Python – Exceptions Handling

Python provides two very important features to handle any unexpected error in your Python programs and to add debugging capabilities in them −

• Exception Handling − This would be covered in this tutorial. Here is a list standard Exceptions available in Python: Standard Exceptions.
• Assertions − This would be covered in Assertions in Python tutorial.
  List of Standard Exceptions −

  Sr.No.	Exception Name & Description
  1	Exception Base class for all exceptions
  2	StopIteration Raised when the next() method of an iterator does not point to any object.
  3	SystemExit Raised by the sys.exit() function.
  4	StandardError Base class for all built-in exceptions except StopIteration and SystemExit.
  5	ArithmeticError Base class for all errors that occur for numeric calculation.
  6	OverflowError Raised when a calculation exceeds maximum limit for a numeric type.
  7	FloatingPointError Raised when a floating point calculation fails.
  8	ZeroDivisionError Raised when division or modulo by zero takes place for all numeric types.
  9	AssertionError Raised in case of failure of the Assert statement.
  10	AttributeError Raised in case of failure of attribute reference or assignment.
  11	EOFError Raised when there is no input from either the raw_input() or input() function and the end of file is reached.
  12	ImportError Raised when an import statement fails.
  13	KeyboardInterrupt Raised when the user interrupts program execution, usually by pressing Ctrl+c.
  14	LookupError Base class for all lookup errors.
  15	IndexError Raised when an index is not found in a sequence.
  16	KeyError Raised when the specified key is not found in the dictionary.
  17	NameError Raised when an identifier is not found in the local or global namespace.
  18	UnboundLocalError Raised when trying to access a local variable in a function or method but no value has been assigned to it.
  19	EnvironmentError Base class for all exceptions that occur outside the Python environment.
  20	IOError Raised when an input/ output operation fails, such as the print statement or the open() function when trying to open a file that does not exist.
  21	IOError Raised for operating system-related errors.
  22	SyntaxError Raised when there is an error in Python syntax.
  23	IndentationError Raised when indentation is not specified properly.
  24	SystemError Raised when the interpreter finds an internal problem, but when this error is encountered the Python interpreter does not exit.
  25	SystemExit Raised when Python interpreter is quit by using the sys.exit() function. If not handled in the code, causes the interpreter to exit.
  26	TypeError Raised when an operation or function is attempted that is invalid for the specified data type.
  27	ValueError Raised when the built-in function for a data type has the valid type of arguments, but the arguments have invalid values specified.
  28	RuntimeError Raised when a generated error does not fall into any category.
  29	NotImplementedError Raised when an abstract method that needs to be implemented in an inherited class is not actually implemented.

  Assertions in Python

  An assertion is a sanity-check that you can turn on or turn off when you are done with your testing of the program.
  The easiest way to think of an assertion is to liken it to a raise-if statement (or to be more accurate, a raise-if-not statement). An expression is tested, and if the result comes up false, an exception is raised.
  Assertions are carried out by the assert statement, the newest keyword to Python, introduced in version 1.5.
  Programmers often place assertions at the start of a function to check for valid input, and after a function call to check for valid output.

  The assert Statement

  When it encounters an assert statement, Python evaluates the accompanying expression, which is hopefully true. If the expression is false, Python raises an AssertionError exception.
  The syntax for assert is −

  assert Expression[, Arguments]

  If the assertion fails, Python uses ArgumentExpression as the argument for the AssertionError. AssertionError exceptions can be caught and handled like any other exception using the try-except statement, but if not handled, they will terminate the program and produce a traceback.

  Example

  Here is a function that converts a temperature from degrees Kelvin to degrees Fahrenheit. Since zero degrees Kelvin is as cold as it gets, the function bails out if it sees a negative temperature −

  def division(num1,num2):
  assert(num2>0),"divisor should be greater than zero"
  return (num1/num2)
  print(division(50,0))

  When the above code is executed, it produces the following result −

  File "demo.py", line 479, in <module>
  print(division(50,0))
  File "demo.py", line 476, in division
  assert(num2>0),"divisor should be greater than zero"
  AssertionError: divisor should be greater than zero

  What is Exception?

