index.lxp@lxpwrap=c6996_252ehtm.htm

GUI Programming with Python

      <TT
CLASS="VARNAME"
>locals</TT
>.</P
><P
>By default, the <TT
CLASS="VARNAME"
>globals</TT
>
      and <TT
CLASS="VARNAME"
>locals</TT
> arguments of
      <TT
CLASS="FUNCTION"
>eval()</TT
>, <TT
CLASS="FUNCTION"
>exec()</TT
> and
      <TT
CLASS="FUNCTION"
>execfile()</TT
> contain the current contents of
      <TT
CLASS="VARNAME"
>globals</TT
> and <TT
CLASS="VARNAME"
>locals</TT
>, but
      you can alter this&#8212; for instance, to restrict access to
      certain application objects.</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
>Playing with <TT
CLASS="FUNCTION"
>eval()</TT
></A
></H2
><P
><TT
CLASS="FUNCTION"
>eval()</TT
> functions as
        if it executes a single line of code in the Python
        interpreter: it returns a value that represents the result of
        the evaluated expression. If the statement you give to
        <TT
CLASS="FUNCTION"
>eval()</TT
> raises an exception, the
        surrounding code gets that exception, too. Playing around with
        <TT
CLASS="FUNCTION"
>eval()</TT
> will give you a feeling for what
        it can do for you.</P
><DIV
CLASS="FIGURE"
></A
><P
><B
>Figure 20-1. Playing with <TT
CLASS="FUNCTION"
>eval()</TT
></B
></P
><PRE
CLASS="SCREEN"
>boud@calcifer:~/doc/pyqt/ch15 &#62; python
Python 2.0 (#1, Mar  1 2001, 02:42:21)
[GCC 2.95.2 19991024 (release)] on linux2
Type "copyright", "credits" or "license" for more information.
&#62;&#62;&#62; eval
&#60;built-in function eval&#62;
&#62;&#62;&#62; eval("1==1")
1
&#62;&#62;&#62; import string
&#62;&#62;&#62; eval("string.split('bla bla bla')")
['bla', 'bla', 'bla']
&#62;&#62;&#62; eval("string.split('bla bla bla')", {}, {})
Traceback (most recent call last):
  File "&#60;stdin&#62;", line 1, in ?
  File "&#60;string&#62;", line 0, in ?
NameError: There is no variable named 'string'
&#62;&#62;&#62; eval("""from qt import *
... s=QString("bla bla bla")
... print str(s).split()
... """)
Traceback (most recent call last):
  File "&#60;stdin&#62;", line 1, in ?
  File "&#60;string&#62;", line 1
    from qt import *
       ^
SyntaxError: invalid syntax
&#62;&#62;&#62;
      </PRE
></DIV
><P
>First, we take a look at what "eval" is
        for a beast. A built-in function. OK, let's try it out. Yes, 1
        equals 1 evaluates to 1, which means TRUE -
        <TT
CLASS="FUNCTION"
>eval</TT
> neatly returns the result of the
        code it executes. Next, having imported the
        <TT
CLASS="FILENAME"
>string</TT
> module, we use it to split a
        string. Here, <TT
CLASS="FUNCTION"
>eval()</TT
> has access to the
        global namespace, which means it can access the module we just
        imported, so <TT
CLASS="FUNCTION"
>string.split()</TT
> evaluates
        just fine. However, if we try to evaluate the same expression,
        but with empty global and local dictionaries, we get a
        <TT
CLASS="VARNAME"
>NameError</TT
> exception - suddenly
        <TT
CLASS="FUNCTION"
>string</TT
> isn't known anymore. Trying to
        evaluate something more complicated, something that is not a
        single expression that returns a value (even if it's only
        <TT
CLASS="VARNAME"
>None</TT
>)  doesn't work at all - which is why
        <TT
CLASS="FUNCTION"
>exec()</TT
> exists.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
>Playing with <TT
CLASS="FUNCTION"
>exec</TT
></A
></H2
><P
>First, <TT
CLASS="FUNCTION"
>exec</TT
> is
        really a statement, not a function, so it doesn't return
        anything.  Just as with <TT
CLASS="FUNCTION"
>eval()</TT
>,
        exceptions are propagated outside the code block you execute.
        You can feed <TT
CLASS="FUNCTION"
>exec</TT
> a string, a compiled
        code object or an open file. The file will be parsed until an
        EOF (end-of-file) occurs, and executed. The same rules hold
        for the global and local namespace dictionaries as with
        <TT
CLASS="FUNCTION"
>eval()</TT
> - but keep in mind that running
        <TT
CLASS="FUNCTION"
>exec</TT
> might add new items to those
        namespaces.</P
><DIV
CLASS="FIGURE"
></A
><P
><B
>Figure 20-2. Playing with <TT
CLASS="FUNCTION"
>exec</TT
></B
></P
><PRE
CLASS="SCREEN"
>boud@calcifer:~/doc/pyqt/ch15 &#62; python
Python 2.0 (#1, Mar  1 2001, 02:42:21)
[GCC 2.95.2 19991024 (release)] on linux2
Type "copyright", "credits" or "license" for more information.
