[Py-howto-checkins] CVS: pyhowto doanddont.tex,1.3,1.4

[Moshe Z. <mo...@us...>]

Update of /cvsroot/py-howto/pyhowto
In directory usw-pr-cvs1:/tmp/cvs-serv16918

Modified Files:
	doanddont.tex 
Log Message:
Adding section about backslashes


Index: doanddont.tex
===================================================================
RCS file: /cvsroot/py-howto/pyhowto/doanddont.tex,v
retrieving revision 1.3
retrieving revision 1.4
diff -C2 -r1.3 -r1.4
*** doanddont.tex	2001/03/23 18:25:21	1.3
--- doanddont.tex	2001/03/25 20:05:22	1.4
***************
*** 90,93 ****
--- 90,120 ----
  the rest of your code. Simply do not do that.
  
+ Bad examples:
+ 
+ \begin{verbatim}
+ >>> for name in sys.argv[1:]:
+ >>>     exec "%s=1" % name
+ >>> def func(s, **kw):
+ >>>     for var, val in kw.items():
+ >>>         exec "s.%s=val" % var  # invalid!
+ >>> execfile("handler.py")
+ >>> handle()
+ \end{verbatim}
+ 
+ Good examples:
+ 
+ \begin{verbatim}
+ >>> d = {}
+ >>> for name in sys.argv[1:]:
+ >>>     d[name] = 1
+ >>> def func(s, **kw):
+ >>>     for var, val in kw.items():
+ >>>         setattr(s, var, val)
+ >>> d={}
+ >>> execfile("handle.py", d, d)
+ >>> handle = d['handle']
+ >>> handle()
+ \end{verbatim}
+ 
  \subsection{from module import name1, name2}
  
***************
*** 100,103 ****
--- 127,154 ----
  one module is reloaded, or changes the definition of a function at runtime. 
  
+ Bad example:
+ 
+ \begin{verbatim}
+ # foo.py
+ a = 1
+ 
+ # bar.py
+ from foo import a
+ if something():
+     a = 2 # danger: foo.a != a 
+ \end{verbatim}
+ 
+ Good example:
+ 
+ \begin{verbatim}
+ # foo.py
+ a = 1
+ 
+ # bar.py
+ import foo
+ if something():
+     foo.a = 2
+ \end{verbatim}
+ 
  \subsection{except:}
  
***************
*** 256,259 ****
--- 307,343 ----
  suited to parsing --- assuming you are ready to deal with the
  \exception{ValueError} they raise.
+ 
+ \section{Using Backslash to Continue Statements}
+ 
+ Since Python treats a newline as a statement terminator,
+ and since statements are often more then is comfortable to put
+ in one line, many people do:
+ 
+ \begin{verbatim}
+ if foo.bar()['first'][0] == baz.quux(1, 2)[5:9] and \
+    calculate_number(10, 20) != forbulate(500, 360):
+       pass
+ \end{verbatim}
+ 
+ You should realize that this is dangerous: a stray space after the
+ \code{\\} would make this line wrong, and stray spaces are notoriously
+ hard to see in editors. In this case, at least it would be a syntax
+ error, but if the code was:
+ 
+ \begin{verbatim}
+ value = foo.bar()['first'][0]*baz.quux(1, 2)[5:9] \
+         + calculate_number(10, 20)*forbulate(500, 360)
+ \end{verbatim}
+ 
+ then it would just be subtly wrong.
+ 
+ It is usually much better to use the implicit continuation inside parenthesis:
+ 
+ This version is bulletproof:
+ 
+ \begin{verbatim}
+ value = (foo.bar()['first'][0]*baz.quux(1, 2)[5:9] 
+         + calculate_number(10, 20)*forbulate(500, 360))
+ \end{verbatim}
  
  \end{document}