import re

s = "Tough Fox"
re.split(r"s(?=[A-Z])", s, maxsplit=1)

['Tough', 'Fox']

In [1]: import re

In [2]: def split(s):
   ...:     return re.split(r'W(?=[A-Z])', s, 1)
   ...:

In [3]: l = ["Tough Fox", "Nice White Cat", "This is a lazy Dog" ]

In [4]: for s in l:
   ...:     print(split(s))
   ...:
['Tough', 'Fox']
['Nice', 'White Cat']
['This is a lazy', 'Dog']

space_split = re.compile(r's+(?=[A-Z])')
 result = space_split.split(inputstring, 1)

>>> import re
>>> space_split = re.compile(r's+(?=[A-Z])')
>>> l = ["Tough Fox", "Nice White Cat", "This is a lazy Dog" ]
>>> for i in l:
...     print space_split.split(i, 1)
...
['Tough', 'Fox']
['Nice', 'White Cat']
['This is a lazy', 'Dog']

>>> import re
>>> l = ["Tough Fox", "Nice White Cat", "This is a lazy Dog" ]
>>> for i in l:
...   print re.findall("[A-Z][^A-Z]*", i)
...
['Tough ', 'Fox']
['Nice ', 'White ', 'Cat']
['This is a lazy ', 'Dog']

>>> for i in l:
...   print re.findall("^[^ ]*|[A-Z].*", i)
...
['Tough', 'Fox']
['Nice', 'White Cat']
['This', 'Dog']