# A few things to remember while coding in Python.

##### - 17 May 2012 -

• Zen of Python

Learning the culture that surrounds a language brings you one step closer to being a better programmer. If you haven’t read the Zen of Python yet open a Python prompt and type import this. For each of the item on the list you can find examples here http://artifex.org/~hblanks/talks/2011/pep20_by_example.html

One caught my attention:

Beautiful is better than ugly

Give me a function that takes a list of numbers and returns only the even ones, divided by two.

  #-----------------------------------------------------------------------

halve_evens_only = lambda nums: map(lambda i: i/2, filter(lambda i: not i%2, nums))

#-----------------------------------------------------------------------

def halve_evens_only(nums):
return [i/2 for i in nums if not i % 2]

• Remember the very simple things in Python

• Swaping two variables:

  a, b = b, a

• The step argument in slice operators. For example:

  a = [1,2,3,4,5]
>>> a[::2]  # iterate over the whole list in 2-increments
[1,3,5]


The special case x[::-1] is a useful idiom for ‘x reversed’.

  >>> a[::-1]
[5,4,3,2,1]


UPDATE: Do keep in mind x.reverse() reverses the list in place and slices gives you the ability to do this:

      >>> x[::-1]
[5, 4, 3, 2, 1]

>>> x[::-2]
[5, 3, 1]

• Don’t use mutables as defaults

  def function(x, l=[]):          # Don't do this

def function(x, l=None):        # Way better
if l is None:
l = []


UPDATE: I realise I haven’t explained why. I would recommend reading the article by Fredrik Lundh. In short it is by design that this happens. “Default parameter values are always evaluated when, and only when, the “def” statement they belong to is executed;”

• Use iteritems rather than items

iteritems uses generators and thus are better while iterating through very large lists.

  d = {1: "1", 2: "2", 3: "3"}

for key, val in d.items()       # builds complete list when called.

for key, val in d.iteritems()   # calls values only when requested.


This is similar with range and xrange where xrange only calls values when requested.

UPDATE: Do note that the iteritems, iterkeys, itervalues are removed from Python 3.x. The dict.keys(), dict.items() and dict.values() return views instead of lists. http://docs.python.org/release/3.1.5/whatsnew/3.0.html#views-and-iterators-instead-of-lists

• Use isinstance rather than type

Don’t do

  if type(s) == type(""): ...
if type(seq) == list or \
type(seq) == tuple: ...


rather:

  if isinstance(s, basestring): ...
if isinstance(seq, (list, tuple)): ...


For why not to do so: http://stackoverflow.com/a/1549854/504262

Notice I used basestring and not str as you might be trying to check if a unicode object is a string. For example:

  >>> a=u'aaaa'
>>> print isinstance(a, basestring)
True
>>> print isinstance(a, str)
False


This is because in Python versions below 3.0 there are two string types str and unicode:

        object
|
|
basestring
/ \
/   \
str  unicode

• Learn the various collections

Python has various container datatypes which are better alternative to the built-in containers like list and dict for specific cases.

Generally most use this:

UPDATE: I’m sure most do not use this. Carelessness from my side. A few may consider writing it this way:

  freqs = {}
try:
freqs[c] += 1
except:
freqs[c] = 1


Some may say a better solution would be:

  freqs = {}
freqs[c] = freqs.get(c, 0) + 1


Rather go for the collection type defaultdict

  from collections import defaultdict
freqs = defaultdict(int)
freqs[c] += 1


Other collections

  namedtuple()	# factory function for creating tuple subclasses with named fields
deque	        # list-like container with fast appends and pops on either end
Counter	        # dict subclass for counting hashable objects
OrderedDict	    # dict subclass that remembers the order entries were added
defaultdict	    # dict subclass that calls a factory function to supply missing values


UPDATE: As noted by a few in Hacker News I could have used Counter instead of defaultdict.

  >>> from collections import Counter
>>> c['a']
5

• When creating classes Python’s magic methods

  __eq__(self, other)      # Defines behavior for the equality operator, ==.
__ne__(self, other)      # Defines behavior for the inequality operator, !=.
__lt__(self, other)      # Defines behavior for the less-than operator, <.
__gt__(self, other)      # Defines behavior for the greater-than operator, >.
__le__(self, other)      # Defines behavior for the less-than-or-equal-to operator, <=.
__ge__(self, other)      # Defines behavior for the greater-than-or-equal-to operator, >=.


There are several others.

• Conditional Assignments

  x = 3 if (y == 1) else 2   It does exactly what it sounds like: "assign 3 to x if y is 1, otherwise assign 2 to x". You can also chain it if you have something more complicated:

x = 3 if (y == 1) else 2 if (y == -1) else 1


Though at a certain point, it goes a little too far.

Note that you can use if … else in any expression. For example:

  (func1 if y == 1 else func2)(arg1, arg2)


Here func1 will be called if y is 1 and func2, otherwise. In both cases the corresponding function will be called with arguments arg1 and arg2.

Analogously, the following is also valid:

  x = (class1 if y == 1 else class2)(arg1, arg2)


where class1 and class2 are two classes.

• Use the Ellipsis when necessary.

UPDATE: As one commenter mentioned in Hacker News “Using Ellipsis for getting all items is a violation of the Only One Way To Do It principle. The standard notation is [:].” I do agree with him. A better example is given using numpy in stackoverflow:

The ellipsis is used to slice higher-dimensional data structures.

It’s designed to mean at this point, insert as many full slices (:) to extend the multi-dimensional slice to all dimensions.

Example:

  >>> from numpy import arange
>>> a = arange(16).reshape(2,2,2,2)


Now, you have a 4-dimensional matrix of order 2x2x2x2. To select all first elements in the 4th dimension, you can use the ellipsis notation

  >>> a[..., 0].flatten()
array([ 0,  2,  4,  6,  8, 10, 12, 14])


which is equivalent to

  >>> a[:,:,:,0].flatten()
array([ 0,  2,  4,  6,  8, 10, 12, 14])


Previous suggestion.

When creating a class you can use __getitem__ to make you class’ object work like a dictionary. Take this class as an example:

  class MyClass(object):
def __init__(self, a, b, c, d):
self.a, self.b, self.c, self.d = a, b, c, d

def __getitem__(self, item):
return getattr(self, item)

x = MyClass(10, 12, 22, 14)


Because of __getitem__ you will be able to get the value of a in the object x by x['a']. This is probably a known fact.

This object is used to extend the Python slicing.(http://docs.python.org/library/stdtypes.html#bltin-ellipsis-object). Thus if we add a clause:

  def __getitem__(self, item):
if item is Ellipsis:
return [self.a, self.b, self.c, self.d]
else:
return getattr(self, item)


We can use x[...] to get a list containing all the items.

  >>> x = MyClass(11, 34, 23, 12)
>>> x[...]
[11, 34, 23, 12]