Using datetime can be problematic because it's the name of both the package and the class.
I'd suggest you try:

from datetime import datetime
datetime.strftime(format)

The first 3 tests work and the rest don't.
Help me please.

Thank you.
Very helpful.

From the description:

Attention: If the number has leading zeros the amount of digits should be considered.

I assume that means that the resulting incremented number should have the same amount of digits.

It's easy to miss details like that.

Can Anybody explain to me please why

'foobar099' should be 'foobar100' but not 'foobar0100' ?

Thank you!