Python Programming – Exceptions

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Python Programming – Exceptions


The exception is a way of breaking out of the normal flow of control of a code block in order to handle the error or other exceptional conditions. An exception is raised at the point where the error is detected.

>>> while True print ' Hello world '
SyntaxError: invalid syntax 
>>> 10 / 0

Traceback (most recent call last) : 
File "<pyshell#l>", line 1, in <module>
    10 / 0 
ZeroDivisionError: integer division or modulo by zero 
>>> 4+tt*3

Traceback (most recent call last) :
File "<pyshe11#2>", line 1, in <module>
NameError: name ' tt ' is not defined 
>>> ' 5 '+7

Traceback (most recent call last) :
File "<pyshe11#3>", line 1, in <module>
' 5 '+7
TypeError: cannot concatenate ' str ' and ' int ' objects

The last line of the error message indicates what went wrong. Exceptions are of different types, and the type is printed as part of the message; the types in the above example are SyntaxError, ZeroDivisionError, NameError, and TypeError. Standard exception names are built-in identifiers (not reserved keywords). The rest of the lines provides detail based on the type of exception and what caused it.

Handling exceptions

If there is some suspicious code that may raise an exception, it can be handled by placing the suspicious code in a try compound statement. After the try clause, include an except clause, followed by a block of code that handles the problem. The following example attempts to open a file and write something in the file.

# ! / usr / bin / python 
try :
     fh = open ( " testflie " , " w " )
     fh.write ( " This is my test file for exception handling ! ! " ) 
except IOError :
print " Error: can\'t find the file or read data " 
print " Written content in the file successfully " 
fh.close ( )

Here are few important points that need to be remembered:

  • A single try statement can have multiple except clauses. This is useful when the try clause contains statements that may throw different types of exceptions.
  • A generic except clause can be provided, which handles any exception.
  • After the except clause(s), and else clause can be included. The code in the else clause is executed, if the code in the try clause does not raise an exception.