Most efficient way to get a key from a dictionary

Let d be a large (but still fits into memory) Python dictionary where we do not know what the keys are. What is the most efficient way (efficient should mean something like the memory used to perform the task is small compared to the size of the dictionary and the speed should at least as fast any of the methods below) to get a key of d (where it does not mater which key you get) and d is unchanged either in content or order (for newer versions of Python) once you are done? This question is not about readability but about the python dictionary objects. For example two methods are:

1. Use the list method

any_key = list(d)[0]

2. Using the popitem method

any_key,y = d.popitem()
d[any_key]=y

So both methods essentially implement a peekkey() method. My basic timeit analysis shows that method 2) is must faster than method 1) and I assume that method 2) uses a lot less memory (but I do not really know if this true yet). Is method 2) “best” or is there something better?

Extra brownie points if you get a fast and a readable method using only Python. Even more points for a C/Python method that accesses the dictionary object directly if that method is significantly faster than the best python method.

Answer

If you do not care about which key you get, and you don’t mean “sample” in the random sense, then just grab the first key using next

key = next(iter(d.keys()))

which, for brevity, is the same as

key = next(iter(d))

Just to test performance, if I generate a dict with 1000 elements

d = {k:k for k in range(1000)}

then benchmarking these two methods, the next approach is about 95% faster

>>> timeit.timeit('sample_key = list(d)[0]', setup='d = {k:k for k in range(1000)}')
5.3303698
>>> timeit.timeit('next(iter(d.keys()))', setup='d = {k:k for k in range(1000)}')
0.18915620000001354