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.

f = open("www")
f.read()

>>> 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()

>>> 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()

# foo.py
a = 1

# bar.py
from foo import a
if something():
    a = 2 # danger: foo.a != a

# foo.py
a = 1

# bar.py
import foo
if something():
    foo.a = 2

try:
    foo = opne("file") # misspelled "open"
except:
    sys.exit("could not open file!")

try:
    foo = opne("file") # will be changed to "open" as soon as we run it
except IOError:
    sys.exit("could not open file")

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 os.path.exists() is made and the time open() is called. That means the last line will throw an 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 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, 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 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):
    with open(file) as fp:
        return fp.readline()

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 than inventing your own.

A useful module very few people know about is os.path. It always has the correct path arithmetic for your operating system, and will usually be much better than whatever you come up with yourself.

Compare:

# ugh!
return dir+"/"+file
# better
return os.path.join(dir, file)

More useful functions in os.path: basename(), dirname() and splitext().

There are also many useful built-in functions people seem not to be aware of for some reason: min() and max() can find the minimum/maximum of any sequence with comparable semantics, for example, yet many people write their own max()/min(). Another highly useful function is reduce(). A classical use of 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 reduce() adds up all the numbers, and the rest is just some pre- and postprocessing.

On the same note, note that float(), int() and long() all accept arguments of type string, and so are suited to parsing — assuming you are ready to deal with the ValueError they raise.

Using Backslash to Continue Statements

Since Python treats a newline as a statement terminator, and since statements are often more than 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))