Manually raising throwing an exception in Python

Question

How can I raise an exception in Python so that it can later be caught via an except block

User · Answer

You should learn the raise statement of python for that  It should be kept inside the try block  Example -  try      raise TypeError             remove TypeError by any other error if you want except TypeError      print  TypeError raised

User · Answer

In Python3 there are 4 different syntaxes for rasing exceptions:

1. raise exception 
2. raise exception (args) 
3. raise
4. raise exception (args) from original_exception

1. raise exception vs. 2. raise exception (args)

If you use raise exception (args) to raise an exception then the args will be printed when you print the exception object - as shown in the example below.

  #raise exception (args)
    try:
        raise ValueError("I have raised an Exception")
    except ValueError as exp:
        print ("Error", exp)     # Output -> Error I have raised an Exception 



  #raise execption 
    try:
        raise ValueError
    except ValueError as exp:
        print ("Error", exp)     # Output -> Error

3.raise

raise statement without any arguments re-raises the last exception. This is useful if you need to perform some actions after catching the exception and then want to re-raise it. But if there was no exception before, raise statement raises TypeError Exception.

def somefunction():
    print("some cleaning")

a=10
b=0 
result=None

try:
    result=a/b
    print(result)

except Exception:            #Output ->
    somefunction()           #some cleaning
    raise                    #Traceback (most recent call last):
                             #File "python", line 8, in <module>
                             #ZeroDivisionError: division by zero

4. raise exception (args) from original_exception

This statement is used to create exception chaining in which an exception that is raised in response to another exception can contain the details of the original exception - as shown in the example below.

class MyCustomException(Exception):
pass

a=10
b=0 
reuslt=None
try:
    try:
        result=a/b

    except ZeroDivisionError as exp:
        print("ZeroDivisionError -- ",exp)
        raise MyCustomException("Zero Division ") from exp

except MyCustomException as exp:
        print("MyException",exp)
        print(exp.__cause__)

Output:

ZeroDivisionError --  division by zero
MyException Zero Division 
division by zero

User · Answer

DON T DO THIS  Raising a bare Exception is absolutely not the right thing to do  see Aaron Hall s excellent answer instead    Can t get much more pythonic than this   raise Exception  I know python      See the raise statement docs for python if you d like more info

User · Answer

Read the existing answers first  this is just an addendum   Notice that you can raise exceptions with or without arguments   Example   raise SystemExit   exits the program but you might want to know what happened So you can use this   raise SystemExit  program exited     this will print  program exited  to stderr before closing the program