&#62;&#62;&#62; globals()
{'__doc__': None, '__name__': '__main__',
 '__builtins__': &#60;module '__builtin__' (built-in)&#62;}
&#62;&#62;&#62; code = """
... import qt
... s = qt.QString("bla bla bla")
... print string.split(str(s))
... """
&#62;&#62;&#62; exec code
Traceback (most recent call last):
  File "&#60;stdin&#62;", line 1, in ?
  File "&#60;string&#62;", line 4, in ?
NameError: There is no variable named 'string'
&#62;&#62;&#62; import string
&#62;&#62;&#62; exec code
['bla', 'bla', 'bla']
&#62;&#62;&#62; globals()
{'__doc__': None, 'string': &#60;module 'string' from
'/usr/lib/python2.0/string.pyc'&#62;, '__name__':
 '__main__', '__builtins__': &#60;module '__builtin__'
 (built-in)&#62;, 'qt': &#60;module 'qt' from
'/usr/lib/python2.0/site-packages/qt.py'&#62;,
 'code': '\012import qt\012s = qt.QString("bla bla bla")\012print
string.split(str(s))\012', 's': &#60;qt.QString instance at 0x8278af4&#62;}
&#62;&#62;&#62;
        </PRE
></DIV
><P
>First, we create a string that contains
        the bit of Python we want to execute. Note how it imports the
        <TT
CLASS="FILENAME"
>qt</TT
> module, and how it uses the
        <TT
CLASS="FILENAME"
>string</TT
> module. Executing the code doesn't
        work: it throws a <TT
CLASS="VARNAME"
>NameError</TT
> because
        <TT
CLASS="FILENAME"
>string</TT
> isn't known. Importing
        <TT
CLASS="FILENAME"
>string</TT
> into the global namespace makes it
        also available to <TT
CLASS="FUNCTION"
>exec</TT
>, of course.
        Executing the <TT
CLASS="VARNAME"
>code</TT
> string now succeeds, and
        a quick peek in <TT
CLASS="VARNAME"
>globals</TT
> learns us that the
        module <TT
CLASS="FILENAME"
>qt</TT
> has been added.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
>Playing with <TT
CLASS="FUNCTION"
>execfile()</TT
></A
></H2
><P
>The <TT
CLASS="FUNCTION"
>execfile()</TT
>
        statement is rarely used; after all, it can't do anything
        beyond what the plain <TT
CLASS="FUNCTION"
>exec</TT
> statement
        already does. <TT
CLASS="FUNCTION"
>execfile()</TT
> functions
        exactly the same as <TT
CLASS="FUNCTION"
>exec</TT
>, except that the
        first argument must be a filename (it doesn't need to be an
        open file object). Note that <TT
CLASS="FUNCTION"
>execfile()</TT
>
        differs from <TT
CLASS="LITERAL"
>import</TT
> in that it doesn't
        create a new module in the global namespace. Note the
        difference between <TT
CLASS="FUNCTION"
>execfile()</TT
> and
        <TT
CLASS="LITERAL"
>import</TT
> in the following output:</P
><DIV
CLASS="FIGURE"
></A
><P
><B
>Figure 20-3. Playing with
      <TT
CLASS="FUNCTION"
>execfile()</TT
></B
></P
><PRE
CLASS="SCREEN"
>Python 2.0 (#1, Mar  1 2001, 02:42:21)
[GCC 2.95.2 19991024 (release)] on linux2
Type "copyright", "credits" or "license" for more information.
&#62;&#62;&#62; execfile("main.py")
Initializing configuration
{'kalam': &#60;kalamapp.KalamApp instance at 0x825f014&#62;,
 'app': &#60;qt.QApplication instance at 0x814a2a4&#62;, 'args': ['']}
Saving configuration
&#62;&#62;&#62; import main
&#62;&#62;&#62; globals()
{..., 'main': &#60;module 'main' from 'main.py'&#62;, ... }
        </PRE
></DIV
><P
>In the middle of all the
        <TT
CLASS="FILENAME"
>qt</TT
> classes the <TT
CLASS="FILENAME"
>main</TT
>
        module of <SPAN
CLASS="APPLICATION"
>Kalam</SPAN
> imports into
        <TT
CLASS="VARNAME"
>globals</TT
>, we find the
        <TT
CLASS="FILENAME"
>main</TT
> module itself, which isn't there if
        we just <TT
CLASS="FILENAME"
>execfile</TT
>
        <TT
CLASS="FILENAME"
>main.py</TT
>.</P
></DIV
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