python argument binders

Question

how can I bind arguments to a python method to store a nullary functor for later invocation? similar to C++'s boost::bind

Answer 1

functools.partial returns a callable based on a function with some or all of the arguments frozen.

~ $ python3.0
Python 3.0rc1 (r30rc1:66511, Sep 19 2008, 00:39:17) 
Type "help", "copyright", "credits" or "license" for more information.
>>> import functools
>>> print_hello = functools.partial(print, "hello, world")
>>> print_hello()
hello, world
>>> print_hello("how are you?")
hello, world how are you?
>>>

(I use Py3k in the example so that I can freeze the print function, since it seems like the simplest example. functools.partial has been available since 2.5.)

Answer 2

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

Answer 3

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()

Answer 4

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

Answer 5

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