--- Python-Docs-2.4/./howto/doanddont.tex	1970-01-01 09:00:00.000000000 +0900
+++ Python-Docs-2.5/./howto/doanddont.tex	2006-07-14 16:20:04.000000000 +0900
@@ -0,0 +1,344 @@
+\documentclass{howto}
+
+\title{Idioms and Anti-Idioms in Python}
+
+\release{0.00}
+
+\author{Moshe Zadka}
+\authoraddress{howto@zadka.site.co.il}
+
+\begin{document}
+\maketitle
+
+This document is placed in the public doman.
+
+\begin{abstract}
+\noindent
+This document can be considered a companion to the tutorial. It
+shows how to use Python, and even more importantly, how {\em not}
+to use Python. 
+\end{abstract}
+
+\tableofcontents
+
+\section{Language Constructs You Should Not Use}
+
+While Python has relatively few gotchas compared to other languages, it
+still has some constructs which are only useful in corner cases, or are
+plain dangerous. 
+
+\subsection{from module import *}
+
+\subsubsection{Inside Function Definitions}
+
+\code{from module import *} is {\em invalid} inside function definitions.
+While many versions of Python do no check for the invalidity, it does not
+make it more valid, no more then having a smart lawyer makes a man innocent.
+Do not use it like that ever. Even in versions where it was accepted, it made
+the function execution slower, because the compiler could not be certain
+which names are local and which are global. In Python 2.1 this construct
+causes warnings, and sometimes even errors.
+
+\subsubsection{At Module Level}
+
+While it is valid to use \code{from module import *} at module level it
+is usually a bad idea. For one, this loses an important property Python
+otherwise has --- you can know where each toplevel name is defined by
+a simple "search" function in your favourite editor. You also open yourself
+to trouble in the future, if some module grows additional functions or
+classes. 
+
+One of the most awful question asked on the newsgroup is why this code:
+
+\begin{verbatim}
+f = open("www")
+f.read()
+\end{verbatim}
+
+does not work. Of course, it works just fine (assuming you have a file
+called "www".) But it does not work if somewhere in the module, the
+statement \code{from os import *} is present. The \module{os} module
+has a function called \function{open()} which returns an integer. While
+it is very useful, shadowing builtins is one of its least useful properties.
+
+Remember, you can never know for sure what names a module exports, so either
+take what you need --- \code{from module import name1, name2}, or keep them in
+the module and access on a per-need basis --- 
+\code{import module;print module.name}.
+
+\subsubsection{When It Is Just Fine}
+
+There are situations in which \code{from module import *} is just fine:
+
+\begin{itemize}
+
+\item The interactive prompt. For example, \code{from math import *} makes
+      Python an amazing scientific calculator.
+
+\item When extending a module in C with a module in Python.
+
+\item When the module advertises itself as \code{from import *} safe.
+
+\end{itemize}
+
+\subsection{Unadorned \keyword{exec}, \function{execfile} and friends}
+
+The word ``unadorned'' refers to the use without an explicit dictionary,
+in which case those constructs evaluate code in the {\em current} environment.
+This is dangerous for the same reasons \code{from import *} is dangerous ---
+it might step over variables you are counting on and mess up things for
+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}
+
+This is a ``don't'' which is much weaker then the previous ``don't''s
+but is still something you should not do if you don't have good reasons
+to do that. The reason it is usually bad idea is because you suddenly
+have an object which lives in two seperate namespaces. When the binding
+in one namespace changes, the binding in the other will not, so there
+will be a discrepancy between them. This happens when, for example,
+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:}
+
+Python has the \code{except:} clause, which catches all exceptions.
+Since {\em every} error in Python raises an exception, this makes many
+programming errors look like runtime problems, and hinders
+the debugging process.
+
+The following code shows a great example:
+
+\begin{verbatim}
+try:
+    foo = opne("file") # misspelled "open"
+except:
+    sys.exit("could not open file!")
+\end{verbatim}
+
+The second line triggers a \exception{NameError} which is caught by the
+except clause. The program will exit, and you will have no idea that
+this has nothing to do with the readability of \code{"file"}.
