FCL/sf/adapt/qemu: comparison symbian-qemu-0.9.1-12/python-2.6.1/Doc/howto/doanddont.rst

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+************************************
+Idioms and Anti-Idioms in Python
+************************************
+:Author: Moshe Zadka
+This document is placed in the public domain.
+.. topic:: Abstract
+This document can be considered a companion to the tutorial. It shows how to use
+Python, and even more importantly, how *not* to use Python.
+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.
+from module import \*
+---------------------
+Inside Function Definitions
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+``from module import *`` is *invalid* inside function definitions. While many
+versions of Python do not 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.
+At Module Level
+^^^^^^^^^^^^^^^
+While it is valid to use ``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::
+f = open("www")
+f.read()
+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 ``from os
+import *`` is present. The :mod:`os` module has a function called :func:`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 --- ``from module import name1, name2``, or keep them in the
+module and access on a per-need basis ---  ``import module;print module.name``.
+When It Is Just Fine
+^^^^^^^^^^^^^^^^^^^^
+There are situations in which ``from module import *`` is just fine:
+* The interactive prompt. For example, ``from math import *`` makes Python an
+amazing scientific calculator.
+* When extending a module in C with a module in Python.
+* When the module advertises itself as ``from import *`` safe.
+Unadorned :keyword:`exec`, :func:`execfile` and friends
+-------------------------------------------------------
+The word "unadorned" refers to the use without an explicit dictionary, in which
+case those constructs evaluate code in the *current* environment. This is
+dangerous for the same reasons ``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::
+>>> 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()
+Good examples::
+>>> 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()
+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 separate 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::
+# foo.py
+a = 1
+# bar.py
+from foo import a
+if something():
+a = 2 # danger: foo.a != a
+Good example::
+# foo.py
+a = 1
+# bar.py
+import foo
+if something():
+foo.a = 2
+except:
+-------
+Python has the ``except:`` clause, which catches all exceptions. Since *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::
+try:
+foo = opne("file") # misspelled "open"
+except:
+sys.exit("could not open file!")
+The second line triggers a :exc:`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 ``"file"``.
+The example above is better written ::
+try:
+foo = opne("file") # will be changed to "open" as soon as we run it
+except IOError:
+sys.exit("could not open file")
+There are some situations in which the ``except:`` clause is useful: for
+example, in a framework when running callbacks, it is good not to let any
+callback disturb the framework.
+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 ::
+def get_status(file):
+if not os.path.exists(file):
+print "file not found"
+sys.exit(1)
+return open(file).readline()
+Consider the case the file gets deleted between the time the call to
+:func:`os.path.exists` is made and the time :func:`open` is called. That means
+the last line will throw an :exc:`IOError`. The same would happen if *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
+:exc:`IOError` escapes to the user, who has to watch the ugly traceback.
+Here is a better way to do it. ::
+def get_status(file):
+try:
+return open(file).readline()
+except (IOError, OSError):
+print "file not found"
+sys.exit(1)
+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, :func:`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 ::
+try:
+status = get_status(log)
+except SystemExit:
+status = None
+So, try to make as few ``except`` clauses in your code --- those will usually be
+a catch-all in the :func:`main`, or inside calls which should always succeed.
+So, the best version is probably ::
+def get_status(file):
+return open(file).readline()
+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 *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). ::
+def get_status(file):
+fp = open(file)
+try:
+return fp.readline()
+finally:
+fp.close()
+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 :mod:`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::
+# ugh!
+return dir+"/"+file
+# better
+return os.path.join(dir, file)
+More useful functions in :mod:`os.path`: :func:`basename`,  :func:`dirname` and
+:func:`splitext`.
+There are also many useful builtin functions people seem not to be aware of for
+some reason: :func:`min` and :func:`max` can find the minimum/maximum of any
+sequence with comparable semantics, for example, yet many people write their own
+:func:`max`/:func:`min`. Another highly useful function is :func:`reduce`. A
+classical use of :func:`reduce` is something like ::
+import sys, operator
+nums = map(float, sys.argv[1:])
+print reduce(operator.add, nums)/len(nums)
+This cute little script prints the average of all numbers given on the command
+line. The :func:`reduce` adds up all the numbers, and the rest is just some
+pre- and postprocessing.
+On the same note, note that :func:`float`, :func:`int` and :func:`long` all
+accept arguments of type string, and so are suited to parsing --- assuming you
+are ready to deal with the :exc:`ValueError` they raise.
+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::
+if foo.bar()['first'][0] == baz.quux(1, 2)[5:9] and \
+calculate_number(10, 20) != forbulate(500, 360):
+pass
+You should realize that this is dangerous: a stray space after the ``\`` 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::
+value = foo.bar()['first'][0]*baz.quux(1, 2)[5:9] \
++ calculate_number(10, 20)*forbulate(500, 360)
+then it would just be subtly wrong.
+It is usually much better to use the implicit continuation inside parenthesis:
+This version is bulletproof::
+value = (foo.bar()['first'][0]*baz.quux(1, 2)[5:9]
++ calculate_number(10, 20)*forbulate(500, 360))