Difference between revisions of "CSC111 Exercises with Exceptions: try/except"
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− | The exercises below deal with protecting Python code that might crash with '''try/ | + | The exercises below deal with protecting Python code that might crash with '''try/except''' statements to make the program robust. |
</bluebox> | </bluebox> | ||
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<source lang="python"> | <source lang="python"> | ||
def example1(): | def example1(): | ||
− | x = | + | for i in range( 3 ): |
− | + | x = int( input( "enter a number: " ) ) | |
− | + | y = int( input( "enter another number: " ) ) | |
+ | print( x, '/', y, '=', x/y ) | ||
def example2( L ): | def example2( L ): | ||
print( "\n\nExample 2" ) | print( "\n\nExample 2" ) | ||
sum = 0 | sum = 0 | ||
+ | sumOfPairs = [] | ||
for i in range( len( L ) ): | for i in range( len( L ) ): | ||
− | + | sumOfPairs.append( L[i]+L[i+1] ) | |
− | print( " | + | print( "sumOfPairs = ", sumOfPairs ) |
− | |||
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− | |||
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− | |||
− | + | def printUpperFile( fileName ): | |
+ | file = open( fileName, "r" ) | ||
+ | for line in file: | ||
+ | print( line.upper() ) | ||
+ | file.close() | ||
def main(): | def main(): | ||
Line 77: | Line 78: | ||
example2( [ 10, 3, 5, 6, "NA", 3 ] ) | example2( [ 10, 3, 5, 6, "NA", 3 ] ) | ||
example3( [ 10, 3, 5, 6 ] ) | example3( [ 10, 3, 5, 6 ] ) | ||
− | + | ||
+ | printUpperFile( "doesNotExistYest.txt" ) | ||
+ | printUpperFile( "./Dessssktop/misspelled.txt" ) | ||
+ | |||
main() | main() | ||
</source> | </source> | ||
<br /> | <br /> | ||
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− | + | ||
==Solution== | ==Solution== | ||
<br /> | <br /> | ||
<source lang="python"> | <source lang="python"> | ||
def example1(): | def example1(): | ||
− | x = | + | while True: |
− | + | try: | |
− | + | x = int( input( "enter a number: " ) ) | |
− | + | y = int( input( "enter another number: " ) ) | |
− | + | print( x, '/', y, '=', x/y ) | |
− | + | break | |
− | + | except ZeroDivisionError: | |
+ | print( "Can't divide by 0!" ) | ||
+ | except ValueError: | ||
+ | print( "That doesn't look like a number!" ) | ||
+ | except: | ||
+ | print( "something unexpected happend!" ) | ||
+ | |||
+ | |||
def example2( L ): | def example2( L ): | ||
print( "\n\nExample 2" ) | print( "\n\nExample 2" ) | ||
+ | print( "L = ", L ) | ||
sum = 0 | sum = 0 | ||
+ | sumOfPairs = [] | ||
for i in range( len( L ) ): | for i in range( len( L ) ): | ||
− | + | try: | |
− | + | sumOfPairs.append( L[i]+L[i+1] ) | |
− | + | except IndexError: | |
− | + | continue | |
− | + | except TypeError: | |
− | print( " | + | continue |
+ | |||
+ | print( "sumOfPairs = ", sumOfPairs ) | ||
− | def | + | def printUpperFile( fileName ): |
− | + | try: | |
− | + | file = open( fileName, "r" ) | |
− | + | except FileNotFoundError: | |
− | + | print( "***Error*** File", fileName, "not found!" ) | |
− | + | return False | |
− | + | ||
− | + | for line in file: | |
− | + | print( line.upper() ) | |
− | + | file.close() | |
− | + | return True | |
def main(): | def main(): | ||
− | + | example1() | |
+ | |||
L = [ 10, 3, 5, 6, 9, 3 ] | L = [ 10, 3, 5, 6, 9, 3 ] | ||
− | + | example2( L ) | |
− | + | ||
− | + | L = [ 10, 3, "NA", 6, 9, 3 ] | |
− | + | example2( L ) | |
+ | open( "doesNotExistYest.txt", "w" ).close() | ||
+ | |||
+ | printUpperFile( "doesNotExistYest.txt" ): | ||
+ | |||
+ | printUpperFile( "./Dessssktop/misspelled.txt" ) | ||
+ | |||
+ | |||
main() | main() | ||
+ | </source> | ||
− | |||
− | |||
<br /> | <br /> | ||
<br /> | <br /> |
Latest revision as of 13:51, 4 April 2014
--D. Thiebaut (talk) 09:05, 2 April 2014 (EDT)
The exercises below deal with protecting Python code that might crash with try/except statements to make the program robust.
Code in need of try/except statements
- The program below is not very robust. We can easily make it crash.
- Observe each function and see how to make it fail, or see why it will fail the way it is called
- Make the program crash. Register the XXXXError that is generated. For example, if the output of the crash looks like this:
Traceback (most recent call last): File "/Users/thiebaut/Desktop/except0.py", line 29, in <module> main() File "/Users/thiebaut/Desktop/except0.py", line 27, in main example3( [ 10, 3, 5, 6 ] ) File "/Users/thiebaut/Desktop/except0.py", line 18, in example3 sum = sum + L[i] IndexError: list index out of range
- what you are interested in is IndexError. This is the exception you want to guard your code against.
try:
........
........
except IndexError:
.........
- Verify that you have made your functions more robust to erroneous input/data.
Code
def example1():
for i in range( 3 ):
x = int( input( "enter a number: " ) )
y = int( input( "enter another number: " ) )
print( x, '/', y, '=', x/y )
def example2( L ):
print( "\n\nExample 2" )
sum = 0
sumOfPairs = []
for i in range( len( L ) ):
sumOfPairs.append( L[i]+L[i+1] )
print( "sumOfPairs = ", sumOfPairs )
def printUpperFile( fileName ):
file = open( fileName, "r" )
for line in file:
print( line.upper() )
file.close()
def main():
example1()
L = [ 10, 3, 5, 6, 9, 3 ]
example2( L )
example2( [ 10, 3, 5, 6, "NA", 3 ] )
example3( [ 10, 3, 5, 6 ] )
printUpperFile( "doesNotExistYest.txt" )
printUpperFile( "./Dessssktop/misspelled.txt" )
main()
Solution
def example1():
while True:
try:
x = int( input( "enter a number: " ) )
y = int( input( "enter another number: " ) )
print( x, '/', y, '=', x/y )
break
except ZeroDivisionError:
print( "Can't divide by 0!" )
except ValueError:
print( "That doesn't look like a number!" )
except:
print( "something unexpected happend!" )
def example2( L ):
print( "\n\nExample 2" )
print( "L = ", L )
sum = 0
sumOfPairs = []
for i in range( len( L ) ):
try:
sumOfPairs.append( L[i]+L[i+1] )
except IndexError:
continue
except TypeError:
continue
print( "sumOfPairs = ", sumOfPairs )
def printUpperFile( fileName ):
try:
file = open( fileName, "r" )
except FileNotFoundError:
print( "***Error*** File", fileName, "not found!" )
return False
for line in file:
print( line.upper() )
file.close()
return True
def main():
example1()
L = [ 10, 3, 5, 6, 9, 3 ]
example2( L )
L = [ 10, 3, "NA", 6, 9, 3 ]
example2( L )
open( "doesNotExistYest.txt", "w" ).close()
printUpperFile( "doesNotExistYest.txt" ):
printUpperFile( "./Dessssktop/misspelled.txt" )
main()