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  Idioms and Anti-Idioms in Python  

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: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))