To resolve the ValueError exception, use the try-except block. The try block lets you test a block of code for errors. The except block lets you handle the error.
ValueError in Python is raised when a user gives an invalid value to a function but is of a valid argument. It usually occurs in mathematical operations that will require a certain kind of value, even when the value is the correct argument. Imagine telling Python to take the square root of a negative integer.
The raise keyword is used to raise an exception. You can define what kind of error to raise, and the text to print to the user.
How do I manually throw/raise an exception in Python? Use the most specific Exception constructor that semantically fits your issue. Be specific in your message, e.g.: raise ValueError('A very specific bad thing happened.
raise ValueError('could not find %c in %s' % (ch,str))
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 > -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')) # -> 6
print(contains('bababa', 'k')) # ->
Traceback (most recent call last):
File "how-to-raise-a-valueerror.py", line 15, in <module>
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')) # -> 6
print(contains('bababa', 'k')) # ->
Traceback (most recent call last):
File "better-way-to-raise-a-valueerror.py", line 9, in <module>
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
>>> 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))
...
>>> contains('bababa', 'k')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 5, in contains
ValueError: could not find 'k' in 'bababa'
>>> contains('bababa', 'a')
5
>>> contains('bababa', 'b')
4
>>> contains('xbababa', 'x')
0
>>>
>>> response='bababa'
... if "K" in response.text:
... raise ValueError("Not found")
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