Python note: make use of built-in space

This is about the ways to make (cross module) global variables.

One way of sharing variables among runtime modules is defining a global module, and let every module import it. The global module will be loaded only once, because Python puts every loaded module into sys.modules and makes sure that it won't be executed again by import statements. Once the global module is loaded, every module could access global.xxx for variables. In this way the variables are shared as global.

The above method takes use of Python module loading mechanism to achieve global sharing. There is a more straightforward way. In Python, searching for variables follows the order of function -> module -> built-in scope. While the variable is not found anywhere in the module, the built-in scope will be looked for. This scope is defined as a global module (or dict).

Suppose we have main module:

main.py

g_nConfig = "blabla"
__builtins__.g_nConfig = g_nConfig

import first_module
import second_module

# other things to do

Then in the subsequent executions within the two imported modules, g_nConfig is accessible.

Now, suppose that there are many such global things to use. You can make a concentrated sharing for them. The easiest form could be a dict.

g_dGlobals = {'g_nConfig' : "bla bla", 'g_oObject1' : object1}

Of course, to make the global powerful, you can make it a class.

Python note: unicode

When you get some error output like this:

'ascii' encoding can not encode ...

The first thing to check is python unicode object.

Python chooses a separate type of object to support unicode, in order to keep string compatibility. Thus python has two kinds of strings: str and unicode. Str objects are the same as the standard C string - an array of chars. It is used in most function calls.

Each char in a computer system can represent 128 different values. For languages like English, the alphabet is below 30. Therefore we can find a mapping between each letter and each char value. Such a mapping is called encoding. ASCII is the most common encoding to map char values into letters.

For languages with many more than 128 letters, such as Chinese and Japanese, many chars need to be combined to represent one character. A problem arises. Because different languages have different interpretations of char values, the same string can be mapped into different letters / characters by different encodings. For example, when viewing one webpage, you can switch your browser to different encodings, and the page will be displayed differently (of course there is only one encoding that is 'correct') Unicode is proposed to solve the encoding clash, and it includes all possible characters / letters in languages. Interestingly, there are also many different UTF encoding versions, include utf-8, utf-16, etc.

Unicode objects in Python are actually strings encoded in utf-8. It can be seen as the abstract representation of the real character / letters, which can be encoded into different computer strings by different encodings. In other words, if strings are viewed as the outside form, Unicode can be viewed as the inside meaning.

Unicode objects can be changed to str object by the method 'encode'. It will translate the meaning to raw strings with certain encodings.

On the contrary, raw strings can be changed to unicode, using method 'decode'. When you know the 'correct' encoding of a raw string, you can tell it to the system and make it an unicode object.

There are methods to help you determine the os encoding. They are sys.getdefaultencoding() and sys.getfilesystemencoding(). Which are self explanatory.

Some methods in python work with str while other work with Unicode. You have no difficulty with those taking both types, but you need to be careful when calling a method that take only str or Unicode params. Also, the return type of a method us often neglected. For example, file.readline() would return a string. If a file is a unicode file, it's still a string encoded in 'utf-8'.

When a unicode object is passed to a method taking string params, or vice versa, the system will try to switch beween them automatically. However because we did not specify encodings beforehand, it will use ascii by default. When the real encoding can't be interpreted by the ascii char set, the exception at the beginning of this article will occur. The steps to take to fix the problem might be: first check the type of the string, using type() method, then try to convert it to the correct type by using encode() or decode, specifying the encoding.

wxPython note: process tab end enter key events for TextCtrl in dialogs

When you place a TextCtrl in a dialog and catch the Key events for it, you will find that enter and tab key events are not processed. When you press tab key, the focus will be switched to the next widget.

The solution is setting the style for the TextCtrl. There are two styles, wx.TE_PROCESS_ENTER and wx.TE_PROCESS_TAB, which default to unset. They will help in processing events.

Haskell note: turorials

I am completely new to Haskell functional programming. I started to play with it simply because of the need in my MSc course. But it seems more and more interesting now.

Functional programming is quite different from "common", i.e. imperative programming, mainly in that it's not executed from the beginning to the end. A functional program can be taken as a set of equations, when calculated together yielding output.

I find this introduction succinct and helpful

http://www.haskell.org/tutorial/

Here are some good summary of the language.

About the grammar

http://www.cs.uu.nl/~afie/haskell/tourofsyntax.html

About the operators

http://www.imada.sdu.dk/~kornerup/DM22/Noter/haskell-operatorer.pdf

wxPython note: how to select many rows in Grid?

This problem puzzled me some time ago, and I forgot the solution again today. Thus I feel it necessary to take it down here.

There is absolutely no way of setting a style like wx.CB_MULTIPLE to specify multiple selection here. wxGrid support multiple row selection by itself, see a reference

http://lists.wxwidgets.org/archive/wx-dev/msg21759.html

The only thing you need to have is specifying the second (hidden!) param of SelectRow - bAppend (I am vague about the name). When it's true you will see the row selected without cleaning other rows.

Of course, this implies that selecting many rows can only be done from program. Thus in order to respond to mouse and keyboard behaviors, event catching and processing are needed. Anyway, it's not uncommon to process events for a grid.

Introduction to MVC

Found a good introduction to the MVC pattern.

http://csis.pace.edu/~bergin/mvc/mvcgui.html

Also see a general explanation at

http://ootips.org/mvc-pattern.html

PHP note: Difference between PHP and Python - scope

Scope in Python is more complex. Firstly, functions. You will encounter local, global and builtin scopes in a function. Local scope is the scope within functions, global scope is the currently running module scope, while builtin scope is the current running python program. Secondly, objects. You will encounter object, class scope. Python searches for variables in sequence of scopes, and looks at a higer level scope when it can't find symbol in the current scope.

Scope in PHP is just simple. Normally you only need a module scope, while in functions there is a local scope. PHP does not search super scopes though. This is somehow because PHP can always find symbols. When variable not defined, PHP will simply set an empty value to it.

PHP note: Difference between PHP and Python - types

PHP and Python are both interpreted languages.

Python has more strict types from PHP. Python does not have implicit type casting. For example, Python will calculate 3/2 as an integer division, and the output could be 1. The only implicit type casting that I find about Python is that Python will change integers to long integers when the value of that integer is too large.

PHP, on the other hand, does implicit type casting all the time. It does not have integer division at all. When it does 3/2 it simply yields the float result 1.5. What is more unacceptable from Python thoughts is that it turns strings to integers automatically. "15abc" + 10 = 25! The same understanding of the world applies to many other places in PHP. Such as when you meet 01081 it will be taken as 8. Because that was an octal number, which was truncated when 8 came up.

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