Find an array slice equaling sum
There's a bug:

&gt;&gt;&gt; a = [1, 2, 3]
&gt;&gt;&gt; findsum(a, 0)
False


but the result should be True because the consecutive slice a[0:0] sums to zero.
The docstring could be clearer. "a consecutive array slice" — of which array? Presumably the parameter a is meant, but it is best to be explicit. Similarly, "Find whether" — and then what? The docstring should say what value is returned.
The function has runtime \$Θ(n^3)\$. That's because sum(a[i:j]) takes an unnecessary copy of the slice, and because it sums it starting at the beginning of the slice. But if you maintained a running sum, then the runtime would come down to \$Θ(n^2)\$. That's easy to implement using itertools.accumulate and  itertools.islice:

from itertools import accumulate, islice

def findsum(a, n):
    """Return True if any consecutive slice of the sequence a sums to n,
    False if none do.

    """
    return n == 0 or any(total == n
                         for i in range(len(a))
                         for total in accumulate(islice(a, i, None)))