How to raise a ValueError

Question

I have this code which finds the largest index of a specific character in a string  however I would like it to raise a ValueError when the specified character does not occur in a string   So something like this   contains  bababa    k     would result in a     ValueError  could not find k in bababa  How can I do this   Here s the current code for my function   def contains string char     list         for i in range 0 len string          if string i     char             list   list    i     return list -1

User · Answer

gt  gt  gt  response  bababa       if  K  in response text          raise ValueError  Not found

User · Answer

gt  gt  gt  def contains string  char           for i in xrange len string  - 1  -1  -1               if string i     char                  return i         raise ValueError  could not find  r in  r     char  string        gt  gt  gt  contains  bababa    k   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt    File   lt stdin gt    line 5  in contains ValueError  could not find  k  in  bababa   gt  gt  gt  contains  bababa    a   5  gt  gt  gt  contains  bababa    b   4  gt  gt  gt  contains  xbababa    x   0  gt  gt  gt

User · Answer

Here s a revised version of your code which still works plus it illustrates how to raise a ValueError the way you want  By-the-way  I think find last    find last index    or something simlar would be a more descriptive name for this function  Adding to the possible confusion is the fact that Python already has a container object method named   contains     that does something a little different  membership-testing-wise   def contains char string  char       largest index   -1     for i  ch in enumerate char string           if ch    char              largest index   i     if largest index  gt  -1     any found          return largest index    return index of last one     else          raise ValueError  could not find   r  in   r   format char  char string    print contains  mississippi    s       - gt  6 print contains  bababa    k       - gt    Traceback  most recent call last     File  how-to-raise-a-valueerror py   line 15  in  lt module gt      print contains  bababa    k      File  how-to-raise-a-valueerror py   line 12  in contains     raise ValueError  could not find    in     format char  char string   ValueError  could not find  k  in  bababa    Update     A substantially simpler way  Wow  Here s a much more concise version   essentially a one-liner   that is also likely faster because it reverses  via    -1   the string before doing a forward search through it for the first matching character and it does so using the fast built-in string index   method  With respect to your actual question  a nice little bonus convenience that comes with using index   is that it already raises a ValueError when the character substring isn t found  so nothing additional is required to make that happen   Here it is along with a quick unit test   def contains char string  char          Ending - 1 adjusts returned index to account for searching in reverse      return len char string  - char string   -1  index char  - 1  print contains  mississippi    s       - gt  6 print contains  bababa    k       - gt    Traceback  most recent call last     File  better-way-to-raise-a-valueerror py   line 9  in  lt module gt      print contains  bababa    k      File  better-way-to-raise-a-valueerror   line 6  in contains     return len char string  - char string   -1  index char  - 1 ValueError  substring not found

User · Answer

raise ValueError  could not find  c in  s     ch str

[python] How to raise a ValueError?

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