  An exception is an event, which occurs during the execution of a program that disrupts the normal flow of the program's instructions. In general, when a Python script encounters a situation that it cannot cope with, it raises an exception. An exception is a Python object that represents an error.
  When a Python script raises an exception, it must either handle the exception immediately otherwise it terminates and quits.

  Exception handling in python

  If you have some suspicious code that may raise an exception, you can defend your program by placing the suspicious code in a try: block. After the try: block, include an except: statement, followed by a block of code which handles the problem as elegantly as possible.

  Syntax

  Here is simple syntax of try....except...else blocks −

  try:
  You do your operations here;
  ......................
  except ExceptionI:
  If there is ExceptionI, then execute this block.
  except ExceptionII:
  If there is ExceptionII, then execute this block.
  ......................
  else:
  If there is no exception then execute this block.

  Here are few important points about the above-mentioned syntax −

• A single try statement can have multiple except statements. This is useful when the try block contains statements that may throw different types of exceptions.
• You can also provide a generic except clause, which handles any exception.
• After the except clause(s), you can include an else-clause. The code in the else-block executes if the code in the try: block does not raise an exception.
• The else-block is a good place for code that does not need the try: block's protection.
  

  Example

  This example opens a file, writes content in the, file and comes out gracefully because there is no problem at all −

  try:
  fh = open("demofle.txt", "w")
  fh.write("This is my test file for exception handling!!")
  except IOError:
  print "Error: can't find file or read data"
  else:
  print "Written content in the file successfully"
  fh.close()

  This produces the following result −

  Written content in the file successfully

  Example

  This example tries to open a file where you do not have write permission, so it raises an exception −

  try:
  fh = open("testfile", "r")
  fh.write("This is my test file for exception handling!!")
  except IOError:
  print "Error: can't find file or read data"
  else:
  print "Written content in the file successfully"

  This produces the following result −

  Error: can't find file or read data

  The except Clause with No Exceptions

  You can also use the except statement with no exceptions defined as follows −

  try:
  You do your operations here;
  ......................
  except:
  If there is any exception, then execute this block.
  ......................
  else:
  If there is no exception then execute this block.

  This kind of a try-except statement catches all the exceptions that occur. Using this kind of try-except statement is not considered a good programming practice though, because it catches all exceptions but does not make the programmer identify the root cause of the problem that may occur.

  The except Clause with Multiple Exceptions

  You can also use the same except statement to handle multiple exceptions as follows −

  try:
  You do your operations here;
  ......................
  except(Exception1[, Exception2[,...ExceptionN]]]):
  If there is any exception from the given exception list,
  then execute this block.
  ......................
  else:
  If there is no exception then execute this block.

  The try-finally Clause

  You can use a finally: block along with a try: block. The finally block is a place to put any code that must execute, whether the try-block
  raised an exception or not. The syntax of the try-finally statement is this −

  try:
  You do your operations here;
  ......................
  Due to any exception, this may be skipped.
  finally:
  This would always be executed.
  ......................

  You cannot use else clause as well along with a finally clause.

  # try --> put the code that might throw an error inside try: (with a : at the end)
  try :
  print (x)
  # except --> in case an error occurs then put the statement or code that will have to executed instead inside except
  except:
  print ("An error occured")
  # finally --> put the statement or code that will have to be executed irrespective of the error
  finally:
  print ("This sentece will defenitely be printed")

  Example

  try:
  fh = open("testfile", "w")
  fh.write("This is my test file for exception handling!!")
  finally:
  print ("Error: can't find file or read data")

  If you do not have permission to open the file in writing mode, then this will produce the following result −

