Sorting digits in a mixed string

[snorri]

Hello.
In a mixed string(digits and English alphabet) sort only the digits.
Example:
"3b2c1" => "1b2c3"
"7Mkx1Intv2n5D" => "1Mkx2Intv5n7D"

I came up with:

s = "7Mkx1Intv2n5D"
arr = list(s)
sorted(arr, key=lambda val:'0123456789'.index(val) if val.isdigit() else ...)

I can't think of a working else condition.

The other idea I had, was to extract all the digits into a list, sort it and go over the string, replacing the digits with values from sorted list.

[buran]

The other idea I had, was to extract all the digits into a list, sort it and go over the string, replacing the digits with values from sorted list.

That looks like working solution, go for it. No need to struggle for oneliner (from Zen of python: Simple is better than complex. Complex is better than complicated.). Also how it would handle case if there are repeating numbers.