Question regarding Higher-Order-Programming in Python

[Mark Fink]

I am about to learn Higher-Order-Programming with Lisp, Haskell, and
Python. Some people who have gone completely out of their mind call
this FP.

In Haskell I learned that when I use map on a list it starts nesting
as soon as I start adding elements. If I do not like the nesting I use
ConcatMap.

In Python I have a similar scenario. I have a generator which creates
some combinatorics of a input dictionary. The list starts nesting. Now
I could use reduce(concat, ...) which would be the correct thing from
a functional perspective. But from a resource utilization perspective
it is not a good idea since the reduce is executing the generator.

I tried to illustrate this using a small example (the often in
combinatorics the real thing would be much bigger that is why I need
to use a generator):

>>> from operator import itemgetter, concat
>>> import itertools as it
>>> from functools import partial
>>>
>>> dims = {'number': [1,2,3], 'letter': ['a', 'b'], 'special': ['+', '-']}
>>> dims
{'special': ['+', '-'], 'number': [1, 2, 3], 'letter': ['a', 'b']}
>>> def get_products(keys):
...     # helper to get products from keys in the following form:
...     # [('bold', True), ('color', 'black')]
...     values  = itemgetter(*keys)(dims)
...     product = it.product(*values)
...     return map(partial(zip, keys), product)
...
>>> comb = it.combinations(dims, 2)
>>> comb_l = list(comb)
>>> comb_l
[('special', 'number'), ('special', 'letter'), ('number', 'letter')]
>>> res = map(get_products, comb_l)
>>> res
[[[('special', '+'), ('number', 1)], [('special', '+'), ('number',
2)], [('special', '+'), ('number', 3)], [('special', '-'), ('number',
1)], [('special', '-'), ('number', 2)], [('special', '-'), ('number',
3)]], [[('special', '+'), ('letter', 'a')], [('special', '+'),
('letter', 'b')], [('special', '-'), ('letter', 'a')], [('special',
'-'), ('letter', 'b')]], [[('number', 1), ('letter', 'a')],
[('number', 1), ('letter', 'b')], [('number', 2), ('letter', 'a')],
[('number', 2), ('letter', 'b')], [('number', 3), ('letter', 'a')],
[('number', 3), ('letter', 'b')]]]  # the resulting list is nested one
level to deep caused by the map(get_products, ..
>>>

My problem is that I want to get single elements from the generator
like [('special', '+'), ('number', 1)]. But this does not work because
the list is now nested to deep.

That is what I expect: (I could get something like that with the
following >>> res = reduce(concat, res)
[[('special', '+'), ('number', 1)], [('special', '+'), ('number', 2)],
[('special', '+'), ('number', 3)], [('special', '-'), ('number', 1)],
[('special', '-'), ('number', 2)], [('special', '-'), ('number', 3)],
[('special', '+'), ('letter', 'a')], [('special', '+'), ('letter',
'b')], [('special', '-'), ('letter', 'a')], [('special', '-'),
('letter', 'b')], [('number', 1), ('letter', 'a')], [('number', 1),
('letter', 'b')], [('number', 2), ('letter', 'a')], [('number', 2),
('letter', 'b')], [('number', 3), ('letter', 'a')], [('number', 3),
('letter', 'b')]]

I have seen the problem many times but so far I could not google a
solution on the web.

By the way do you know any substantial example using FP in Python
(generators, imap, ifilter, ...)?