Which pairs of the following Python programs are equivalent?

$$ P $=$def f$($x$)$: return str$($len$($x$)+1)$

$$ Q $=$def f$($x$)$: return str$($len$($x$+\text{'}$a$\text{'}))$

$$ R $=$def f$($x$)$: L$=1$ while L$!=$len$($x$)$: L$=$L$+1$ return str

$($L$+1)$