First experiments with python’s functools.partial

I started playing with one of the python2.5 features, namely functools.partial() that allows one to effectively “construct variants of existing functions that have some of the parameters filled in” (quote from Functional Programming HOWTO).

In the code below I am trying to preset the second paramater of the operator.lt() built-in function.

 1  #!/usr/bin/env python2.5
 2  """
 3  python 2.5 partial functions experiments (part 1)
 4  """
 5  import operator
 6  import functools
 7
 8  if __name__ == '__main__':
 9      #>>> help(operator.lt)
10      #Help on built-in function lt in module operator:
11      #
12      #lt(...)
13      #    lt(a, b) -- Same as a<b.
14
15      #>>> operator.lt
16      #<built-in function lt>
17
18      less_than_two = functools.partial(operator.lt, b=2)
19      print "1 < 2: ", less_than_two(1)
20      print "2 < 2: ", less_than_two(2)

However, when I run the code I get the following error:

mhr@playground2:~/src/published$ python2.5 partialf3.py
1 < 2:
Traceback (most recent call last):
  File "partialf3.py", line 19, in <module>
    print "1 < 2: ", less_than_two(1)
TypeError: lt() takes no keyword arguments

For some reason python’s built-in functions do not accept keyword arguments. Hence, my attempt to preset the second parameter (but leave the first alone (line 18 above)) does not work.

Now, I understand I can achieve the same effect by using the opposite (greater than) operator (operator.gt) in conjunction with preset positional parameters (see line 18 below).

 1  #!/usr/bin/env python2.5
 2  """
 3  python 2.5 partial functions experiments (part 2)
 4  """
 5  import operator
 6  import functools
 7
 8  if __name__ == '__main__':
 9      #>>> help(operator.gt)
10      #Help on built-in function gt in module operator:
11      #
12      #gt(...)
13      #    gt(a, b) -- Same as a>b.
14
15      #>>> operator.gt
16      #<built-in function gt>
17
18      less_than_two = functools.partial(operator.gt, 2)
19      print "1 < 2: ", less_than_two(1)
20      print "2 < 2: ", less_than_two(2)

This yields the expected behaviour:

mhr@playground2:~/src/published$ python2.5 partialf_2_5.py
1 < 2:  True
2 < 2:  False

The refusal on the part of python’s built-in functions to accept keyword arguments seems odd since it introduces somewhat of an inconsistency (built-in functions differing from “normal” functions in that regard).

I am wondering why this seeming inconsistency was introduced to python, a language that prides itself on a clean design.

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