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os.path.split seems to be returning wrong

I can't understand what os.path.split is doing. I'm debugging a program (specifically git's interface with Perforce: git-p4) and seeing that os.path.split is splitting the incoming path in ways the script isn't expecting, and also seems inconsistent with the documentation. I made some simpler tests and can't figure out what it's doing myself.
The path I want to split is //a/b (The path is actually a Perforce path, not a local filesystem path), and I need b in the second half of the returned pair. I'm running on Windows, and suspect the issue has something to do with the path not looking very Windows-esque. When I tried running my test code in an online sandbox it worked as expected unlike my Windows machine.
I've read the documentation:
os.path.split(path)
Split the pathname path into a pair, (head, tail) where tail is the last pathname component and head is everything leading up to that. The tail part will never contain a slash; if path ends in a slash, tail will be empty. If there is no slash in path, head will be empty. If path is empty, both head and tail are empty. Trailing slashes are stripped from head unless it is the root (one or more slashes only). In all cases, join(head, tail) returns a path to the same location as path (but the strings may differ). Also see the functions dirname() and basename().
My test code:
import os
print os.path.split("//a")
print os.path.split("//a/b")
print os.path.split("//a/b/c")
What I'd expect:
('//', 'a')
('//a', 'b')
('//a/b', 'c')
What I actually get on a couple online sandboxes:
('//', 'a')
('//a', 'b')
('//a/b', 'c')
What I actually get on my PC:
('//', 'a')
('//a/b', '')
('//a/b/', 'c')
Python 2 because the git-p4 code is written for Python 2.
So my first question is just for my own understanding. What's going wrong here? An OS difference?
And then beyond my own curiosity, I need a fix. I've been able to modify git-p4, but I'd of course prefer to edit it as little as possible as I'm not trying to understand it! I'm not a python expert. Is there a comparable method that can get ('//a', 'b') returned?

You are using the wrong tool to handle these paths. On Windows, paths that start with //foo/bar or \\foo\bar are seen as UNC network paths, and os.path.split() will first use os.path.splitdrive() to make sure the UNC portion is not split. The UNC or drive portion is then re-attached after splitting the remainder.
You can use the posixpath module instead, to get the POSIX behaviour:
import posixpath
posixpath.split(yourpaths)
Quoting from the top of the os.path module documentation:
Note: Since different operating systems have different path name conventions, there are several versions of this module in the standard library. The os.path module is always the path module suitable for the operating system Python is running on, and therefore usable for local paths. However, you can also import and use the individual modules if you want to manipulate a path that is always in one of the different formats. They all have the same interface:
posixpath for UNIX-style paths
ntpath for Windows paths
[...]
On Windows, os.path is the same module as ntpath, the online sandboxes must all have been POSIX systems.
Treating your Perforce paths as POSIX paths is fine, provided you always use forward slashes as path separators.

running python code in an external shell using sublimetext?

First of all, I am new to programming.
To run python code in an external shell window, I followed the instructions given on this page
link
My problem is that if I save the python file in any path that contains a folder name with a space, it gives me this error:
C:\Python34\python.exe: can't open file 'C:\Program': [Errno 2] No such file or directory
Does not work:
C:\Program Files\Python Code
Works:
C:\ProgramFiles\PythonCode
could someone help me fix the problem???
Here is the code:
import sublime
import sublime_plugin
import subprocess
class PythonRunCommand(sublime_plugin.WindowCommand):
def run(self):
command = 'cmd /k "C:\Python34\python.exe" %s' % sublime.active_window().active_view().file_name()
subprocess.Popen(command)

subprocess methods accept a string or a list. Passing as a string is the lazy way: just copy/paste your command line and it works. That is for hardcoded commands, but things get complicated when you introduce parameters known at run-time only, which may contain spaces, etc...
Passing a list is better because you don't need to compose your command and escape spaces by yourself. Pass the parameters as a list so it's done automatically and better that you could do:
command = ['cmd','/k',r"C:\Python34\python.exe",sublime.active_window().active_view().file_name()]
And always use raw strings (r prefix) when passing literal windows paths or you may have some surprises with escape sequences meaning something (linefeed, tab, unicode...)
In this particular case, if file associations are properly set, you only need to pass the python script without any other command prefix:
command = [sublime.active_window().active_view().file_name()]
(you'll need shell=True added to the subprocess command but it's worth it because it avoids to hardcode python path, and makes your plugin portable)

How do you define a local path in python

Good morning, I can indicate how to enter a path of internal hard disk in python, currently use the statement:
file = GETfile() or 'http://**********'
I would like to put a path to a local file, but it does not work, where am I wrong?
file = GETfile() or 'D:\xxx\xxxx\playlist\playlist.m3u'

