Tag Archives: python

Django Q function

When querying objects in django the simplest way is to use the filter() query.

If we have a set of systems for example and we want to filter out those that have a SPARC architecture we could do:

And if we wanted to narrow that down to only sparc machines that use NIS for name resolution we could do:

But that is narrowing down the selection. It’s an AND. What were asking for is systems that are a SPARC _AND_ use NIS. And although we can use exclude() to reject some results it’s still and AND operation. What about an OR query?

What if we wanted to select systems that had either a T5 or a T7 processor? Then we need Q queries.

Q queries take the same form of argument as filter() & exclude() etc. but can be combined using OR operators. Here’s the Q query for our model that looks at the cpu_implementation for T5 or T7:

To build our query we just use this Q expression in a filter:

Python: Creating a singleton (to control an Arduino)

A while ago I was writing a tool in python to communicate with a sensor on an Arduino. The idea was to have a class for the Arduino that would hold the connection and handle the reading and writing to the serial port. However that way I needed to have the object maintain the connection and the open COM port (this was on windows), but this meant that I could not have multiple objects. Why? Because you can only have one connection to a COM port. The solution – use a Singleton design pattern.

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Python : generating unittests on the fly

When creating python unit tests you need to create a method in a unittest class. The problem however is when you don’t know until the code is running what these tests will be. In my particular case I need to run a series of tests against a number of Solaris packages. But I won’t know what the packages will be in advance. Continue reading Python : generating unittests on the fly

Python: Searching for a string within a list – List comprehension

The simple way to search for a string in a list is just to use ‘if string in list’. eg:

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