[gridspy]

Sure, x.append doesn't return a value. So x=x.append is None. You learn that one pretty quick.

Perhaps use x = x + [1] instead?

But yeah, surprises like this (unexpected return values) are actually a pretty strong case for using a language like Rust or C++ where the compiler will tell you this before you run the program.

[ausjke]

x=1; id(x); x+=1; id(x); # different id, means x points to new location

x=[1,2],id(x), x+=[3], id(x) ; # same id, means x stay at the original location

there should be a blog page to list those surprises for beginners :)