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General Remarks
Directory Structure
The source is divided into the core sources, containing the basic
graphics classes, the UI code and some utility modules. 
devguide-22.htmlImport filters  and other devguide-20.htmlplugins are maintained outside of this core source. Other external packages
provide direct access to Xlib functions and objects (
#N4Pax ) and stream filter objects ( #N5Filter ).
sketch-<version>/The top level directory holds the main scripts sketch.py and
sk2ps.py
Sketch/The top-level directory for the core sources. This works as a
Python package which exports many of the commonly needed support
and graphics objects (classes, functions, constants etc.).
Ideally, the Sketch package should export every thing a
`normal' plugin may need. This is not yet the case, as some
interfaces are not very stable yet and may need to be
redesigned. Once they have reached a certain level of stability,
they will be moved into 
Sketch.
Base/Contains the most fundamental modules such as the
modules for file IO, undo/redo, the plugin manager and
code for user preferences.
Modules/The C-Modules.
The modules in 
Base/ and Modules/ are accessible as
Sketch.modulename. These directories are not
currently subpackages.
Graphics/The subpackage for the graphics modules. 
It contains all of the builtin graphics classes:
primitives, compound objects, special effects
(blend/mask/...), the document object, ...
It also holds the graphics devices for drawing into the
window or PostScript file.
The Graphics modules are accesible as
Sketch.Graphics.modulename. Many of the
objects defined in Graphics modules are exported
directly by the Sketch module.
UI/The subpackage for user interface related code.
Contains the user interface code including the
application object.
Lib/A subpackage for some support modules that are not
really Skencil specific, such as a Type 1 font
interpreter (simply extracts the outline information)
and a simple parser for PostScript files adhering to the
DSC.
Pixmaps/Pixmap files used by Skencil.
Plugins/The subdirectories contain the various devguide-20.htmlplugins :
		
Filters/Contains the devguide-22.htmlimport filters and export filters for SK-files and other formats.
Objects/ devguide-24.htmlPlugin objects .
Resources/The subdirectories contain platform/UI independent resources:
Fontmetrics/Fontmetrics (*.afm) for standard PostScript fonts
and `font directory files' (
*.sfd) that describe
fonts for Skencil.
Misc/Files that define arrows (*.arrow), dashes
(
*.dashes), palettes (*.spl) and
Tk-resources.
Pax/Modules that allow Skencil direct access to Xlib. This is a
heavily modified version of the Xt-module.
Filter/A module for stream filters. A stream filter behaves just like a
(non seekable) file and comes in two variants: An encoding
filter encodes the data written to it and writes the encoded
data to a data target (a file or another filter); a decoding
filter reads data from a data source (a file or another filter)
and provides it in decoded form.
These stream filters are modeled after the filters in
PostScript Level 2.
Don't confuse these filters with the Import filters in the #N3Plugins/Filters/  directory
(although import filters might use stream filters to decode the
data).
Examples/Some example drawings.
Naming Conventions
Module names are lowercase.
Functions have identifiers that are either
lowercase_with_underscores or Capitalized. This is a bit of a
mess, but generally the former should be preferred.
Variables, local or global, are also lowercase_with_underscores.
Module internal names (for functions, variables, etc.) shouldstart with
an underscore (`
_') (this is a Python convention that affects
import statements and module finalization).
Class names are Capitalized. Public methods are also
Capitalized, protected methods are lowercase_with_underscores,
private methods start with two underscores (another Python convention).
Instance variables are also 
lowercase_with_underscores.
Constants are often UPPERCASE (an exception are the X-constants in
Pax/X.py)
Python does not enforce encapsulation and the distinction between
public, protected and private methods and instance variables is more or
less a convention programmers are expected to adhere to.
Coding Guidelines
This not a `coding style' or something similar you are required to
adhere to, just some remarks on writing maintainable and readable Python
code (IMHO).
Import statements
Don't use from module import *
It is difficult to find out whether the code really depends on objects
exported by the module. Editing the source or moving some parts of the
code to other modules may result in code that doesn't need that module
any more, so the import statement should be removed.
Using `import module' or `from moduleimport 
foo, bar' makes this easier.
Multiple assignments
Don't use multiple assignments.
Multiple assignments are assignments like this:
	a, b = 0, 1
I think this is harder to read than
	a = 0
	b = 1
or, if you really want only one line, 
	a = 0; b = 1
In the current interpreter (Python 1.5.1) the multiple assignment is
even slower than two single assignments (for local variables; the
interpreter actually builds a tuple and unpacks it immediately)
Multiple assignments are convenient, though, if the first assignment has
side effects influencing the second assignment. If you want to swap the
values of the variables 
a and b you may write
	a, b = b, a
Note that even in this case the traditional idiom for swapping
variables, 
temp = a; a = b; b = temp is faster than multiple
assignment (see 
Tools/swapbench.py).
Truth Values, __getattr__ and __len__
In various places, instance or class attributes have the value Noneto indictate that the attribute has no particular value, while, if the
attribute is set, its value is an instance object (one such attribute is
devguide-13.html#N2HierarchyNode 's attribute parent). Functions and
methods often return 
None instead of an instance object for similar
reasons.
There are more cases, and, indeed, the following applies whenever you
have an object that may be 
None or an instance object.
In all these cases, you have to test whether the object you have is
None or not. It is tempting to write e.g.
	if self.parent:
		self.parent.SomePublicMethod()
Don't do that. Test whether the object is None:
	if self.parent is not None:
		self.parent.SomePublicMethod()
There are two reasons for this:
Firstly, if the object is an instance object, Python tries to determine
if it is true or false by calling its 
__nonzero__ method and, if it
doesn't have that method, its 
__len__ method. Now, most objects in
Skencil don't have these methods, so Python tries to call the
__getattr__ method, which many objects, including all graphics
objects, have, twice, for 
__nonzero__ and __len__. This
overhead can be avoided by testing for identity with 
None.
Secondly, even if you know that the object has no __len__ or
__nonzero__ methods (and, hence, is always considered `true'), it
may have them in future versions (the compound graphics objects might
implement the Python sequence protocol and, accordingly, the
__len__ method). In that case, code that simply tests whether the
object is true would break, as the object might be false even though it
is 
not None.
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