Python Argument Binders

How can I bind arguments to a Python method to store a nullary functor for later invocation? Similar to C++'s boost::bind.

For example:

def add(x, y):
    return x + y

add_5 = magic_function(add, 5)
assert add_5(3) == 8

[functools.partial][1] returns a callable wrapping a function with some or all of the arguments frozen.

import sys
import functools

print_hello = functools.partial(sys.stdout.write, "Hello world\n")

print_hello()

Hello world

The above usage is equivalent to the following lambda.

print_hello = lambda *a, **kw: sys.stdout.write("Hello world\n", *a, **kw)

[1]: http://docs.python.org/2/library/functools.html#functools.partial "Python 2 Documentation: functools module: partial function"

I'm not overly familiar with boost::bind, but the partial function from functools may be a good start:

>>> from functools import partial

>>> def f(a, b):
...     return a+b

>>> p = partial(f, 1, 2)
>>> p()
3

>>> p2 = partial(f, 1)
>>> p2(7)
8

If functools.partial is not available then it can be easily emulated:

>>> make_printer = lambda s: lambda: sys.stdout.write("%s\n" % s)
>>> import sys
>>> print_hello = make_printer("hello")
>>> print_hello()
hello

Or

def partial(func, *args, **kwargs):
    def f(*args_rest, **kwargs_rest):
        kw = kwargs.copy()
        kw.update(kwargs_rest)
        return func(*(args + args_rest), **kw) 
    return f

def f(a, b):
    return a + b

p = partial(f, 1, 2)
print p() # -> 3

p2 = partial(f, 1)
print p2(7) # -> 8

d = dict(a=2, b=3)
p3 = partial(f, **d)
print p3(), p3(a=3), p3() # -> 5 6 5

lambdas allow you to create a new unnamed function with less arguments and call the function!

>>> def foobar(x,y,z):
...     print "%d, %d, %d" % (x,y,z)
>>> foobar(1,2,3) # call normal function

>>> bind = lambda x: foobar(x, 10, 20) # bind 10 and 20 to foobar
>>> bind(1) # print 1, 10, 20

>>> bind = lambda: foobar(1,2,3) # bind all elements  
>>> bind()  # print 1, 2, 3

edit

https://docs.python.org/2/library/functools.html#functools.partial

if you are planning to use named argument binding in the function call this is also applicable:

>>> from functools import partial
>>> barfoo = partial(foobar, x=10)
>>> barfoo(y=5,z=6)
21

Please note that if you bind arguments from the left you need to call the arguments by name. If you bind from the right it works as expected.

>>> barfoo(5,6) 
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: foobar() got multiple values for keyword argument 'x'
>>> f = partial(foobar, z=20)
>>> f(1,1)
22        

This would work, too:

def curry(func, *args):
    def curried(*innerargs):
       return func(*(args+innerargs))
    curried.__name__ = "%s(%s, ...)" % (func.__name__, ", ".join(map(str, args)))
    return curried

>>> w=curry(sys.stdout.write, "Hey there")
>>> w()
Hey there

Functors can be defined this way in Python. They're callable objects. The "binding" merely sets argument values.

class SomeFunctor( object ):
    def __init__( self, arg1, arg2=None ):
        self.arg1= arg1
        self.arg2= arg2
    def __call___( self, arg1=None, arg2=None ):
        a1= arg1 or self.arg1
        a2= arg2 or self.arg2
        # do something
        return

You can do things like

x= SomeFunctor( 3.456 )
x( arg2=123 )

y= SomeFunctor( 3.456, 123 )
y()

The question asks generally about binding arguments, but all answers are about functions. In case you are wondering, partial also works with class constructors (i.e. using a class instead of a function as a first argument), which can be useful for factory classes. You can do it as follows:

from functools import partial

class Animal(object):
    def __init__(self, weight, num_legs):
        self.weight = weight
        self.num_legs = num_legs
        
animal_class = partial(Animal, weight=12)
snake = animal_class(num_legs = 0)
print(snake.weight) # prints 12