[Python] 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?

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