You have a Python script or package that is running perfectly. After you make some alterations, "all of a sudden" you can't run it because it can't find a variable or name:
AttributeError: 'module' object has no attribute 'foobar'
But if you search for that name, it doesn't …
“Suddenly” (as is traditional to commence these sort of stories), I started having problems with some of the Python installations on my local machine. As is the way of things, I had three distinct sources of Python:
You need to convert a time difference into a floating point quantity, like 2.5 days. Or you may want to do something related like round a fdatetime difference up or down, e.g. 3 or 2 days for our previous example.
When you subtract a …
Buried in the Python standard library, unicodedata contains most the information needed to interrogate and translate unicode characters. Unfortunately, it's underdocumented. More accurately, the docs are a terse list of what it does, but not why you might want to use it or how you use it. Unfortunately it's also …
Not the first time I've been stung by this, so it's worth recording the problem.
A function or call inexplicably returns not as the expected type but as a tuple containing the said type. After a furious amount of debugging, you determine that everything is acting as it should …
Setuptools and the single file
Problems with making an installer for a simple module
Although setuptools is a complex thing, and there has has been widespread complaints about its architecture, for the most part, I've had few problems with it. However, I recently encountered a crusty and under-documented corner of …
In a way, tuples are one of the most magical parts of Python. How does the interpreter distinguish between a method call, a bracketed portion of a mathematical expression and an actual tuple?:
calc_midpoint (1, None, [2, 3], False) x = w + (x * y) a = (b, c)
The really smart thing …
So I was writing a web application using WSGI and - after working for a long time - it started erroring out with:
AssertionError: write() argument must be string
from deep inside the WSGI handler. Now, this usually means that you're returning unicode, which WSGI can't handle. However, it's quite happy to …
Who would of thunk it? print is a reserved word everywhere.
Perhaps this is buried in some specification, but it seems that you can't use print as the name of a method of a class. (It makes sense you can't override the global name, but in an object? Perhaps it's …
Recently, I had the need to get an informative name from a range of Python objects for generating a useful error message. Where problems started was that these objects could include classes (new-style and old), functions, lambdas and built-in types. And here the logic started getting tricky.
So, epydoc used to be the neatest and bestest documentation tool available for Python. It produced documentation from introspecting code and thus was the easiest way to the most accurate API documentation. However, it looks like the project has been abandoned. Attempts to run it over code result in sometimes …
Let me explain: Ete2, a Python module for representing phylogenies, has a number of dependencies (MySQLdb, Numpy, PyQt, etc.) that it doesn’t necessarily need and it can be installed without them. If you don’t use the associated functionality, you won’t need these dependencies. But, irritatingly, ete2 tries …
Over the years, Apple has fallen in and out of love with Applescript, its "official" scripting language for MacOS. True, Applescript isn't going to go away any day now and true, it is a very simple language and easy to use. But if you don't want to have to learn …
In which an oddity in SQLAlchemy is spotted, and it turns out to be a bug not a misunderstanding.
The merge function in SQLAlchemy lets an object seem to get pushed to the database, but actually stores and returns a copy of the object. This is handy for when …
How to make a good XML solution for Python better.
Python was late to adopt XML and even then the libraries provided weren't as powerful or easy to use as those available elsewhere. Fortunately, with Python 2.5, a subset of the ElementTree library was incorporated into the standard library …
Matplotlib is cool. Did I say it was cool? I meant very cool. While principally a plotting library, it can be used for image manipulation and drawing.
Which is where our first problem occurs (as of the 0.8.4 version of matplotlib and possibly earlier). Should you try …
Not that I'm picking on ElementTree or anything but ...
Normally XML documents will declare their character encoding in their opening tag. And, normally, this will be utf8. (This may be a standard.) So this is a common sight:
xml version='1.0' encoding='utf8'?>
An oddity in ElementTree is that …
Good news: you can get reliable and easy access to a proper, fully-featured database from within Python.
Bad news: you may have to tinker to get it to (a) install properly. And if not done properly, this may cause it to (b) not work properly.
Good news: Download a .dmg …
Pydee is new and nifty graphical frontend for using Ipython. Unfortunately, there's no monolithic binary package available for installing (at least not on the Mac), and so users have to install and compile all the prerequisites. Given the the number of steps and small but important details, it's easy to …
One of those weird things that always slips my mind and always seems less obvious than it should be.
Parsing a date (object) from a string representation in Python seems oddly neglected or cumbersome as compared to the rest of the standard library that surrounds it. (Witness the number of …
Add this to list of list of things I'm surprised I didn't know about Python. How do you use a percent / modulo ''%'' symbol in a string?
I had to do this the other day and it stumped me. How could it be that in <insert large number of years> I've …
While some people have reporting installing PIL on MacOSX without problems, there are legion reports that say otherwise. Frequently this revolve around PIL installing without JPEG support. There are a myriad of solutions (involving editing make files, configure flags, using fink to install dependencies, using package managers, casting goat entrails …
I've gotten used to manipulating members of Python objects with getattr, setattr etc. But a recent similar problem had me stumped. I had a class that I wanted to create a large number of similar behaving properties on (they would all manipulate an internal dictionary):
>>> f = Foo() >>> f.contributor = 'xyz' …
The crude "lets drop a print statement in" approach keeps being useful even with advanced dynamic languages. Here's a (slightly) improved version for ruby that pretty prints any number of passed objects and (importantly) where the print call was made from:
def dbg (*args)
print("dbg: #{caller()[0]}: ")
args.each …
Around version 2.2, Python rejigged its classes with some useful extensions. Unfortunately these enhancements have been explained so poorly that they appear in little published code.
One such enhancement is __slots__. An attribute of this name in a class restricts what attributes can be created in objects of that …