\ is a escape character. You have three options.
1) use /. This, as a bonus works for linux as well:
'D:/xxx/xxxx/playlist/playlist.m3u'
2) escape the backslash
'D:\\xxx\\xxxx\\playlist\\playlist.m3u'
3) use raw strings:
r'D:\xxx\xxxx\playlist\playlist.m3u'

A correct answer is already given, but some additional information when working with local drive paths on Windows operating system.
Personally I would go with the r'D:\dir\subdir\filename.ext' format, however the other two methods already mentioned are valid as well.
Furthermore, file operations on Windows are limited by Explorer to a 256 character limit. Longer path names will usually result in an OS error.
However there is a workaround, by pre fixing "\\?\" to a long path.
Example of a path which does not work:
D:\reallyreallyreallyreallyreallylonglonglonglongdir\reallyreallyreallyreallyreallylonglonglonglongdir\reallyreallyreallyreallyreallylonglonglonglongdir\reallyreallyreallyreallyreallylonglonglonglongdir\reallyreallyreallyreallyreallylonglonglonglongdir\reallyreallyreallyreallyreallylonglonglonglongdir\reallyreallyreallyreallyreallylonglonglonglongdir\reallyreallyreallyreallyreallylonglonglonglongdir\filename.ext
Same file path which does work:
\\?\D:\reallyreallyreallyreallyreallylonglonglonglongdir\reallyreallyreallyreallyreallylonglonglonglongdir\reallyreallyreallyreallyreallylonglonglonglongdir\reallyreallyreallyreallyreallylonglonglonglongdir\reallyreallyreallyreallyreallylonglonglonglongdir\reallyreallyreallyreallyreallylonglonglonglongdir\reallyreallyreallyreallyreallylonglonglonglongdir\reallyreallyreallyreallyreallylonglonglonglongdir\filename.ext
so the following code I use to change filenames to include the "\\?\":
import os
import platform
def full_path_windows(filepath):
if platform.system() == 'Windows':
if filepath[1:3] == ':\\':
return u'\\\\?\\' + os.path.normcase(filepath)
return os.path.normcase(filepath)
I use this for every path to file (or directories), it will return the path with a prefix. The path does not need to exist; so you can use this also before you create a file or directory, to ensure you are not running into the Windows Explorer limitations.
HTH

Does os.path.sep affect the tarfile module?

Is the path separator employed inside a Python tarfile.TarFile object a '/' regardless of platform, or is it a backslash on Windows?
I basically never touch Windows, but I would kind of like the code I'm writing to be compatible with it, if it can be. Unfortunately I have no Windows host on which to test.

A quick test tells me that a (forward) slash is always used.
In fact, the tar format stores the full path of each file as a single string, using slashes (try looking at a hex dump), and python just reads that full path without any modification. Likewise, at extraction time python hard-replaces slashes with the local separator (see TarFile._extract_member).
... which makes me think that there are surely some nonconformant implementations of tar for Windows that create tarfiles with backslashs as separators!?

Cannot read in files

I have a small problem with reading in my file. My code:
import csv as csv
import numpy
with open("train_data.csv","rb") as training:
csv_file_object = csv.reader(training)
header = csv_file_object.next()
data = []
for row in csv_file_object:
data.append(row)
data = numpy.array(data)
I get the error no such file "train_data.csv", so I know the problem lies with the location. But whenever I specify the pad like this: open("C:\Desktop...etc) it doesn't work either. What am I doing wrong?

If you give the full file path, your script should work. Since it is not, it must be that you have escape characters in your path. To fix this, use a raw-string to specify the file path:
# Put an 'r' at the start of the string to make it a raw-string.
with open(r"C:\path\to\file\train_data.csv","rb") as training:
Raw strings do not process escape characters.
Also, just a technical fact, not giving the full file path causes Python to look for the file in the directory that the script is launched from. If it is not there, an error is thrown.

When you use open() and Windows you need to deal with the backslashes properly.
Option 1.) Use the raw string, this will be the string prefixed with an r.
open(r'C:\Users\Me\Desktop\train_data.csv')
Option 2.) Escape the backslashes
open('C:\\Users\\Me\\Desktop\\train_data.csv')
Option 3.) Use forward slashes
open('C:/Users/Me/Desktop/train_data.csv')
As for finding the file you are using, if you just do open('train_data.csv') it is looking in the directory you are running the python script from. So, if you are running it from C:\Users\Me\Desktop\, your train_data.csv needs to be on the desktop as well.