Computer Science 125
6 October 2020
What exceptions could occur from the following Python code?
1.
a = float(input("Please enter a number: "))
b = float(input("Please enter another number: "))
fraction = a/b
F = open("myfile.txt", "w")
F.write(fraction)
F.close()
How could you handle the exceptions in the code above?
2.
What exceptions could occur from the following Python code? How
could you handle them?
3.
import random
def mayCauseError():
L = [5, 12]
r = random.randrange(0, 3)
n = L[r]
s = random.randrange(0, 3)
a = n/s
return a
try:
a = float(input("Please enter a number: "))
b = float(input("Please enter another number: "))
fraction = a/b
except ValueError:
print("Sorry, cannot convert that to a number.")
except ZeroDivisionError:
print("Sorry, cannot divide by zero")
try:
F = open("myfile.txt", "w")
F.write(fraction)
F.close()
except PermissionError:
print("Sorry, cannot write file.)
def mayCauseError():
L = [5, 12]
r = random.randrange(0, 3)
if r < len(L):
n = L[r]
else:
print("Sorry!")
return None
import random
def mayCauseError():
L = [5, 12]
r = random.randrange(0, 3)
n = L[r]
s = random.randrange(0, 3)
a = n/s
return a
def main():
try:
mayCauseError()
except IndexError:
print("Sorry, an index error occurred.")
main()
Day14 Page 1
What does the raise statement do in Python?
4.
return None
s = random.randrange(0, 3)
a = n/s
return a
mayCauseError()
import random
class myNewError(Exception):
pass
def mayCauseError():
raise myNewError
L = [5, 12]
r = random.randrange(0, 3)
n = L[r]
s = random.randrange(0, 3)
a = n/s
return a
def main():
try:
mayCauseError()
except IndexError:
print("Sorry, an index error occurred.")
except myNewError:
print("Your error occurred.")
main()
Day14 Page 2
PRACTICE WITH EXCEPTIONS – SOLUTIONS
1. The following line of code might raise a ValueError:
def inputInteger():
n = int(input("Enter an integer: "))
return n
Call this function from within a try statement and use except to catch any ValueError
that occurs. If a ValueError occurs, then display a helpful error message.
def inputInteger():
n = int(input("Enter an integer: "))
return n
try:
inputInteger()
except ValueError:
print("Sorry, that input does not seem to be an integer.")
2. Modify your previous code so that, if the user does not enter an integer, the program asks
the user for another integer. The program should continue asking until the user enters an
integer.
repeat = True
while repeat:
try:
n = inputInteger()
repeat = False
except ValueError:
print("Sorry, that input does not seem to be an integer.
Please try again.")
print("You entered: ", n)
3. Write a function called oops that intentionally raises an IndexError exception when called.
Then call your function from within a try statement and use except to catch the error.
def oops():
raise IndexError
print("PRACTICE PROBLEM 3:")
try:
oops()
except IndexError:
print("An IndexError occurred.")
4. Bonus: The following code retrieves a file from a web server:
import urllib.request
result = urllib.request.urlopen( someURL )
However, the function urllib.request.urlopen() can raise various exceptions. For
example, it raises a ValueError if it cannot interpret its argument as a valid URL. It also
raises a urllib.error.HTTPError if the server cannot find the requested file. To see this
for yourself, run the following code:
import urllib.request
urllib.request.urlopen( "test" ) # this line raises a ValueError
urllib.request.urlopen( "http://www.mlwright.org/nofile.txt" )
# this line above raises a urllib.error.HTTPError
Write a program that asks the user for a URL and then uses urllib.request.urlopen() to
request that URL. If urllib.request.urlopen() raises a ValueError or
urllib.error.HTTPError, then your program should handle the exception and print a
helpful message for the user.
If no error occurs, you can (usually) print the HTML file returned like this:
byteData = result.read()
strData = byteData.decode("utf8")
print(strData)
Once you have the data from the file stored in a Python string, you can extract information
from it. Experiment with using Python to read web pages!
import urllib.request
try:
f =
urllib.request.urlopen("http://www.mlwright.org/teaching/cs121s17/in
dex.php")
print(f)
print("URL:", f.geturl()) # optional
print("INFO:", f.info()) # optional
print("CODE:", f.getcode()) # optional
bytedata = f.read()
strdata = bytedata.decode("utf8")
print(strdata)
except ValueError:
print("Invalid URL.")
except urllib.error.HTTPError:
print("A HTTP Error occurred")
except urllib.error.URLError:
print("URL not found")
