As pointed out, strings are immutable and depending on F(c) being mostly c or not, different approaches might work best.
Eg. for a single replacement (I haven't seen this one mentioned), slicing might work best: s[:i-1] + F(s[i]) + s[i+1:]. This is obviously good for a small number of replacements too, though you might want to switch to "".join(iterable) for better performance.
In general, immutability of strings requires a more functional approach than in-place editing.
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[ 2.7 ms ] story [ 17.5 ms ] threadEg. for a single replacement (I haven't seen this one mentioned), slicing might work best: s[:i-1] + F(s[i]) + s[i+1:]. This is obviously good for a small number of replacements too, though you might want to switch to "".join(iterable) for better performance.
In general, immutability of strings requires a more functional approach than in-place editing.