raw_input returns a Unicode string, including the empty string.
[Enter no input]
If you do not want spaces, then either filter them (which you may not yet know how to do), or test for them.
user = raw_input()
if ' ' in user: ...
This could be inside a loop that only ends when the input is valid.
To see if a string is all digits,
To see if a string is all alphabet characters,
The former will return False if any characters other than numbers are present in the string. It does not consider a decimal point to be a digit so floats will return False, as well.
The latter will return False if any characters other than letters are present in the string.