How to check if a string "is" an int?

[Duncan Booth]

Fredrik Lundh wrote:

> no, but all characters in the string belongs to the "digit" character
> class, which is what the "is" predicates look for.
> 

then gave examples including:

>>>> "Life of Brian".istitle()
> False

I don't see how istitle() matches your definition of what the "is" 
predicates look for.