Python basics

5 08 2008

Python‘s great, really. It takes a little bit of a mode-change before you get used to it, but I’m really quite taken with it. I’ve got experience with C/++, Java, Perl, PHP, Javascript, and a little in Ruby (and I can hammer things out in a number of others, but I wouldn’t claim to “know” the language), and I’m simply impressed with Python.One major plus: Python was built to be a command-line tool.  Thus, you can “run” Python and it will be in command-line form, where you can enter one line at a time.  Sounds rather daft until you’re sitting on a hard chair at 3am trying to guess at how a method behaves.  Python makes this sort of guesswork trivial, as you can just open the command prompt (or terminal, if you’re on some Unix-based system) and run “python”.  You have the ability to simply test a command several times over, to observe its behavior.This INVALUABLE to a person like myself.  There are times when (let’s face it), the online documentation is a bunch of crap, or (in my case), the documentation on the panda3d library’s methods is incomplete.  You have the power to just *do* the command and see for yourself, rather than trying to debug your current program for strange behavior, or trying to built a seperate program for the sole purpose of testing a function or method.  I’ll be honest, I usually have a Python process running in my command prompt while I program in my IDE off on the other monitor.

Some quick observations about Python: 

  • Global scoping can be weird.  Try to be as encapsulated as possible.  Python’s scoping is more like “static” (if you’re familiar with the C++ idea of that term).  There’s no cross-file variable/function/method sharing.
  • Functions” are created with the “def” keyword.
  • Any functions (“def”s) that are within a Class always take “self” as their first arguement, by convention.  When you call these functions/methods, they do NOT require that you pass an arguement to fill the “self” variable in your declaration’s arguement list.  This is resolved automatically.  It is like the “this” pointer of C++.  It’s a reference to the class object that the “def” belongs to.
  • Any variables that should be persistently available in a class should be accessed by the “self.myVar” syntax.
  • A method defined by “def __init__(self):” will act as the class’s constructor.
  • Triple-quoted strings (using three single quote or double quote characters in a row) can be used as something called a “docstring“, and should be placed directly under each class and def statement’s first line.  Triple-quoted strings are really just immune to linebreaks.  That’s all that’s special about  them.  They just happen to be used for this “docstring” very commonly.
  • There are no braces in Python code. Indentation is the way that Python knows what block you’re in.  All statements that should be within an “if” block should be indented the same amount–no more, no less.
  • General lines do not require a semicolon at the end. This is because Python was made for line-by-line console programming.  Making every line end with a “;” is a silly notion.  Just push “enter” on the prompt to finish the line.  However, including a “;” at the end allows you to execute multiple statements on a single line–functionality only really useful from the command prompt.
  • Blocked statements, like “if”, “else”, “elif”, “while”, “for”, “def”, “class”, etc, require a colon at the end. This seems utterly backwards to the normal programming standards out there, but again, on the console, it makes sense; this is how Python knows that you want to enter more information to be within the block that you’ve just tried to define.
  • Everything in Python is by reference. Sounds annoying at first, but it makes the programs a bit more lightweight on the memory footprint.  Variable assignment, if dealing with Objects, is by reference:
    >>> a = myClassObject()
    >>> b = a
    >>> b.setX(3)
    >>> a.getX()
    3.0
    >>>
  • Comments are made with a pound, or hash, sign “#”.  There are no multi-line comments.  You could, however, be a jacker and abuse the triple-quote string to get multi-line “comments” in your code.  It’s nicer just to put a “#” at the front of each line.  C’mon… how lazy ARE you?

And those are some of the basics.