  Error: can't find file or read data

  Same example can be written more cleanly as follows −

  try:
  fh = open("testfile", "w")
  try:
  fh.write("This is my test file for exception handling!!")
  finally:
  print ("Going to close the file")
  fh.close()
  except IOError:
  print ("Error: can't find file or read data")

  When an exception is thrown in the try block, the execution immediately passes to the finally block. After all the statements in the finally block are executed, the exception is raised again and is handled in the except statements if present in the next higher layer of the try-except statement.

  try:
  #print x
  print(y)
  except NameError: # This except block will be executed only when there is NameError type of error
  print ("This is undefined variable")
  except: # As no type of error is specified here for all the other type of error this block is executed
  print ("Someother kind of error")
  finally: # Irrespective of any error this block is executed
  print ("finally print this anyway...")

  This produces the following result −

  This is undefined variable
  finally print this anyway...

  Argument of an Exception

  An exception can have an argument, which is a value that gives additional information about the problem. The contents of the argument vary by exception. You capture an exception's argument by supplying a variable in the except clause as follows −

  try:
  You do your operations here;
  ......................
  except ExceptionType, Argument:
  You can print value of Argument here...

  If you write the code to handle a single exception, you can have a variable follow the name of the exception in the except statement. If you are trapping multiple exceptions, you can have a variable follow the tuple of the exception.
  This variable receives the value of the exception mostly containing the cause of the exception. The variable can receive a single value or multiple values in the form of a tuple. This tuple usually contains the error string, the error number, and an error location.

  Example

  Following is an example for a single exception −

  # Define a function here.
  def temp_convert(var):
  try:
  return int(var)
  except ValueError, Argument:
  print("The argument does not contain numbersn", Argument)
  # Call above function here.
  temp_convert("xyz");

  This produces the following result −

  The argument does not contain numbers
  invalid literal for int() with base 10: 'xyz'

  Raising an Exceptions

  You can raise exceptions in several ways by using the raise statement. The general syntax for the raise statement is as follows.

  Syntax

  raise [Exception [, args [, traceback]]]

  Here, Exception is the type of exception (for example, NameError) and argument is a value for the exception argument. The argument is optional; if not supplied, the exception argument is None.
  The final argument, traceback, is also optional (and rarely used in practice), and if present, is the traceback object used for the exception.

  Example

  An exception can be a string, a class or an object. Most of the exceptions that the Python core raises are classes, with an argument that is an instance of the class. Defining new exceptions is quite easy and can be done as follows −

  def functionName( level ):
  if level  matchObj.group() : ", matchObj.group()
  else:
  print("No match!!")
  searchObj = re.search( r'dogs', line, re.M|re.I)
  if searchObj:
  print ("search --> searchObj.group() : ", searchObj.group())
  else:
  print ("Nothing found!!")

  When the above code is executed, it produces the following result −

  No match!!
  search --> searchObj.group() :  dogs

  Search and Replace

  One of the most important re methods that use regular expressions is sub.

  Syntax

  re.sub(pattern, repl, string, max=0)

  This method replaces all occurrences of the RE pattern in string with repl, substituting all occurrences unless max provided. This method returns modified string.

  Example

  import re
  phone = "694%2-9#59-85!^9% # This is Phone Number"
  # Remove anything other than digits
  num = re.sub(r'D', "", phone)
  print ("Phone Num : ", num)

