Question

I was wondering whether for most examples it is more ‘pythonic’ to use lambda or the partial function?

For example, I might want to apply imap on some list, like add 3 to every element using:

imap(lambda x : x + 3, my_list)

Or to use partial:

imap(partial(operator.add, 3), my_list)

I realize in this example a loop could probably accomplish it easier, but I’m thinking about more non-trivial examples.

In Haskell, I would easily choose partial application in the above example, but I’m not sure for Python. To me, the lambda seems the the better choice, but I don’t know what the prevailing choice is for most python programmers.

Answer 1

To be truly equivalent to imap, use a generator expression:

(x + 3 for x in mylist)

Like imap, this doesn’t immediately construct an entire new list, but instead computes elements of the resulting sequence on-demand (and is thus much more efficient than a list comprehension if you’re chaining the result into another iteration).

If you’re curious about where partial would be a better option than lambda in the real world, it tends to be when you’re dealing with variable numbers of arguments:

>>> from functools import partial
>>> def a(*args):
...     return sum(args)
... 
>>> b = partial(a, 2, 3)
>>> b(6, 7, 8)
26

The equivalent version using lambda would be…

>>> b = lambda *args: a(2, 3, *args)
>>> b(6, 7, 8)
26

which is slightly less concise – but lambda does give you the option of out-of-order application, which partial does not:

>>> def a(x, y, z):
...    return x + y - z
...
>>> b = lambda m, n: a(m, 1, n)
>>> b(2, 5)
-2