+
+The example above is better written
+
+\begin{verbatim}
+try:
+    foo = opne("file") # will be changed to "open" as soon as we run it
+except IOError:
+    sys.exit("could not open file")
+\end{verbatim}
+
+There are some situations in which the \code{except:} clause is useful:
+for example, in a framework when running callbacks, it is good not to
+let any callback disturb the framework.
+
+\section{Exceptions}
+
+Exceptions are a useful feature of Python. You should learn to raise
+them whenever something unexpected occurs, and catch them only where
+you can do something about them.
+
+The following is a very popular anti-idiom
+
+\begin{verbatim}
+def get_status(file):
+    if not os.path.exists(file):
+        print "file not found"
+        sys.exit(1)
+    return open(file).readline()
+\end{verbatim}
+
+Consider the case the file gets deleted between the time the call to 
+\function{os.path.exists} is made and the time \function{open} is called.
+That means the last line will throw an \exception{IOError}. The same would
+happen if \var{file} exists but has no read permission. Since testing this
+on a normal machine on existing and non-existing files make it seem bugless,
+that means in testing the results will seem fine, and the code will get
+shipped. Then an unhandled \exception{IOError} escapes to the user, who
+has to watch the ugly traceback.
+
+Here is a better way to do it.
+
+\begin{verbatim}
+def get_status(file):
+    try:
+        return open(file).readline()
+    except (IOError, OSError):
+        print "file not found"
+        sys.exit(1)
+\end{verbatim}
+
+In this version, *either* the file gets opened and the line is read
+(so it works even on flaky NFS or SMB connections), or the message
+is printed and the application aborted.
+
+Still, \function{get_status} makes too many assumptions --- that it
+will only be used in a short running script, and not, say, in a long
+running server. Sure, the caller could do something like
+
+\begin{verbatim}
+try:
+    status = get_status(log)
+except SystemExit:
+    status = None
+\end{verbatim}
+
+So, try to make as few \code{except} clauses in your code --- those will
+usually be a catch-all in the \function{main}, or inside calls which
+should always succeed.
+
+So, the best version is probably
+
+\begin{verbatim}
+def get_status(file):
+    return open(file).readline()
+\end{verbatim}
+
+The caller can deal with the exception if it wants (for example, if it 
+tries several files in a loop), or just let the exception filter upwards
+to {\em its} caller.
+
+The last version is not very good either --- due to implementation details,
+the file would not be closed when an exception is raised until the handler
+finishes, and perhaps not at all in non-C implementations (e.g., Jython).
+
+\begin{verbatim}
+def get_status(file):
+    fp = open(file)
+    try:
+        return fp.readline()
+    finally:
+        fp.close()
+\end{verbatim}
+
+\section{Using the Batteries}
+
+Every so often, people seem to be writing stuff in the Python library
+again, usually poorly. While the occasional module has a poor interface,
+it is usually much better to use the rich standard library and data
+types that come with Python then inventing your own.
+
+A useful module very few people know about is \module{os.path}. It 
+always has the correct path arithmetic for your operating system, and
+will usually be much better then whatever you come up with yourself.
+
+Compare:
+
+\begin{verbatim}
+# ugh!
+return dir+"/"+file
+# better
+return os.path.join(dir, file)
+\end{verbatim}
+
+More useful functions in \module{os.path}: \function{basename}, 
+\function{dirname} and \function{splitext}.
+
+There are also many useful builtin functions people seem not to be
+aware of for some reason: \function{min()} and \function{max()} can
+find the minimum/maximum of any sequence with comparable semantics,
+for example, yet many people write their own
+\function{max()}/\function{min()}. Another highly useful function is
+\function{reduce()}. A classical use of \function{reduce()}
+is something like
+
+\begin{verbatim}
+import sys, operator
+nums = map(float, sys.argv[1:])
+print reduce(operator.add, nums)/len(nums)
+\end{verbatim}
+
+This cute little script prints the average of all numbers given on the
+command line. The \function{reduce()} adds up all the numbers, and
+the rest is just some pre- and postprocessing.
+
+On the same note, note that \function{float()}, \function{int()} and
+\function{long()} all accept arguments of type string, and so are
+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}
