Jim DeLaHunt, world-ready

I post on various forums around the net. Sometimes I am able to tap into such inspiration that I want to add that essay to my portfolio. Such was the case here. The question: Why does Unicode have separate codepoints for characters with identical glyphs? My response begins: The short answer to this question is, “Unicode encodes characters, not glyphs”. But like many questions about Unicode, a related answer is “plain text may be plain, but it’s not simple”.… Continue Reading »

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I was contributing to the FFmpeg project recently. They keep their source code in a Git repo, accessed via SSL. I had an awkward error message:

SSL reported: PKIX path building failed:
sun.security.provider.certpath.SunCertPathBuilderException:
unable to find valid certification path to requested target

The problem was that my tool handling the SSL communication lacked the SSL certificate which validated the communication with the project. I could dismiss the error and proceed without validating the SSL security. The better solution was to supply the right SSL certificate to the communication tool, so that it could validate the SSL security with no awkwardness. Here’s how I accomplished that.  This post is offered as search engine fodder, in hopes that others will benefit from these instructions. Continue Reading »

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LiClipse is the developers tool I use for writing Python code. Based on the Eclipse IDE, it accepts numerous plugins to support other programming languages like Java and C, and related tools, such as the Git version control system. Eclipse is mostly Java language code, and it runs on a JRE (Java Runtime Environment). Last month, I wanted to contribute code to a git repository which I accessed via HTTPS. That worked more smoothly if I could put an SSL certificate into the JRE, and I’ll skip the details of why for now.

So I looked up the Java Home of the JRE installed on my Mac OS X laptop (short answer: it’s the path output by running /usr/libexec/java_home). I installed the SSL certificate there. It did not work. That was a sign that LiClipse did not use that JRE. Did it perhaps include its own JRE?  After some investigation, I found out the answer: yes!

Here’s the explanation. I hope this helps others. Continue Reading »

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I recently had a problem, “When I run ffmpeg in the background, how do I prevent suspended (tty output)?”. I solved it. Here is my solution, in the hopes that it will help others seeing the same problem.

I have a sh script which calls ffmpeg on several files. When I try to run this script in the background, redirecting output to a file, the job starts but then immediately suspends:

% bin/mp3convert.sh path/a/b &> ~/tmp/log.txt &
[1] 93352
% [1]  + suspended (tty output)  bin/mp3convert.sh path/a/b &>

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I guess I’m exactly the right age to have this problem: hundreds of liner notes from CD albums, stripped from their jewel cases and also CD boxed-sets and many different CD gatefold cases. If I were any younger, I’d be subscribing to some music streaming service, or downloading pirated albums as MP3 files. If I were any older, I’d be building elaborate shelf units to store the hundreds of intact jewel cases, and keeping a multi-disc CD player running to play the music.  But here I am, old enough to buy CDs as a way to pay the artists for their work, but young enough to want to rip the CDs into music files, upload them to a file server, and play them via computer or from my smartphone.

All of this leaves me with a problem: having put the ripped CDs onto nice compact 100-disc spindles, where do I put the liner notes and booklets so that I have access to them if I want ?  I can throw out the regular or the 2-disc jewel cases, because those are generic. But the artwork is indispensable.  On the other hand, I don’t need to keep it out on a open shelf to browse. It’s fine for me to put it away, and retrieve it only when I need it.  And now I am happy to report a solution: a box, of just the right size to hold liner notes or CD box-sets or gatefold cases efficiently, easily available from shipping materials suppliers, and very affordably priced. I post this in the hopes of helping some else who is trying to solve the same problem. Continue Reading »

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Recently I was writing a Python-language tool, and some of my doctests (text fixtures, within module comments) were failing. When I tried to import the StringIO module in my test, I got a quite annoying message, “Got nothing”, and the test didn’t work as I wanted. I asked StackOverflow. User wim there gave me a crucial insight, but didn’t explain the underlying cause of my problem. I read the doctest code, and came up with an explanation that satisfied me. I am posting it here, as an aid to others. The gist of the insight: What looks like a multi-line doctest fixture is in fact a succession of single-line doctest “Examples”, some which return no useful result but which set up state for later Examples. Each single-line Example should each have a >>> prefix, not a ... prefix. But, there are Examples that require the ... prefix. The difference lies in Python’s definition of an Interactive Statement.

