Python Lambda Count/Loop Function

Lambdas are just another way of defining a function

def foo(x):
    return x + x

is the same as

foo = lambda x: x + x

So let's start with a function to do what you want:

def first_missing(items, base):
    for number in itertools.count():
        text = base + '_' + str(number)
        if text not in items:
             return text

The first thing to note is that you can't use loops inside a lambda. So we'll need to rewrite this without a loop. Instead, we'll use recursion:

def first_missing(items, base, number = 0):
        text = base + '_' + str(number)
        if text not in items:
             return text
        else:
             return first_missing(items, base, number + 1)

Now, we also can't use an if/else block in a lambda. But we can use a ternary expression:

def first_missing(items, base, number = 0):
        text = base + '_' + str(number)
        return text if text not in items else first_missing(items, base, number + 1)

We can't have local variables in a lambda, so we'll use a trick, default arguments:

def first_missing(items, base, number = 0):
        def inner(text = base + '_' + str(number)):
            return text if text not in items else first_missing(items, base, number + 1)
        return inner()

At this point we can rewrite inner as a lambda:

def first_missing(items, base, number = 0):
        inner = lambda text = base + '_' + str(number): text if text not in items else first_missing(items, base, number + 1)
        return inner()

We can combine two lines to get rid of the inner local variable:

def first_missing(items, base, number = 0):
    return (lambda text = base + '_' + str(number): text if text not in items else first_missing(items, base, number + 1))()

And at long last, we can make the whole thing into a lambda:

first_missing = lambda: items, base, number = 0: (lambda text = base + '_' + str(number): text if text not in items else first_missing(items, base, number + 1))()

Hopefully that gives you some insight into what you can do. But don't ever do it because, as you can tell, lambdas can make your code really hard to read.

You can combine a lambda with itertools.dropwhile:

import itertools
n = itertools.dropwhile(lambda n: 'test_name_dup_%d' % n in testlist, range(1, len(testlist))).next()

As to your last question, you can write a generator for names, like:

def possible_names(prefix):
    yield prefix
    yield prefix + '_dup'
    n = 0
    while True:
        n += 1
        yield '%s_dup_%d' % (prefix, n)

and then use this generator with dropwhile:

unique_name = itertools.dropwhile(lambda x: x in testlist, possible_names('test_name')).next()
print unique_name

You are a bit off the track. Lambdas are nothing but "simple" functions, often used for their fast syntax in functional programming. They are the perfect companion built-in functions "map", "reduce", "filter" but also for more complicated functions defined into itertools. Therefore the most useful thing to do with them is to generate/manipulate iterable objects (especially lists). Note that lambdas will slow down your code in most of the cases if compared to list comprehensions/normal loops and will make it harder to be read. Here is an example of what you want to do with lambdas.

>>> filter(lambda i: i!=(0 if len(testlist[i].split("_"))==3 else int(testlist[i].split("_")[-1])), range(len(testlist)))[0]
2

Or you could use more complicated functions with itertools. Anyhow I strongly suggest you not to use lambdas for this kind of assignments, as the readability is terrible. I'd rather use a well structured for loop, which is also faster.

[Edit]

To prove that lambdas+builtins are not faster than list comprehensions: consider a simple problem, for x in range(1000) create a list of x shifted by 5.

$ python -m timeit 'map(lambda x: x>>5, range(1000))' 1000 loops, best of 3: 225 usec per loop

$ python -m timeit '[x>>5 for x in range(1000)]'10000 loops, best of 3: 99.1 usec per loop

You have a >100% performance increase without lambdas.

I prefer the list comprehension or iterator method. Makes for easy one liners that I feel are pretty easy to read and maintain. Quite frankly, lambdas belong some places, here I believe its less elegant a solution.

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

from itertools import count
print next('%s%d' % (prefix, i) for i in count(1) if '%s%d' % (prefix, i) not in testlist)

This returns the first not-found instance in the sequence, which I think is the cleanest.

Of course, if you prefer a list from a definite range, you can modify it to become a list comprehension:

print ['%s%d' % (prefix, i) for i in xrange(0,5) if '%s%d' % (prefix, i) not in testlist]

returns:

['test_name_dup_0', 'test_name_dup_2', 'test_name_dup_4']