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devguide-2.html
The Design of Skencil
devguide.html
Developer's Guide
devguide-4.html
Coordinate Systems
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.html
Import filters
and other
devguide-20.html
plugins
are maintained outside of this core source. Other external packages
provide direct access to Xlib functions and objects (
#N4
Pax
) and stream filter objects (
#N5
Filter
).
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.html
plugins
:
Filters/
Contains the
devguide-22.html
import filters
and export filters for SK-files and other formats.
Objects/
devguide-24.html
Plugin 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
#N3
Plugins/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
module
import
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
None
to 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#N2
HierarchyNode
'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
.
devguide-2.html
The Design of Skencil
devguide.html
Developer's Guide
devguide-4.html
Coordinate Systems
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