Brief
=====
In this easy kata your function has to take a **string** as input and **return a string** with everything removed (*whitespaces* included) but the **digits**. As you may have guessed **empty strings** are to be returned as they are & if the input string contains no digits then the output will be an **empty string**.By the way , you have to watch out for **non-string** inputs too.Return **'Invalid input !'** for them.    
Hint
====
If you're writing more than 1 line of code, then think again! ;)   
Good luck!

def digit_all (x):
    if isinstance(x,str):
        return "".join( x for x in x if x.isnumeric())
    else:
        return 'Invalid input !'

def digit_all (x):
    if isinstance(x,str):
        return "".join( x for x in x if x.isnumeric())
    else:
        return 'Invalid input !'