sanjaysiddhanti / variants

Library providing syntactic sugar for creating variant forms of a canonical function

variants

variants is a library that provides syntactic sugar for creating alternate forms of functions and other callables, in the same way that alternate constructors are class methods that provide alternate forms of the constructor function.

To create a function with variants, simply decorate the primary form with @variants.primary, which then adds the .variant decorator to the original function, which can be used to register new variants. Here is a simple example of a function that prints text, with variants that specify the source of the text to print:

import variants

@variants.primary
def print_text(txt):
    print(txt)

@print_text.variant('from_file')
def print_text(fobj):
    print_text(fobj.read())

@print_text.variant('from_filepath')
def print_text(fpath):
    with open(fpath, 'r') as f:
        print_text.from_file(f)

@print_text.variant('from_url')
def print_text(url):
    import requests
    r = requests.get(url)
    print_text(r.text)

print_text and its variants can be used as such:

print_text('Hello, world!')                 # Hello, world!

# Create a text file
with open('hello_world.txt', 'w') as f:
    f.write('Hello, world (from file)')

# Print from an open file object
with open('hello_world.txt', 'r') as f:
    print_text.from_file(f)                 # Hello, world (from file)

# Print from the path to a file object
print_text.from_filepath('hello_world.txt') # Hello, world (from file)

# Print from a URL
hw_url = 'https://ganssle.io/files/hello_world.txt'
print_text.from_url(hw_url)                 # Hello, world! (from url)

Differences from singledispatch

While variants and singledispatch are both intended to provide alternative implementations to a primary function, the overall aims are slightly different. singledispatch transparently dispatches to variant functions based on the type of the argument, whereas variants provides explicit alternative forms of the function. Note that in the above example, both print_text.from_filepath and print_text.from_url take a string, one representing a file path and one representing a URL.

Additionally, the variants is compatible with singledispatch, so you can have the best of both worlds; an example that uses both:

@variants.primary
@singledispatch
def add(x, y):
    return x + y

@add.variant('from_list')
@add.register(list)
def add(x, y):
    return x + [y]

Which then automatically dispatches between named variants based on type:

>>> add(1, 2)
3
>>> add([1], 2)
[1, 2]

But also exposes the explicit variant functions:

>>> add.from_list([1], 2)
[1, 2]
>>> add.from_list()
      7 @add.register(list)
      8 def add(x, y):
----> 9     return x + [y]

TypeError: unsupported operand type(s) for +: 'int' and 'list'

It is important to note that the variants decorators must be the outer decorators.

Installation

To install variants, run this command in your terminal:

$ pip install variants

Requirements

This is a library for Python, with support for versions 2.7 and 3.4+.