index.lxp@lxpwrap=c88_252ehtm.htm

GUI Programming with Python

></LI
><LI
><P
>Built-in regular expression engine
          that works on both regular and Unicode strings.
          </P
></LI
><LI
><P
>Use of indentation instead of braces
          <TT
CLASS="FUNCTION"
>begin</TT
>/<TT
CLASS="FUNCTION"
>end</TT
> pairs to
          delimit blocks of code. This practically forces readable
          code.</P
></LI
></UL
><P
>Your Python code resides in files, ending
      with <TT
CLASS="FILENAME"
>.py</TT
> suffix. These files can be grouped
      in modules, in the form of directories with an indexfile called
      <TT
CLASS="FILENAME"
>__init__.py</TT
>, and you can import elements
      from modules and files in other files. There is one file you use
      to start your application. It will usually simply import the
      necessary modules and start the application explicitly in a
      <TT
CLASS="FUNCTION"
>main (args)</TT
> function.</P
><P
>Maybe the introduction is bit <SPAN
><I
CLASS="EMPHASIS"
>early</I
></SPAN
> to
      start with actual code examples, but let's have an example of a
      Python bootstrap script anyway:</P
><DIV
CLASS="EXAMPLE"
></A
><P
><B
>Example 1-1. Bootstrapping a Python application</B
></P
><PRE
CLASS="PROGRAMLISTING"
>#!/usr/bin/env python                                      <IMG
SRC="images/callouts/1.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(1)"></A
>
#
# bootstrap.py
#

import sys                                                 <IMG
SRC="images/callouts/2.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(2)"></A
>
from myapp import SomeClass                                <IMG
SRC="images/callouts/3.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(3)"></A
>

def main(args):                                            <IMG
SRC="images/callouts/4.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(4)"></A
>
    class=SomeClass(args)
    class.exec_loop()

if __name__=="__main__":                                   <IMG
SRC="images/callouts/5.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(5)"></A
>
    main(sys.argv)
        </PRE
><DIV
CLASS="CALLOUTLIST"
><DL
COMPACT="COMPACT"
><DT
><A href="index.lxp@lxpwrap=c88_252ehtm.htm#HASHBANG"><IMG
SRC="images/callouts/1.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(1)"></A
></DT
><DD
>The
              so-called &#8216;hash-bang' trick is useful on Unix systems
              only. If the first line of any text file starts with #!,
              then the system will try to execute the application that
              follows the #! with the rest of the file as input. In
              this case, the <B
CLASS="COMMAND"
>env</B
> utility starts
              <B
CLASS="COMMAND"
>python</B
>, which runs the rest of the
              script.</DD
><DT
><A href="index.lxp@lxpwrap=c88_252ehtm.htm#IMPSYS"><IMG
SRC="images/callouts/2.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(2)"></A
></DT
><DD
>The
              standard Python module <B
CLASS="COMMAND"
>sys</B
> handles
              tasks like passing on command-line arguments and lots of
              other things. Here we import the module, so we can pass
              the command-line arguments to the application. </DD
><DT
><A href="index.lxp@lxpwrap=c88_252ehtm.htm#IMPMYAPP"><IMG
SRC="images/callouts/3.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(3)"></A
></DT
><DD
>All
              application code is in separate modules; the first of
              these we import here.</DD
><DT
><A href="index.lxp@lxpwrap=c88_252ehtm.htm#DEFMAIN"><IMG
SRC="images/callouts/4.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(4)"></A
></DT
><DD
>This is
              the definition of the main function. By encapsulating
              this code in a function, it won't get run if this file
              were imported from another file.</DD
><DT
><A href="index.lxp@lxpwrap=c88_252ehtm.htm#IFNAME"><IMG
SRC="images/callouts/5.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(5)"></A
></DT
><DD
>In this
              line, we check if this is a top-level script, instead of
              a file imported from another file. This is done by
              looking at the variable <TT
CLASS="VARNAME"
>__name__</TT
>. If
              this is the toplevel file, then the
              <TT
CLASS="FUNCTION"
>main(args)</TT
> is run.</DD
></DL
></DIV
></DIV
><P
>Python is, like Java, a language that is
      compiled to bytecode. Python uses a virtual machine to run the
      bytecode. This virtual machine is written in C and interprets
      each byte-code instruction, translates it to real machine code
      and then runs it. The Python virtual machine differs from the
      Java virtual machine in that the byte-code instructions are a
      bit more high-level, and that there are no JIT-compilers that
      pre-compile chunks of byte-code to native machine code.</P
><P
>The translation from Python code to
      byte-code only happens once: Python saves a compiled version of
      your code in another file with the extension
      <TT
CLASS="FILENAME"
>.pyc</TT
>, or an optimized compiled version of
      your code that removes assert statements and line-number
      tracking in a file with the extension
      <TT
CLASS="FILENAME"
>.pyo</TT
>.</P
><P
>However, that is only done with Python
      files that are imported from other files: the bootstrap script
      will be compiled to bytecode every time you run it, but python
      will create a <TT
CLASS="FILENAME"
>myapp.pyc</TT
> from a file
      <TT
CLASS="FILENAME"
>myapp.py</TT
> (which is not shown here).</P
><P
>Interpreted languages, even byte-code
      interpreted languages, have a reputation for sluggishness. On
      the other hand, modern computers have a well-deserved reputation
      for excessive processing power. The combination means that an
      application written in a interpreted language can be fast enough
      for almost any needs.</P
><P
>Certainly, anyone who has ever tried to
      use a full-scale Java GUI application will know the exact
      meaning of the expression &#8216;slow as frozen treacle'. There
      are several reasons for the abominable slowness of Java
      applications, the most important of which is the fact that all
      Java Swing gui elements are also written in Java. Every pixel is
      put on screen by Java. Python, on the other hand, makes clever
      use of available GUI libraries that are coded in C or C++ and
      thus run as native machine code.</P
><P
>The ease with which Python can make use
      of native libraries is one of its strong points. Thanks to this
      extensibility, you can write the logic of your application in
      Python, and later rewrite the bottlenecks in C or C++. But even
      without writing extension libraries, I have never encountered
      any problem with the performance of a Python application.
    </P
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