Check parameter type (4.4.4-4.4.6)

NLTK

It is not necessary to declare type of variables in Python. As a result, unexpected behavior might happen.

>>> def tag(word):
...     if word in ['a', 'the', 'all']:
...             return 'det'
...     else:
...             return 'noun'
... 
>>> tag('the')
'det'
>>> tag('aho')
'noun'
>>> tag(['aho', 'new', 'old'])
'noun'

This is the revised version of the same function. In case the parameter value is neither string nor unicode, an error is triggered.

>>> def tag(word):
...     assert isinstance(word, basestring), "argument to tag() must be a string"
...     if word in ['a', 'the', 'all']:
...             return 'det'
...     else:
...             return 'noun'
... 
>>> tag(['aho', 'new', 'old'])                                              
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in tag
AssertionError: argument to tag() must be a string
>>> tag(u'モリー')
'noun'

Another bad example:

>>> def freq_words(url, freqdist, n):
...     text = nltk.clean_url(url)
...     for word in nltk.word_tokenize(text):
...             freqdist.inc(word.lower())
...     print freqdist.keys()[:n]
... 
>>> constitution = "http://www.archives.gov/national-archives-experience"\
...     "/charters/constitution_transcript.html"
>>> fd = nltk.FreqDist()
>>> freq_words(constitution, fd, 20)
['the', 'of', 'charters', ',', 'bill', 'constitution', 'declaration', 'rights', '-', '.', 'freedom', 'impact', 'independence', 'making']

Why nltk.FreqDist() is called outside of the function? Then change it.

>>> def freq_words(url):
...     freqdist = nltk.FreqDist()
...     text = nltk.clean_url(url)
...     for word in nltk.word_tokenize(text):
...             freqdist.inc(word.lower())
...     return freqdist
... 
>>> fd = freq_words(constitution)
>>> print fd.keys()[:20]
['the', 'of', 'charters', ',', 'bill', 'constitution', 'declaration', 'rights', '-', '.', 'freedom', 'impact', 'independence', 'making']

This link is the general rule of function documentations.
http://www.python.org/dev/peps/pep-0257/