Building Skills in Python: A Programmer’s Introduction to Python by Steven F. Lott (© 2002, 2005, 2007, 2008 Steven F. Lott).

Note: this a collection of scraps, describing how values (de)referencing works in Python, and describing when your variable is either a value or a reference. Primary source of knowledge for this post (also here).

Python passes references-to-objects by value (like Java), and everything in Python is an object. This sounds simple, but then you will notice that some data types seem to exhibit pass-by-value characteristics, while others seem to act like pass-by-reference… what’s the deal?

It is important to understand mutable and immutable objects. Some objects, like strings, tuples, and numbers, are immutable. Altering them inside a function/method will create a new instance and the original instance outside the function/method is not changed. Other objects, like lists and dictionaries are mutable, which means you can change the object in-place. Therefore, altering an object inside a function/method will also change the original object outside.

Immutable variables – such as integers [strings, numerics and tuples are immutables] – are passed by value. That is, if your function accepts some integer argument, you are safe assuming that your function won’t be able to modify your integer. Mutable variables – such as dictionaries and lists – are passed by reference, and so if your function accepts mutable argument, it may modify the contents of that mutable variable outside the scope of the function.

When doing :
s = “Hello ”
s += “World”
… you are not modifying the string object bound to s, but creating a new string object and binding it to s.

If using object’s methods within a called function, variable is considered “passed by reference” – it is modified out of the function’s scope. If using assignment on a mutable object, it is created a-new within the function, and global value isn’t modified.

When you call a function with an arg, a “local variable” is created, which references the object passed as the argument. (well… an entry with the formal parameter name as key and a reference to the object passed in is created in the ‘local’ dict).

So, rebinding this local symbol does not impact the binding in the caller’s namespace – because the symbol lives in another namespace.

*But* – and if the object referenced is mutable of course – modifying the object in the function… well, just modifies the object, because it’s the *same* object that is bound to (‘referenced by’, if you prefer) both symbols (the one in the caller’s namespace and the one in the function’s namespace). So yes, the object *is* modified when the function returns.

These are all excerpts from the Python documentation.

To synchronously and simultaneously loop over two sequences:

To loop over a sequence with both key and value:
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Sample script (copypasted from Well House Consultants training course):
click the PLAIN TEXT header for copy-pasteable version

You can also define the Python ksort() function similar to that found in PHP:
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Andy Dustman (used to blog at dustman.net) gave a presentation on Python and MySQL at the MySQL Users Conference 2005, Santa Clara, CA. The presentation is an excellent collection of examples for those who use the MySQLdb Python module.
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Here’s an excellent description of various methods to handle date and time in Python scripts.

I just had a small problem with my Python script:

1. I have a list of tuples, called records_cache; each tuple looks like this:
(note: for copy-pasting, click the PLAIN TEXT box headers)

2. I need to INSERT only some of those values into MySQL table, using the MySQLdb executemany() method. The values I need are utr_start, utr_end, ensembl_gene_id (in exactly this order).

To do this, I need to create another list of tuples, but with shorter 3-item tuples. Let us call the new list genes_update.

Here is the elegant, though probably not the most efficient, solution:
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