Developer's Guide: General Remarks

The Design of Skencil | Developer's Guide | 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. Import filters and other plugins are maintained outside of this core source. Other external packages provide direct access to Xlib functions and objects (Pax) and stream filter objects (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 plugins:

    `Filters/`

      Contains the import filters and export filters for SK-files and other formats.

    `Objects/`

      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 `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 `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`.

The Design of Skencil | Developer's Guide | Coordinate Systems