The Question

I posted a question much like this to StackOverflow:

Why is importing a module breaking my doctest (Python 2.7)?

I tried to use a StringIO instance in a doctest in my class, in a Python 2.7 program. Instead of getting any output from the test, I get a response, “Got nothing”.

This simplified test case demonstrates the error:

#!/usr/bin/env python2.7
# encoding: utf-8

class Dummy(object):
    '''Dummy: demonstrates a doctest problem
    >>> from StringIO import StringIO
    ... s = StringIO()
    ... print("s is created")
    s is created
    '''

if __name__ == "__main__":
    import doctest
    doctest.testmod()


Expected behaviour: test passes.

Observed behaviour: test fails, with output like this:

% ./src/doctest_fail.py
**********************************************************************
File "./src/doctest_fail.py", line 7, in __main__.Dummy
Failed example:
    from StringIO import StringIO
    s = StringIO()
    print("s is created")
Expected:
    s is created
Got nothing
**********************************************************************
1 items had failures:
    1 of 1 in __main__.Dummy
***Test Failed*** 1 failures.


Why is this doctest failing? What change to I need to make in order to be able to use StringIO-like functionality (a literal string with a file interface) in my doctests?

(I had originally suspected the StringIO module of being part of the problem. My original question title was, “Why is use of StringIO breaking my doctest (Python 2.7)”. When I realised that suspicion was incorrect, I edited the question on StackOverflow.)

The Answer

StackOverflow expert wim was quick with the crucial insight: “It’s the continuation line syntax (...) that is confusing doctest parser.” Wim then rewrote my example so that it functioned correctly. Excellent!  Thank you, wim.

The Explanation

I wasn’t satisfied, however. I know from  didn’t explain the underlying cause of my problem. I read the doctest code, and came up with an explanation that satisfied me. Below is an improved version of the answer I posted to StackOverflow at the time.

The example fails, because it uses the PS2 syntax (...) instead of PS1 syntax (>>>) in front of separate simple statements.

Change ... to >>>:


#!/usr/bin/env python2.7
# encoding: utf-8

class Dummy(object):
    '''Dummy: demonstrates a doctest problem
    >>> from StringIO import StringIO
    >>> s = StringIO()
    >>> print("s is created")
    s is created
    '''

if __name__ == "__main__":
    import doctest
    doctest.testmod()


Now the corrected example, renamed doctest_pass.py, runs with no errors. It produces no output, meaning that all tests pass:

% ./src/doctest_pass.py

Why is the >>> syntax correct? The Python Library Reference for doctest, 25.2.3.2. How are Docstring Examples Recognized? should be the place to find the answer, but it isn’t terribly clear about this syntax.

Doctest scans through a docstring, looking for “Examples”. Where it sees the PS1 string >>>, it takes everything from there to the end of the line as an Example. It also appends any following lines which begin with the PS2 string ... to the Example (See: _EXAMPLE_RE in class doctest.DocTestParser, lines 584-595). It takes the subsequent lines, until the next blank line or line starting with the PS1 string, as the Wanted Output.

Doctest compiles each Example as a Python “interactive statement”, using the compile() built-in function in an exec statement (See: doctest.DocTestRunner.__run(), lines 1314-1315).

An “interactive statement” is a statement list ending with a newline, or a Compound Statement. A compound statement, e.g. an if or try statement, “in general, […spans] multiple lines, although in simple incarnations a whole compound statement may be contained in one line.” Here is a multi-line compound statement:

if 1 > 0:
    print("As expected")
else:
    print("Should not happen")


A statement list is one or more simple statements on a single line, separated by semicolons.


from StringIO import StringIO
s = StringIO(); print("s is created")


So, the question’s doctest failed because it contained one Example with three simple statements, and no semicolon separators. Changing the PS2 strings to PS1 strings succeeds, because it turns the docstring into a sequence of three Examples, each with one simple statement. Although these three lines work together to set up one test of one piece of functionality, they are not a single test fixture. They are three tests, two of which set up state but do not really test the main functionality.

