An idea for fast function composition

Arnaud Delobelle arnodel at googlemail.com
Sat Feb 16 22:47:51 CET 2008


Hi all,

Recently there was a thread about function composition in Python (and
this was probably not the first).  The fast way to create a
(anonymous) composite function

     f1 o f2 o ... o fn

in Python is via

     lambda x: f1(f2(...fn(x)...)),

but according to some this is neither the most compact nor the most
readable.  Below I define a 'compose' function such that the above can
be written

     compose(f1, f2, ...., fn),

the resulting function being as fast as the lambda version (or maybe
faster?).  'getcomposer' is a helper function (which in most cases
will amount to a dictionary lookup).

----------------------------------
def getcomposer(nfunc, _cache={}):
     "getcomposer(n) -> lambda f1, ..., fn:(lambda x: f1(...fn(x)...))"
     try:
         return _cache[nfunc]
     except KeyError:
         fnames = ['f%s' % i for i in range(nfunc)]
         call = ''.join('%s(' % f for f in fnames)
         args = ','.join(fnames)
         cstr = 'lambda %s:(lambda x:%sx%s)' % (args, call, ')'*nfunc)
         composer = _cache[nfunc] = eval(cstr)
         return composer

def compose(*functions):
     "compose(f1, ..., fn) -> lambda x: f1(f2(...fn(x)...))"
     return getcomposer(len(functions))(*functions)


# Test

def double(x): return 2*x
def square(x): return x**2
def succ(x): return x+1

f1 = compose(double, square, succ, float)
f2 = lambda x: double(square(succ(float(x))))

def benchmark(f, n=1000000):
     from time import time
     from itertools import imap
     t0 = time()
     for _ in imap(f1, xrange(n)): pass
     t1 = time()
     return t1-t0

print 'compose', benchmark(f1)
print 'lambda ', benchmark(f2)
----------------------------------

marigold:python arno$ python -i simple_compose.py
compose 1.84630298615
lambda  1.86365509033
 >>> import dis
 >>> dis.dis(f1)
   1           0 LOAD_DEREF               0 (f0)
               3 LOAD_DEREF               3 (f1)
               6 LOAD_DEREF               1 (f2)
               9 LOAD_DEREF               2 (f3)
              12 LOAD_FAST                0 (x)
              15 CALL_FUNCTION            1
              18 CALL_FUNCTION            1
              21 CALL_FUNCTION            1
              24 CALL_FUNCTION            1
              27 RETURN_VALUE
 >>> dis.dis(f2)
  23           0 LOAD_GLOBAL              0 (double)
               3 LOAD_GLOBAL              1 (square)
               6 LOAD_GLOBAL              2 (succ)
               9 LOAD_GLOBAL              3 (float)
              12 LOAD_FAST                0 (x)
              15 CALL_FUNCTION            1
              18 CALL_FUNCTION            1
              21 CALL_FUNCTION            1
              24 CALL_FUNCTION            1
              27 RETURN_VALUE

f1 and f2 are almost exaclty the same but array lookups (LOAD_DEREFs)
in f1 replace dictionary lookups (LOAD_GLOBALs) in f2.  A C version of
'compose' could easily be written that doesn't require the use of a
python lambda-function (as created by 'getcomposer').

-- 
Arnaud




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