Functional Infix Implementation in Python

Question

I have a current implementation like this:

class Infix(object):
  def __init__(self, func):
    self.func = func
  def __or__(self, other):
    return self.func(other)
  def __ror__(self, other):
    return Infix(partial(self.func, other))
  def __call__(self, v1, v2):
    return self.func(v1, v2)

@Infix
def Map(data, func):
  return list(map(func,data))

This is great, it works as expected, however I want to also expand this implementation to allow for a left side only solution. If somebody can showcase a solution AND explanation, that would be phenomenal.

Here is an example of what I would like to do...

 valLabels['annotations'] \
    |Map| (lambda x: x['category_id']) \
    |Unique|

Where Unique is defined as below...

@Infix
def Unique(data):
    return set(data)

Thanks!

Isn't that an unavoidable syntax error? You can't end a python expression with |. — drhagen
– drhagen, Commented Aug 15, 2016 at 20:54
Have you looked at coconut either for your use case, or at their source code? They have an infix notation, for instance, and I believe their syntax is a superset of Python. — Two-Bit Alchemist
– Two-Bit Alchemist, Commented Aug 15, 2016 at 20:56
@Two-BitAlchemist nah, I'm trying to stay with as much vanilla python stuff I have due to a variety of c integrations etc, just gets hairy to move outside that. — David Crook
– David Crook, Commented Aug 15, 2016 at 21:24

RoadieRich · Accepted Answer · 2016-08-16 14:25:51Z

2

If you don't mind dropping the final |, you could do

class InfixR(object):
  def __init__(self, func):
    self.func = func
  def __ror__(self, other):
    return self.func(other)
  def __call__(self, v1):
    return self.func(v1)

@InfixR
def Unique(data):
  return set(data)

Then your expression would look like

valLabels['annotations'] \
    |Map| (lambda x: x['category_id']) \
    |Unique

Your original Infix class is (technically) abusing the bitwise or operator: |Map| is nothing special, it's just value | Map | my_lambda, a "bitwise or" of a list, an object, and a lambda, with a couple of spaces removed, and some newlines inserted (using \ to prevent the interpretter from trying to treat each line separately).

In custom classes, you can implement many of the usual operators using __double_underscore__ methods, in the case of bitwise or, they are __or__ and __ror__.

When the python interpreter encounters the | operator, it first looks at the object to the right, to see if it has a __or__ method. It then calls left.__or__(right). If that is not defined or returns NotImplemented, it looks at the object on the right, for __ror__ (reversed or), and calls right.__ror__(left).

The other part of this is the decorator notation.

When you say

@Infix
def Unique(data):
    return set(data)

The interpretter expands that into

def Unique(data):
    return set(data)

Unique = Infix(Unique)

So you get an Infix instance, which has __or__ and __ror__ methods, and a __call__ method. As you might guess, my_obj.__call__() is called when you invoke my_oby().

edited Aug 16, 2016 at 14:25

answered Aug 15, 2016 at 21:03

RoadieRich

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1 Comment

David Crook Over a year ago

You mind adding a little explanation about whats up with this section of code? I've mashed this together from a variety of sources and not really sure whats going on with it.

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