Invalid literal for int with base 10: ''

Question

>>> n = ''.join(i for i in x if i.isdigit())
>>> n.isdigit()
True
>>> x.isdigit()
False

>>> previous = 0
>>> next = 100
>>> answer = 0


>>> for i in range(0,100):
...     answer += int(n[previous:next])
...     previous = next
...     next += 100
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ValueError: invalid literal for int() with base 10: ''

Why am I getting this error ? As you can see n is digit..

Solution

n might be numeric, but at some stage you're going past the length of n such that n[previous:next] contains no characters at all. The empty string '' cannot be converted to an int, hence the error which tells the full story: invalid literal for int() with base 10: ''.

>>> int('')
Traceback (most recent call last):
  File "<input>", line 1, in <module>
ValueError: invalid literal for int() with base 10: ''