======File System=====
Bits of advice about working with filesystem using Python.

=====Working with directories=====

It is quite likely that you will want to either use data from a file or to write data into a file. The filename will usually be in one variable (i.e. //fname//) and the path in another (i.e. //fpath//). To access the file, you have to join those with a correct separator. For Unix/Linux it is /, for Windows it is \, which, however, has to written as \\. (Because \ is used to write special characters)

You do not have to worry about OS detection, just use standard module //os//. I.E. the //os.sep// variable always contains the right separator for the case.
So, the following code
<code python>
>>> import os
>>> fpath = 'filepath'
>>> fname = 'filename'
>>> fpath + os.sep + fname
</code>
will have this output under Windows
<code python>
'filepath\\filename'
</code>
and this output under Linux
<code python>
'filepath/filename'
</code>

We could also use the //os.path.join()// function. Thus
<code python>
>>> os.path.join(fpath, fname)
</code>
will give same output as the above examples using //os.sep//. Another advantage of using //os.path.join()// is also that it deals with cases when fpath already contains a separator.

=====Saving file to current directory=====
You will also want your code to be able to save some data for later use. The most logical place for this is the same directory your program is in (or its subdirectory). You will probably think of using relative path, which derives the file location from the current directory. So, if you run your script //script.py// which contains
<code python>
f = open('data.txt','w')
</code>
the new file file, //data.txt// will be created in the same directory //script.py// is in.

Problems begin, when your script //script.py// is imported in other modules and those try to use it, because in this case, current directory is the directory the main script is run from.

If you want to always save the new file relative to the imported module, you can use 
<code python>
fpath = os.path.dirname( os.path.abspath(__file__) )
</code>
However, this construction can fail, if we do not import the module, but run it, i.e. using //execfile()//, because in this case the //file// variable does not get set.

Completely universal way to do this uses //inspect// module.
<code python>
import inspect
fpath = os.path.dirname( inspect.getfile( inspect.currentframe() ) )
</code>