The Datafile PD-CD 5/DATAFILE_PDCD5.iso/utilities/p/python/pyhtmldoc/h/how_it_wor

Text File | 1996-11-14 | 3KB | 67 lines

<TITLE>How It Works -- Python library reference</TITLE>
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<H1>8.2. How It Works</H1>
Some changes were made to the interpreter:
<P>
<UL>
<LI>•  sys.settrace(func) sets the global trace function
<LI>•  there can also a local trace function (see later)
</UL>
Trace functions have three arguments: (<VAR>frame</VAR>, <VAR>event</VAR>, <VAR>arg</VAR>)
<P>
<DL>
<DT><B><VAR>frame</VAR></B><DD>is the current stack frame
<P>
<DT><B><VAR>event</VAR></B><DD>is a string: <CODE>'call'</CODE>, <CODE>'line'</CODE>, <CODE>'return'</CODE>
or <CODE>'exception'</CODE>
<P>
<DT><B><VAR>arg</VAR></B><DD>is dependent on the event type
<P>
</DL>
A trace function should return a new trace function or None.
Class methods are accepted (and most useful!) as trace methods.
<P>
The events have the following meaning:
<P>
<DL>
<DT><B><CODE>'call'</CODE></B><DD>A function is called (or some other code block entered).  The global
trace function is called; arg is the argument list to the function;
the return value specifies the local trace function.
<P>
<DT><B><CODE>'line'</CODE></B><DD>The interpreter is about to execute a new line of code (sometimes
multiple line events on one line exist).  The local trace function is
called; arg in None; the return value specifies the new local trace
function.
<P>
<DT><B><CODE>'return'</CODE></B><DD>A function (or other code block) is about to return.  The local trace
function is called; arg is the value that will be returned.  The trace
function's return value is ignored.
<P>
<DT><B><CODE>'exception'</CODE></B><DD>An exception has occurred.  The local trace function is called; arg is
a triple (exception, value, traceback); the return value specifies the
new local trace function
<P>
</DL>
Note that as an exception is propagated down the chain of callers, an
<CODE>'exception'</CODE> event is generated at each level.
<P>
Stack frame objects have the following read-only attributes:
<P>
<DL>
<DT><B>f_code</B><DD>the code object being executed
<DT><B>f_lineno</B><DD>the current line number (<CODE>-1</CODE> for <CODE>'call'</CODE> events)
<DT><B>f_back</B><DD>the stack frame of the caller, or None
<DT><B>f_locals</B><DD>dictionary containing local name bindings
<DT><B>f_globals</B><DD>dictionary containing global name bindings
</DL>
Code objects have the following read-only attributes:
<P>
<DL>
<DT><B>co_code</B><DD>the code string
<DT><B>co_names</B><DD>the list of names used by the code
<DT><B>co_consts</B><DD>the list of (literal) constants used by the code
<DT><B>co_filename</B><DD>the filename from which the code was compiled
</DL>