[Python] concurrent.futures.ProcessPoolExecutor hangs when the function is a lambda or nested...

Can anyone provide insight into why using lambda or a nested function (f) would make concurrent.futures.ProcessPoolExecutor hang in the following code example?

import concurrent.futures


def f2(s):
return len(s)


def main():
def f(s):
return len(s)

data = ["a", "b", "c"]

with concurrent.futures.ProcessPoolExecutor(max_workers=1) as pool:
# results = pool.map(f, data) # hangs
# results = pool.map(lambda d: len(d), data) # hangs
# results = pool.map(len, data) # works
results = pool.map(f2, data) # works

print(list(results))


if __name__ == "__main__":
main()

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