By the way, you can see the number of Examples which doctest recognises by using the -v flag. Note that it says, “3 tests in __main__.Dummy“. One might think of the three lines as one test unit, but doctest sees three Examples. The first two Examples have no expected output. When the Example executes and generates no output, that counts as a “pass”.


% ./src/doctest_pass.py -v
Trying:
    from StringIO import StringIO
Expecting nothing
ok
Trying:
    s = StringIO()
Expecting nothing
ok
Trying:
    print("s is created")
Expecting:
    s is created
ok
1 items had no tests:
    __main__
1 items passed all tests:
    3 tests in __main__.Dummy
3 tests in 2 items.
3 passed and 0 failed.
Test passed.


Within a single docstring, the Examples are executed in sequence. State changes from each Example are preserved for the following Examples in the same docstring. Thus the import statement defines a module name, the s = assignment statement uses that module name and defines a variable name, and so on. The doctest documentation, 25.2.3.3. What’s the Execution Context?, obliquely discloses this when it says, “examples can freely use … names defined earlier in the docstring being run.”

The preceding sentence in that section, “each time doctest finds a docstring to test, it uses a shallow copy of M’s globals, so that … one test in M can’t leave behind crumbs that accidentally allow another test to work”, is a bit misleading. It is true that one test in M can’t affect a test in a different docstring. However, within a single docstring, an earlier test will certainly leave behind crumbs, which might well affect later tests.

But there is an example doctest, in the Python Library Reference for doctest, 25.2.3.2. How are Docstring Examples Recognized?, which uses ... syntax. Why doesn’t it use >>> syntax? Because that example consists of an if statement, which is a compound statement on multiple lines. As such, its second and subsequent lines are marked with the PS2 strings.  It’s unfortunate that this is the only example of a multi-line fixture in the documentation, because it can be misleading about when to use PS1 instead of PS2 strings.

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I use the GnuCash free software accounting software. Like many accounting tools, it can import bank or credit card transactions, and has a way to learn the correct mapping from transaction data to my own account structure. And, sometimes the tool gets the mapping wrong, and needs to be reset.  Here is how I was able to perform this reset. I post it here in the hopes it will help others. Fasten your seatbelt, it involves some awfully technical command-line tools, including XSLT processing. Continue Reading »

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I just resolved a problem installing a Python module pycdio on my Mac OS X 10.10.1 “Yosemite” operating system. The error message was obscure: “Error: Unable to find ‘python.swg'”, and “Error: Unable to find ‘typemaps.i'”. The solution involved something non-obvious about how Mac Ports handles swig. Here’s my notes, in hopes of helping others seeing this error.

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I use and value a good spreadsheet application the way chefs use and value good knives. I have countless occasions to do ad-hoc data processing and conversion, and I tend to turn to spreadsheets even more often I turn to a good text editor. I know a lot of ways to get the job done with spreadsheets. But recently I learned a new trick. I’m delighted to share it with you here.

The situation: you have an HTML document, with a list of linked text. Imagine a list of projects, each with a link to a project URL (the names aren’t meaningful):

• Example “Aphids” Project
• Example “Beeswax” Project
• Example “Clover” Project

The task is to convert this list of formatted links into a table, with the project name in column A, and the URL in column B.  The trick is to use an OpenOffice macro, which exposes the URL (and other facets of formatted text) as OpenOffice functions. Continue Reading »

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I post on various forums around the net, and a few of my posts there get some very gratifying kudos. I’ve been a diligent contributor to StackOverflow, the Q-and-A site for software developers. I’m in the top 15% of contributors overall, and one of the top 25 answerers of Unicode-related questions. Here’s my second best-voted answer in StackOverflow so far.

The question, How do I get SQLAlchemy to correctly insert a unicode ellipsis into a mySQL table?,  was asked by user kvedananda in February 2012. In abbreviated form, it was:

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