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
singledispatch are both intended to provide alternative implementations
to a primary function, the overall aims are slightly different.
dispatches to variant functions based on the type of the argument, whereas
explicit alternative forms of the function. Note that in the above example, both
print_text.from_url take a string, one representing a file
path and one representing a URL.
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(, 2) [1, 2]
But also exposes the explicit variant functions:
>>> add.from_list(, 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.
To install variants, run this command in your terminal:
$ pip install variants
This is a library for Python, with support for versions 2.7 and 3.4+.