Programming Python, Part II

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Having covered some advanced features in Part I, it's time to include some basics.
Loops

One of the Python's looping constructs is for. It is designed to iterate over lists, sets, maps and other iterable objects. In this case, it takes all the items in self._posts and, one by one, assigns them to the variable post. In the body of the for, which is executed on each iteration, we can use the variable post.

The body of the for, as with other constructs that need a piece of code, is delimited by nothing more than the indentation. Here's an example:

>>> the_list = [1,2,3,"a","b"]
>>> for item in the_list:
...     print item
...
1
2
3
a
b
>>>

Various tasks are solved with a loop. One such task is doing something for each member of a collection, like we did in the previous example. For those types of tasks, the for construct is excellent.

Another common practice is to perform an action a given number of times—for example, printing “Hello, world” three times. To do that we can use:


>>> a = 0
>>> while a < 3:
...     print "Hello world"
...     a = a + 1
...
Hello world
Hello world
Hello world
>>>

Another loop construct is while, and it will continue to run its body until the check—that is, the expression after while and before the colon—becomes false.

We can rethink the previous loop as iterating over a list containing the numbers 0–9. There's a way to do it with a for construct:

>>> for a in range(0,3):
...     print "Hello world"
...
Hello world
Hello world
Hello world
>>>>

This is shorter and arguably more readable. What is while useful for then? It is useful any time you don't really know when you are going to stop the loop. Here are some examples:

  • Reading characters from a file until you encounter the End of File (EOF).

  • Reading commands from a user until the user enters the quit command.

  • Reading temperatures from a sensor until the temperature is too high.

  • Reading events from a user interface until the user presses the X button at the top of the window to close the program.

There's a pattern forming here—doing something until something else happens. That's what while is good for.

Some time ago, when we didn't have as many choices in programming languages and we ended up using C most of the time, the while construct tended to be much more useful than the for construct. But today, with a powerful for construct, nice functions such as range and the possibility of putting an iterator around anything, for is being used much more than while.

Here's one last example for your enjoyment:

>>> for l in "Hello World":
...     print l + " ",
...
H  e  l  l  o     W  o  r  l  d

Conditionals

In the fourth line of some previous sample code, if post.is_public(), we have another new construct—an if. This allows programs to make choices based on data. It needs a boolean value and a piece of code. The code is run only if the boolean is True. If you provide something that is not a boolean, Python does its best to interpret it as a boolean. For example, the number 0 is interpreted as False, but all the other numbers as True. Here are some examples:

>>> if True:
...     print "It is true!"
...
It is true!
>>> if False:
...     print "Is it false?"
...
>>> 

We can perform many different types of comparisons on different kinds of objects. Note that the equality operator is ==, not = (that is, two equal signs):

>>> a = 10
>>> if a == 10:
...     print "Ten!"
...
Ten!

There are other comparisons, such as greater than (>), less than (<) and different (!=). You can experiment with comparisons directly on the REPL:

>>> 3 == 4
False
>>> 10 != 5
True
>>> 4 >= 1
True

It is common to run a piece of code if something is true and another piece of code if it is false. For example, we could do the following:

if a == 10:
    print "A is ten."
if a != 10:
    print "A is not ten."

This has a big problem. If we change a to b in the first case, we have to remember to change it in the second. And, the same should be done for any other little changes we do. The solution is an extension to the if construct:

if a == 10:
    print "A is ten."
else:
    print "A is not ten."

The piece of code after the else will be executed if the first piece wasn't executed.

Another common situation is having various conditionals for different cases. In that case, we use a string of ifs:

if a == 10:
    print "A is ten."
elif a == 0:
    print "A is zero."
elif a != 30:
    print "A is not thirty."
else:
    print "Who cares about a ?"

elif is the contraction of “else if”, and indeed, the previous code could be written as:

if a == 10:
    print "A is ten."
else:
    if a == 0:
        print "A is zero."
    else:
        if a != 30:
            print "A is not thirty."
        else:
            print "Who cares about a ?"

But, that is ugly and prone to errors. If you have 10 or 15 different cases, you'll need a 29"-widescreen monitor just to view it. (Not that I have anything against such a monitor. I'd like to have one.)

If you come from other languages that have a switch or select or case construct and are wondering where they are in Python, I'm sorry to disappoint you. Python doesn't have such constructs. There's a proposal to include them, but it hasn't been implemented yet. Right now, the solution is to use a chain of ifs, elifs and elses. After you use this a few times, it's not so bad.

Now that you know about else, here's an interesting tidbit: for and while also can have elses. What do they do? Run Python, and try it out until you discover for yourself. While programming, you'll need to run a lot of code to find out how many undocumented, obscure, almost black-magic, things work, so starting with something simple will help you get some training.

______________________

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error?

csshef's picture

cool = blog.Post("Cool", "Python is cool")

I can't get the above line to work. Is that correct?

error?

ejournal's picture

Were you using Emacs and following the instructions in part I? If so, the class "Post" will be part of the module "blog". If not, you must arrange for that yourself - the article explains.

If I understand correctly. I'm not a Python programmer. :)

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