User · Answer

How do I manually throw raise an exception in Python   Use the most specific Exception constructor that semantically fits your issue  Be specific in your message  e g   raise ValueError  A very specific bad thing happened     Don t raise generic exceptions Avoid raising a generic Exception  To catch it  you ll have to catch all other more specific exceptions that subclass it  Problem 1  Hiding bugs raise Exception  I know Python      Don t  If you catch  likely to hide bugs   For example  def demo bad catch        try          raise ValueError  Represents a hidden bug  do not catch this           raise Exception  This is the exception you expect to handle       except Exception as error          print  Caught this error      repr error     gt  gt  gt  demo bad catch   Caught this error  ValueError  Represents a hidden bug  do not catch this     Problem 2  Won t catch And more specific catches won t catch the general exception  def demo no catch        try          raise Exception  general exceptions not caught by specific handling       except ValueError as e          print  we will not catch exception  Exception       gt  gt  gt  demo no catch   Traceback  most recent call last     File  quot  lt stdin gt  quot   line 1  in  lt module gt    File  quot  lt stdin gt  quot   line 3  in demo no catch Exception  general exceptions not caught by specific handling  Best Practices  raise statement Instead  use the most specific Exception constructor that semantically fits your issue  raise ValueError  A very specific bad thing happened    which also handily allows an arbitrary number of arguments to be passed to the constructor  raise ValueError  A very specific bad thing happened    foo    bar    baz     These arguments are accessed by the args attribute on the Exception object  For example  try      some code that may raise our value error   except ValueError as err      print err args   prints   message    foo    bar    baz        In Python 2 5  an actual message attribute was added to BaseException in favor of encouraging users to subclass Exceptions and stop using args  but the introduction of message and the original deprecation of args has been retracted  Best Practices  except clause When inside an except clause  you might want to  for example  log that a specific type of error happened  and then re-raise  The best way to do this while preserving the stack trace is to use a bare raise statement  For example  logger   logging getLogger   name     try      do something in app that breaks easily   except AppError as error      logger error error      raise                   just this        raise AppError        Don t do this  you ll lose the stack trace   Don t modify your errors    but if you insist  You can preserve the stacktrace  and error value  with sys exc info    but this is way more error prone and has compatibility problems between Python 2 and 3  prefer to use a bare raise to re-raise  To explain - the sys exc info   returns the type  value  and traceback  type  value  traceback   sys exc info    This is the syntax in Python 2 - note this is not compatible with Python 3  raise AppError  error  sys exc info   2    avoid this    Equivalently  as error  is  the second object  raise sys exc info   0   sys exc info   1   sys exc info   2   If you want to  you can modify what happens with your new raise - e g  setting new args for the instance  def error        raise ValueError  oops     def catch error modify message        try          error       except ValueError          error type  error instance  traceback   sys exc info           error instance args    error instance args 0       lt modification gt             raise error type  error instance  traceback  And we have preserved the whole traceback while modifying the args  Note that this is not a best practice and it is invalid syntax in Python 3  making keeping compatibility much harder to work around    gt  gt  gt  catch error modify message   Traceback  most recent call last     File  quot  lt stdin gt  quot   line 1  in  lt module gt    File  quot  lt stdin gt  quot   line 3  in catch error modify message   File  quot  lt stdin gt  quot   line 2  in error ValueError  oops   lt modification gt   In Python 3  raise error with traceback sys exc info   2    Again  avoid manually manipulating tracebacks  It s less efficient and more error prone  And if you re using threading and sys exc info you may even get the wrong traceback  especially if you re using exception handling for control flow - which I d personally tend to avoid   Python 3  Exception chaining In Python 3  you can chain Exceptions  which preserve tracebacks  raise RuntimeError  specific message   from error  Be aware   this does allow changing the error type raised  and this is not compatible with Python 2   Deprecated Methods  These can easily hide and even get into production code  You want to raise an exception  and doing them will raise an exception  but not the one intended  Valid in Python 2  but not in Python 3 is the following  raise ValueError   message    Don t do this  it s deprecated   Only valid in much older versions of Python  2 4 and lower   you may still see people raising strings  raise  message    really really wrong  don t do this   In all modern versions  this will actually raise a TypeError  because you re not raising a BaseException type  If you re not checking for the right exception and don t have a reviewer that s aware of the issue  it could get into production  Example Usage I raise Exceptions to warn consumers of my API if they re using it incorrectly  def api func foo          foo should be either  baz  or  bar   returns something very useful         if foo not in  ALLOWED ARGS          raise ValueError   foo  wrong  use  quot baz quot  or  quot bar quot   format foo repr foo     Create your own error types when apropos   quot I want to make an error on purpose  so that it would go into the except quot   You can create your own error types  if you want to indicate something specific is wrong with your application  just subclass the appropriate point in the exception hierarchy  class MyAppLookupError LookupError          raise this when there s a lookup error for my app     and usage  if important key not in resource dict and not ok to be missing      raise MyAppLookupError  resource is missing  and that is not ok

User · Answer

Another way to throw an exceptions is assert  You can use assert to verify a condition is being fulfilled if not then it will raise AssertionError  For more details have a look here   def avg marks       assert len marks     0  List is empty       return sum marks  len marks   mark2    55 88 78 90 79  print  Average of mark2   avg mark2    mark1      print  Average of mark1   avg mark1

User · Answer

Just to note  there are times when you DO want to handle generic exceptions  If you re processing a bunch of files and logging your errors  you might want to catch any error that occurs for a file  log it  and continue processing the rest of the files  In that case  a try      foo    except Exception as e      print str e     Print out handled error  block is a good way to do it  You ll still want to raise specific exceptions so you know what they mean  though

User · Answer

For the common case where you need to throw an exception in response to some unexpected conditions  and that you never intend to catch  but simply to fail fast to enable you to debug from there if it ever happens     the most logical one seems to be AssertionError   if 0  lt  distance  lt   RADIUS       Do something  elif RADIUS  lt  distance       Do something  else      raise AssertionError  Unexpected value of  distance     distance

[python] Manually raising (throwing) an exception in Python

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