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enigma2 python coding rules


1.) The Zen of Python

The Zen of Python, by Tim Peters

Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than *right* now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!

2.) Exceptions

Exceptions are great!

They are great to nullify any performance. Don't ever use them in functions
which are called more than once or twice. 

Don't use them when a failure is expected. Use them for unexpected failures,
not as a method for coroutines.

C++ wrapped stuff usually doesn't throw exceptions, but returns None or
stuff like that.

Exceptions trough C++ are evil, too. Don't throw exceptions on callbacks
made from C++!

Exceptions are still great!

For example, to hide your coding errors. Don't wrap your code in a
try:/except:-clause because it behaves differently each time. The proper
solution is to fix your code!

Sometimes, exceptions are ok.

Really. For example, when opening a file, which you expect to be there. It's
ok if there is an assertion failure, i.e. something which can't be, unless
something is seriously broken (i.e. buggy, not misconfigured)!

As a rule of thumb,

"except:" (that is, without a specification which exception to catch) is
generally FORBIDDEN.

(Of course, unless you know what you're doing. So if you're feeling smarter
than this document, do whatever you like.)

3.) exec, eval, or other uses of dynamic code

Be aware that any call to exec/eval/... opens a backdoor. That sucks. Plus,
it starts the parser, which is SLOOOW. There are generally very few reasons
to call exec/eval.

 - importing a "plugin", or resolving a dynamic reference (for example in a
   menu.xml)

There is __import__. "exec 'import ' + string" is not so good.

 - dynamically resolving function names

There is "getattr". For example an "eval('blub.' + x)" can also be written
as "blub.getattr(x)", which is a LOT faster as it doesn't need to start a
parser. It's also easier to debug.

 - dynamic code, loaded from a menu.xml.

You can compile() code, and call that later. The backdoor warning still
applies, of course.

4.) formatting rules

Please use tabs (that is, \t) for indenting. 

Empty lines should be either empty, or indented like the line before / line
after. Empty lines are definitely preferred to save bandwidth.

An ascii file ends with \n, and preferrable not with other empty lines.

That means: make sure the last line doesn't contain any characters, thanks.

5.) usage of 'print'

While it's great to dump out debug stuff, especially if your code can crash,
expect your code to be stable at some point.

At that point, others might get annoyed by the debug output created by your
code. That's no problem, they can remove it, but they have to find them
first.

Using "print obj" with obj being some object, preferably a complex one, is a
good way to ensure that nobody is able to remove your debug output - because
nobody finds it!

Please, always prepend something before which can be grepped. Anything, just
not nothing. Going trough all prints to find the offending one is definitely
no fun. Something like "print 'obj', obj" is fine. Something like "print 
'mySpecialPlugin actionmap is', actionMap" is even better.

6.) usage of 'import'

Please avoid 'import *'.
Use "from foo import bar" only if bar is clearly identifiable to belong to
foo (e.g.: it's ok to "from Screens.MessageBox import MessageBox", but it's
not ok to do "from os import path". Use "import os.path", then os.path.fnc.
Of course "from os.path import *" is even worse.)

7.-99.) Threads are bad.

(Unless they are worker threads. And sleep()ing is definitely not working.
So if you every having a thread which sleeps, you did something wrong. There
are eTimers, and not knowing how to use them is no excuse to throw aways all
enigma code design concepts.)