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Safari | Python Developer's Handbook -> Code Examples





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Code Examples

This first example returns the square root of a given input value. If the input value is negative or if it is a character, two traceback messages are displayed.

Listing 4.1 Square root (File squareroot.py)
 1: ###
 2: # Program: Square root
 3: # Author: Andre S Lessa
 4: ###
 5:
 6: ### import modules
 7:
 8: import sys, traceback, math
 9:
10: try:
11:     n = float(raw_input("Please, enter a number: "))
12:     print "The sqrt of %f is %f" % (n, math.sqrt(n))
13:
14: except (ValueError, TypeError, OverflowError):
15:     print "-----------------------------------------"
16:     print "This is the standard traceback message:"
17:     print ""
18:     traceback.print_exc()
19:
20:     print "-----------------------------------------"
21:     print "This is the customized traceback message:"
22:     print ""
23:     info = sys.exc_info()
24:     exc_type = info[0]
25:     exc_value = info[1]
26:     exc_traceback = info[2]
27:
28:     trace = traceback.extract_tb(sys.exc_traceback)
29:     print "Exception Type:  ", exc_type
30:     print "Error Message:   ", exc_value
31:     print "File name:       ", trace[0][0]
32:     print "Error message:   ", trace[0][1]
33:     print "Line:            ", trace[0][2]
34:     print "Function:        ", trace[0][3]
35: else:
36:     print "Everything went just fine."

The except clause in line 14 covers ValueError, OverflowError, and TypeError exceptions.

The else clause in line 35 is only executed when no exception is raised.

The next lines show the two traceback messages that are displayed by this program: Python standard traceback message and a customized version.

					
C:\python> s:\python\squareroot.py
Please, enter a number: i
-----------------------------------------
This is the standard traceback message:

Traceback (innermost last):
  File "s:\python\squareroot.py", line 11, in ?
   n = float(raw_input("Please, enter a number: "))
ValueError: invalid literal for float(): i
-----------------------------------------
This is the customized traceback message:

Exception Type:   exceptions.ValueError
Error Message:    invalid literal for float(): i
File name:        s:\python\squareroot.py
Error message:    11
Line:             ?
Function:         n = float(raw_input("Please, enter a number: "))

				

This example uses multiple except clauses (lines 17 and 20). It also takes advantage of the assert command to raise a debug exception (line 15).

Listing 4.2 Internet country codes (File countrycode.py)
 1: ###
 2: # Program: Country code
 3: # Author: Andre S Lessa
 4: ###
 5:
 6: ### import modules
 7:
 8: import sys, string
 9:
10: matrix = {
 "brazil":"br","france":"fr","argentina":"ar","usa":"us"
}
11:
12: def getcode(country):
13:      try:
14:          data = matrix[string.lower(country)]
15:          assert data != "br", "You cannot select this country " + 
}
                                  "for this action!"
16:          return data
17:      except KeyError:
18:          print sys.exc_type, ":", "%s is not in the list." % 
}
                   sys.exc_value
19:          print
20:      except AssertionError, b:
21:          print b
22:          print
23:
24: while 1:
25:     country = raw_input("Enter the country name or press x to exit: ")
26:     if country == "x":
27:         break
28:     code = getcode(country)
29:     if code != None:
30:         print "%s's country code is %s" % (country, code)
31:         print

The following screen dump shows the execution of this program. Note that the program doesn't end after an exception has been raised.

					
C:\>python s:\python\ countrycode.py
Enter the country name or press x to exit: Mexico
exceptions.KeyError : mexico is not in the list.

Enter the country name or press x to exit: USA
USA's country code is us

Enter the country name or press x to exit: Brazil
You cannot select this country for this action!

Enter the country name or press x to exit: Argentina
Argentina's country code is ar

Enter the country name or press x to exit: x

C:\Python>

				

See more exception handling cases in the final section of the next chapter.


Last updated on 1/30/2002
Python Developer's Handbook, © 2002 Sams Publishing

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2002, O'Reilly & Associates, Inc.

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