Question

I’m sorry if this is a question answered elsewhere. Searching through Google and Stackforum I didn’t find anything from which I could extrapolate the answers; but I feel like part of that is me.

I’m trying to work out lambdas as a concept, and as part of that I’m kinda looking for ways to use it.

SO, if this is a colossally stupid thing to do with lambda from a function standpoint, feel free to let me know and explain. But either way, I still want to know the answer/still want to know how to do this with the python language.

So, for testing purposes I have:

my_test = 'test_name'
testlist = ['test_name', 'test_name_dup', 'test_name_dup_1', 'test_name_dup_3']

I’m looking to use lambda to create one function that loops through and returns the first test_name_# that isn’t in the testlist. The functionality will eventually be applied to filenames, but for testing purposes I had to get away from actually reading the filenames–gave me too many more ways to mess something up.

But my_test has to be able to change, and the test list will be a list of filepaths.

So, I’m looking for a function like:

new_name = lambda x: my_test + '_' + str(x)

But the initial value should be x = 1, and it should continue until new_name is not in testlist. Seems like:

bool(new_name not in testlist)

might be something work with.

But I can’t figure out a way to set the initial x to 1, and have it loop through with (x+1) until the bool is true.

I know this is possible as I’ve found some CRAZY lambda examples out there that are looping through lines in a file. I just couldn’t quite make sense of them (and didn’t have any way to play with them as they were dealing with things outside my programming level.

On a related note, could I add values to the beginning of this loop? (i.e. can I have it check for test_name, then test_name_dup, then test_name_dup_#)?

Thanks in advance for the help! Lambdas (while very cool) totally mess with my head.

Answer 1

There’s no need to use a lambda in this case, a simple for loop will do:

my_test  = 'test_name_dup'  
testlist = ['test_name', 'test_name_dup','test_name_dup_1', 'test_name_dup_3']

for i in xrange(1, len(testlist)):
    if my_test + '_' + str(i) not in testlist:
        break

print my_test + '_' + str(i)
> test_name_dup_2

If you really, really want to use a lambda for this problem, you’ll also have to learn about itertools, iterators, filters, etc. I’m gonna build on thg435’s answer, writing it in a more idiomatic fashion and explaining it:

import itertools as it

iterator = it.dropwhile(
    lambda n: '{0}_{1}'.format(my_test, n) in testlist,
    it.count(1))

print my_test + '_' + str(iterator.next())
> test_name_dup_2

The key to understanding the above solution lies in the dropwhile() procedure. It takes two parameters: a predicate and an iterable, and returns an iterator that drops elements from the iterable as long as the predicate is true; afterwards, returns every element.

For the iterable, I’m passing count(1), an iterator that produces an infinite number of integers starting from 1.

Then dropwhile() starts to consume the integers until the predicate is false; this is a good opportunity for passing an in-line defined function – and here’s our lambda. It receives each generated integer in turn, checking to see if the string test_name_dup_# is present in the list.

When the predicate returns false, dropwhile() returns and we can retrieve the value that made it stop by calling next() on it.