CSc 120. Introduction to Computer Programming II. 07: Excep*ons. Adapted from slides by Dr. Saumya Debray

Transcription

1 CSc 120 Introduction to Computer Programming II Adapted from slides by Dr. Saumya Debray 07: Excep*ons

2 EXERCISE Type in the following code: def foo(): n = int(input("enter a number:")) print("n = ", n) print("reciprocal = ", str(1/n)) Run the code Call foo() and enter a number 2

3 Errors and exceptions in Python A Python program can have two kinds of errors:* Syntax errors: the code is not legal Python syntax detected before the program is run Excep*ons: the code is legal Python syntax but something goes wrong when the program is run An excep%on is an error that is only detected at run *me. * This does not count logic errors, which the Python system cannot detect 3

4 Some common exceptions FileNotFoundError file name or directory cannot be found IndexError an index into a string or list is out of bounds KeyError a non-existent key used to access a dic*onary TypeError arguments to an opera*on are of the wrong type ValueError type is OK but the value is not. E.g.: int("abc") 4

5 Handling exceptions try raise catch excep*on may occur excep*on occurs catch and handle the excep*on 5

6 Handling exceptions Example: try: code that might raise an excep%on except: code to handle the excep%on 6

7 Handling exceptions Example: try: infile = open(filename) except: print("could not open file: " + filename) 7

8 Handling exceptions Example: >>> f = open("no]here.txt") Traceback (most recent call last): File "<pyshell#6>", line 1, in <module> f = open("no]here.txt") FileNotFoundError: [Errno 2] No such file or directory: 'no]here.txt' >>> >>> try: f = open("no]here.txt") except: print("error: file not found") Error: file not found >>> 8

9 EXERCISE Add try and except statements to handle an excep*on that may occur. def foo(): n = int(input("enter a number:")) print("n = ", n) print("reciprocal = ", str(1/n)) try: code that might raise an excep%on except: code to handle the excep%on 9

10 EXERCISE-sol Run the code and enter a non-digit value. What's the problem? def foo(): try: n = int(input("enter a number:")) print("n = ", n) print("reciprocal = ", str(1/n)) except: print("divide-by-zero error") 10

11 Handling exceptions Example: try: This will catch any excep*on raised in the try block This may not always be desirable code that might raise an excep%on except: code to handle the excep%on 11

12 Handling exceptions 12

13 Handling exceptions 13

14 Handling exceptions 14

15 Handling exceptions CULPRIT: Catching all excep%ons (BAD STYLE) The file was read! The error message doesn't make sense 15

16 Handling exceptions Deals with a specific excep%on Does not mislead on other excep%ons 16

17 EXERCISE Modify your code to catch a ZeroDivisionError. def foo(): try: n = int(input("enter a number:")) print("n = ", n) print("reciprocal = ", str(1/n)) except: print("error: Divide-by-zero error") 17

18 Handling multiple exceptions 1 Handle mul%ple excep%ons in the same way Behavior for both excep%ons is the same 18

19 Handling multiple exceptions 2 Handle mul%ple excep%ons in different ways 19

20 Handling multiple exceptions 2 20

21 Exception propagation an unhandled excep*on is passed along from a func*on to its caller un*l (a) it is handled; or (b) it reaches the top level of execu*on 21

22 EXERCISE Download: h]p://www2.cs.arizona.edu/classes/cs120/fall18/notes/propagate.py def fun1(x): return 1/x def fun2(x): return 1 + fun1(x) def main(): z = fun2(3) print(z) z = fun2(0) print(z) main() Make 2 copies of the program. 1- Modify the code to catch the excep*on in fun2(). 22

23 EXERCISE-(cont.) In which func*on does the error occur? Which func*on catches the error? 23

24 def fun1(x): return 1/x EXERCISE Download: h]p://www2.cs.arizona.edu/classes/cs120/summer18/notes/propagate.py def fun2(x): return 1 + fun1(x) def main(): z = fun2(3) print(z) z = fun2(0) print(z) main() 2- Modify the code to catch the excep*on in main(). 24

25 EXERCISE-(cont.) Call order: main() à calls fun2() à calls fun1() In which func*on does the error occur? Which func*on catches the error? The error occurs in fun1(). When it's not "handled" there, Python goes to the caller of fun1(), which is fun2(). If not handled there, Python goes to the caller of fun2(), which is main(). 25

26 Dealing with exceptions If possible and appropriate, try to recover from the excep*on depends on the problem spec, nature of the excep*on If recovery is not possible, exit the program import sys... sys.exit(1) exits the program with error code 1 (this indicates that an error occurred to any other program that may be using this program) 26

27 Example import sys def read_input(filename): try: fileobj = open(filename) except IOError: print( ERROR: could not open file + filename) sys.exit(1) for line in fileobj:...process contents of file... 27

28 Else clause (optional) Executed if no excep*ons are raised.... for fname in names_list: try: f = open(fname) except IOError: print("cannot open ", fname) else: print("length of", fname, "is", len(f.readlines())) f.close() 28

29 Exceptions: summary Avoid naked except if at all possible catch and handle specific excep*ons by name other excep*ons will propagate up to the caller Keep the try except separa*on as small as possible makes the code easier to understand avoids inadvertent masking of excep*ons Recover from the excep*on if possible; otherwise exit with error code 1 29