  When the above code is executed, it produces the following result −

  Phone Num :  6942959859

  more examples:

  import re
  regexp = '''    name:prutor
  email:prutor.ai@gmail.com
  mob.no:963*530*4444
  name:john
  email:john@gmail.com
  mob.no:963*530*6666
  Mr. prutor
  Mr. john
  ms. ammy
  '''
  pattern1 = re.compile(r'd{3}.d{3}.d{4}')#d mean digit {3} mean how many consecutive digits,
  #. means whatever charecter there it takes
  matches = pattern1.finditer(regexp)
  for match in matches:
  print(match)
  print("-----------------------------Second Pattern-------------------------------------------")
  pattern2 = re.compile(r'd{3}[-]d{3}.d{4}') #[-] mean only '-' should come after three digits
  matches = pattern2.finditer(regexp)
  for match in matches:
  print(match)
  print("-----------------------------Third Patern---------------------------------------------")
  pattern3 = re.compile(r'(Mr|ms).sw+')  #. if we type  before .(dot) it will not work like regular expression like in pattern 1
  matches = pattern3.finditer(regexp)
  for match in matches:
  print(match)
  print("-----------------------------fourth Patern---------------------------------------------")
  pattern4 = re.compile('(Mr|ms).sw+')
  matches = pattern4.finditer(regexp)
  for match in matches:
  print(match.group(0))

  Regular Expression Modifiers: Option Flags

  Regular expression literals may include an optional modifier to control various aspects of matching. The modifiers are specified as an optional flag. You can provide multiple modifiers using exclusive OR (|), as shown previously and may be represented by one of these −

  Sr.No.	Modifier & Description
  1	re.I Performs case-insensitive matching.
  2	re.L Interprets words according to the current locale. This interpretation affects the alphabetic group (w and W), as well as word boundary behavior(b and B).
  3	re.M Makes $ match the end of a line (not just the end of the string) and makes ^ match the start of any line (not just the start of the string).
  4	re.S Makes a period (dot) match any character, including a newline.
  5	re.U Interprets letters according to the Unicode character set. This flag affects the behavior of w, W, b, B.
  6	re.X Permits "cuter" regular expression syntax. It ignores whitespace (except inside a set [] or when escaped by a backslash) and treats unescaped # as a comment marker.

  Regular Expression Patterns

  Except for control characters, (+ ? . * ^ $ ( ) [ ] { } | ), all characters match themselves. You can escape a control character by preceding it with a backslash.
  Following table lists the regular expression syntax that is available in Python −

  Sr.No.	Pattern & Description
  1	^ Matches beginning of line.
  2	$ Matches end of line.
  3	. Matches any single character except newline. Using m option allows it to match newline as well.
  4	[...] Matches any single character in brackets.
  5	[^...] Matches any single character not in brackets
  6	re* Matches 0 or more occurrences of preceding expression.
  7	re+ Matches 1 or more occurrence of preceding expression.
  8	re? Matches 0 or 1 occurrence of preceding expression.
  9	re{ n} Matches exactly n number of occurrences of preceding expression.
  10	re{ n,} Matches n or more occurrences of preceding expression.
  11	re{ n, m} Matches at least n and at most m occurrences of preceding expression.
  12	a| b Matches either a or b.
  13	(re) Groups regular expressions and remembers matched text.
  14	(?imx) Temporarily toggles on i, m, or x options within a regular expression. If in parentheses, only that area is affected.
  15	(?-imx) Temporarily toggles off i, m, or x options within a regular expression. If in parentheses, only that area is affected.
  16	(?: re) Groups regular expressions without remembering matched text.
  17	(?imx: re) Temporarily toggles on i, m, or x options within parentheses.
  18	(?-imx: re) Temporarily toggles off i, m, or x options within parentheses.
  19	(?#...) Comment.
  20	(?= re) Specifies position using a pattern. Doesn't have a range.
  21	(?! re) Specifies position using pattern negation. Doesn't have a range.
  22	(?> re) Matches independent pattern without backtracking.
  23	w Matches word characters.
  24	W Matches nonword characters.
  25	s Matches whitespace. Equivalent to [tnrf].
  26	S Matches nonwhitespace.
  27	d Matches digits. Equivalent to [0-9].
  28	D Matches nondigits.
  29	A Matches beginning of string.
  30	Z Matches end of string. If a newline exists, it matches just before newline.
  31	z Matches end of string.
  32	G Matches point where last match finished.
  33	b Matches word boundaries when outside brackets. Matches backspace (0x08) when inside brackets.
  34	B Matches nonword boundaries.
  35	n, t, etc. Matches newlines, carriage returns, tabs, etc.
  36	1...9 Matches nth grouped subexpression.
  37	10 Matches nth grouped subexpression if it matched already. Otherwise refers to the octal representation of a character code.

