Let's take a look at some examples of using os,path.isabs():

Obviously, the first path is no problem, but it is followed by a string of characters I typed casually. Why do some return True and others return False?

I checked the documentation of Python 3.5 and found the following sentence:

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.

That is to say, in the WIndow system, if the input string starts with "/", os.path.isabs() will return True, so the fourth example can understand.

What's the difference between the second and the third? Shouldn't all return False?

After consulting the data, I found the following sentence:

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;

Although it's python2, it should be no different from python3. As you can see from the above sentence, on Windows systems, os.path.isabs will also return True for strings starting with "/ /".

Then we can know:

This explains why the second returns False and the third returns True.