面这段话：

The current os.path.isabs documentation says:
> isabs(path) 
>    Return True if path is an absolute pathname (begins with a slash). 
The “begins with a slash” part is incorrect since certain systems use a
different pathname notation.
For example, on Macintosh (where os.sep == “:”) this is an absolute
pathname:
hardDriveName:folderName1:folderName2:fileName.ext
…and this is a relative one:
:folderName1:fileName.ext
Moreover, on Windows os.path.isabs(‘\\’) returns True since ‘\\’ is an
alias for the current drive letter (e.g. C:\\) hence, independently from
what said before, the documentation should include also the “backslash”
term.
It turns out that on Windows there are really 4 different kinds of paths:
1) Completely relative, e.g. foo\bar
2) Completely absolute, e.g. c:\foo\bar or \\server\share
3) Halfbreeds with no drive, e.g. \foo\bar
4) Halfbreeds relative to the current working directory on a specific drive, e.g. c:foo\bar
Python 2.5’s os.path.isabs() method considers both (2) and (3) to be absolute;

虽然是python2的，但是这和python3应该没什么区别。从上面这段话可以知道，在Windows系统上，以“  //  “开头的字符串，os.path.isabs也会返回True。

然后我们又可以知道：

这就能解释为什么第二个返回False而第三个返回True了。