  Matches nth grouped subexpression if it matched already. Otherwise refers to the octal representation of a character code.

  Regular Expression Examples

  Literal characters

  Sr.No.	Example & Description
  1	python Match "python".

  Character classes

  Sr.No.	Example & Description
  1	[Pp]ython Match "Python" or "python"
  2	rub[ye] Match "ruby" or "rube"
  3	[aeiou] Match any one lowercase vowel
  4	[0-9] Match any digit; same as [0123456789]
  5	[a-z] Match any lowercase ASCII letter
  6	[A-Z] Match any uppercase ASCII letter
  7	[a-zA-Z0-9] Match any of the above
  8	[^aeiou] Match anything other than a lowercase vowel
  9	[^0-9] Match anything other than a digit

  Special Character Classes

  Sr.No.	Example & Description
  1	. Match any character except newline
  2	d Match a digit: [0-9]
  3	D Match a nondigit: [^0-9]
  4	s Match a whitespace character: [ trnf]
  5	S Match nonwhitespace: [^ trnf]
  6	w Match a single word character: [A-Za-z0-9_]
  7	W Match a nonword character: [^A-Za-z0-9_]

  .
  ## Repetition Cases

  Sr.No.	Example & Description
  1	ruby? Match "rub" or "ruby": the y is optional
  2	ruby* Match "rub" plus 0 or more ys
  3	ruby+ Match "rub" plus 1 or more ys
  4	d{3} Match exactly 3 digits
  5	d{3,} Match 3 or more digits
  6	d{3,5} Match 3, 4, or 5 digits

  ## Nongreedy repetition
  This matches the smallest number of repetitions −

  Sr.No.	Example & Description
  1	Greedy repetition: matches "perl>"
  2	Nongreedy: matches "" in "perl>"

  ## Grouping with Parentheses

  Sr.No.	Example & Description
  1	Dd+ No group: + repeats d
  2	(Dd)+ Grouped: + repeats Dd pair
  3	([Pp]ython(, )?)+ Match "Python", "Python, python, python", etc.

  ## Backreferences
  This matches a previously matched group again −

  Sr.No.	Example & Description
  1	([Pp])ython&1ails Match python&pails or Python&Pails
  2	(['"])[^1]*1 Single or double-quoted string. 1 matches whatever the 1st group matched. 2 matches whatever the 2nd group matched, etc.

  ## Alternatives

  Sr.No.	Example & Description
  1	python|perl Match "python" or "perl"
  2	rub(y|le)) Match "ruby" or "ruble"
  3	Python(!+|?) "Python" followed by one or more ! or one ?

  ## Anchors
  This needs to specify match position.

  Sr.No.	Example & Description
  1	^Python Match "Python" at the start of a string or internal line
  2	Python$ Match "Python" at the end of a string or line
  3	APython Match "Python" at the start of a string
  4	PythonZ Match "Python" at the end of a string
  5	bPythonb Match "Python" at a word boundary
  6	brubB B is nonword boundary: match "rub" in "rube" and "ruby" but not alone
  7	Python(?=!) Match "Python", if followed by an exclamation point.
  8	Python(?!!) Match "Python", if not followed by an exclamation point.

  ## Special Syntax with Parentheses

  Sr.No.	Example & Description
  1	R(?#comment) Matches "R". All the rest is a comment
  2	R(?i)uby Case-insensitive while matching "uby"
  3	R(?i:uby) Same as above
  4	rub(?:y|le)) Group only without creating 1 backreference