[SOLVED] How do you extract key/value from a function in a dict comprehension?

How do you extract key/value from a function in a dict comprehension?

I have a function foo that generates a key and value from a string. I want to use that function in a dict comprehension, but haven't figured out how to extract multiple values from a single iteration of the function.

The catch is that foo could be expensive or stateful or executed remotely, so I can't call it more than once per iteration.

Example:

def foo(s):             # simple placeholder for an expensive function
    return s[0], s[1:]


print([foo(s) for s in ['abcd', 'efg', 'hi']])
print({foo(s) for s in ['abcd', 'efg', 'hi']})
print({foo(s)[0]: foo(s)[1] for s in ['abcd', 'efg', 'hi']})

Output:

[('a', 'bcd'), ('e', 'fg'), ('h', 'i')]   # List comprehension works
{('a', 'bcd'), ('e', 'fg'), ('h', 'i')}   # Set comprehension works
{'a': 'bcd', 'e': 'fg', 'h': 'i'}         # Desired dict output, but function is called twice

How can I extract k: v from a single invocation of the function?

Solution

How about this with dict() and a generator expression? The dict constructor can take an iterable of tuples, with each tuple representing a key and associated value.

print(dict(foo(s) for s in ['abcd', 'efg', 'hi']))

Working demo: https://www.online-python.com/ME7cP0L6Hw