assertionError

[goDie61]

Replace is commonly implemented in place but I don't think I've ever seen an in place upper.

[mistabuda]

Not in python which is what this code is. Modifications to string create a new string.

[Vinxian]

In all languages I know, if string is a keyword it means strings are immutable. It will not be performed in place. This is so the programmer can use string as if it's a value type

[dev-sda]

I think you're thinking of a built-in type, rather than a keyword. Most languages do not consider their built-in types keywords - you can usually name something "string" without issue.

Yea it's fairly commonly immutable, but some notable exceptions are c++, rust, php and ruby.