[Tutor] empty delimiters, and None

[ugajin at talktalk.net]

I run Inkscape (an Open Source vector drawing application) on OSX (Snow leopard)  installed via .dmg pkg. The application ships with a collection of Python extensions, one of which Measure Path, failed to execute. 

I could not find a reference for this issue in the Inkscape archives, and I tried and worked with other workarounds including a javascript (outside Inkscape) to find the length of Bézier paths, but eventually I raised the issue and contacted launchpad/inkscape (question #239599) but to limited avail. After much continued searching I then found this Bug #803791 report which describes (as an aside) the exact same issue together with a solution. 

I know next to nothing about Python and I shall be glad if any Python guru can make clear (in layman's terms where possible) what the problem is/was and how the fix works. 

I remembered an alternative posting that I had read, which reported that Python dislikes empty delimiters. So I took a slightly different approach to the one mentioned in the above bug report. Instead of removing line 35 from the Measure Path script I replaced the empty delimiters with the Python constant None. To my great delight as well as my own amazement the 'fix' worked. 

The problem may or may not be limited to OSX. 

On trying to run the script (before the fix) I had the following traceback message: 
File "measure.py", line 35, in <module>
locale.setlocale(locale.LC_ALL, '')
File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/locale.py", line 494, in setlocale
return _setlocale(category, locale)
locale.Error: unsupported locale setting

If I booted from the Unix file I received the following feedback:
Setting Language: .UTF-8
(process:82224): Gtk-WARNING **: Locale not supported by C library.
Using the fallback 'C' locale.

Line 35 (above) shows the empty delimiter. The fix that I applied was to replace line 35 in the Measure.py script with:
locale.setlocale(locale.LC_ALL, None). The Measure Path script then executes correctly. No other action is/was required.

I note also the Python 2 library entry for Built-in Constants as:
None
The sole value of types.NoneType. None is frequently used to represent the absence of a value, as when default arguments are not passed to a function.
Changed in version 2.4: Assignments to None are illegal and raise a SyntaxError.

And under standard type hierarchy as:
None
This type has a single value. There is a single object with this value. This object is accessed through the built-in name None. It is used to signify the absence of a value in many situations, e.g., it is returned from functions that don’t explicitly return anything. Its truth value is false.
I have run the various scripts in a Python Validator, and line 35 (both versions) pass unremarked, although there are a few other code errors and warnings and many pep8 warnings. 

I have also looked at locale.py Line 494 of which is the last line of a def (def function?) I include this below, hopefully this may save you searching for locale.py (Pyhon 2.6) should you need it and wish to answer the above questions, it may help.

def setlocale(category, locale=None):

    """ Set the locale for the given category.  The locale can be
        a string, a locale tuple (language code, encoding), or None.

        Locale tuples are converted to strings the locale aliasing
        engine.  Locale strings are passed directly to the C lib.

        category may be given as one of the LC_* values.

    """
    if locale and type(locale) is not type(""):
        # convert to string
        locale = normalize(_build_localename(locale))
    return _setlocale(category, locale)


Many, many thanks for your interest and considering matters.

-A
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