I have seen other posts/videos in order to figure out how to solve this issue I am having but with no success. I am trying to raise an exception if the third parameter(p) is a string datatype but all my attempts in order to achieve this have not been successful and was looking for some help on what I am doing wrong.
class Friends(Ben):
def __init__(self, frank, greg, p):
Ben.__init__(self, frank, greg)
self.p = p
try:
if p == str:
raise TypeError("This is a string!")
except:
print("This not a string")
There are a number of oddities in your piece of code; however, to address the issue of raising an Exception, you use raise to do that, as other answers indicate.
The try/except syntax is used to catch and handle exceptions when they occur. As an example, here is a reworked, standalone version of your code snippet that illustrates this.
class Friends:
def __init__(self, frank, greg, p):
if isinstance(p, str):
raise TypeError(p, "This is a string!")
self.p = p
try:
friends = Friends('Frank', 'Greg', 'dubious_string')
except TypeError as e:
print("Hey, I caught the error!")
# print the exception
print(e)
# raise the exception again
raise e
Output:
Hey, I caught the error!
('dubious_string', 'This is a string!')
Traceback (most recent call last):
File "tmp.py", line 14, in <module>
raise e
File "tmp.py", line 10, in <module>
friends = Friends('Frank', 'Greg', 'dubious_string')
File "tmp.py", line 5, in __init__
raise TypeError(p, "This is a string!")
TypeError: ('dubious_string', 'This is a string!')
I'm not sure what you are trying to do with the class, but I think this is what you are looking for:
#p = 10
p = "some string"
if type(p) == str:
raise Exception("This is a string")
class Ben:
def __init__(self):
pass
class Friends(Ben):
def __init__(self,frank,greg,p):
if isinstance(p, str):
raise TypeError('Why are you giving me strings!')
Ben.__init__(frank,greg)
self.p = p
group = Friends('frank', 'greg', 'evil_string')
Output:
Traceback (most recent call last):
File "C:\Users\StackOverflow\Desktop\temp.py", line 15, in <module>
group = Friends('frank', 'greg', 'evil_string')
File "C:\Users\StackOverflow\Desktop\temp.py", line 10, in __init__
raise TypeError('Why are you giving me strings!')
TypeError: Why are you giving me strings!
Related
I am working on a project for university and I am having trouble with raising errors.
I am supposed to create several functions that will eventually be re-used in even more functions that I am about to create. I am supposed to use Raise ValueError when some specific arguments are being used. Yet, when I use the initial functions to define new functions, the ValueError that gets raised is the initial one and not the new one.
def first_function(arg1):
if isinstance(arg1, tuple):
return True
else:
raise ValueError("This is not a tuple")
def second_function(arg2):
if first_function(arg2):
print("That Is Indeed a Tuple")
else:
raise ValueError("This is really not a tuple")
argument = "(1, 1)"
print(second_function(argument))
# output: ValueError: This is not a tuple
# desired output: ValueError: This is really not a tuple
How can I fix this? I am not supposed to repeat code. I am supposed to keep re-using the functions I build, yet the errors seem to interfere.
So, technically, the "This is not a tuple" error is functioning correctly. The error will stop the code - where the error happens - and report on the stack in that state. This is desirable.
That said, you can use a try/except/finally to catch the first error and pass it back to the second function, which is what I think you're trying to do.
def first_function(arg1):
if isinstance(arg1, tuple):
return True
else:
raise ValueError("This is not a tuple")
def second_function(arg2):
# The try statement wraps the if/then/else
try:
# When you call first_function, it will error
# That error will be recognized and passed to
# second_function's "except" statement
if first_function(arg2):
print("That Is Indeed a Tuple")
# This else is probably not necessary
# Since first_function will either work (return True), or
# raise an error, you will never have a situation where
# "if first_function(arg2):" will ever reach this else
# else:
# raise ValueError("This is not a tuple")
except ValueError:
raise ValueError("This is really not a tuple")
argument = "(1, 1)"
print(second_function(argument))
Traceback (most recent call last):
File "/home/rightmire/eclipse-workspace/junkcode/test.py", line 18, in second_function
if first_function(arg2):
File "/home/rightmire/eclipse-workspace/junkcode/test.py", line 6, in first_function
raise ValueError("This is not a tuple")
ValueError: This is not a tuple
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/home/rightmire/eclipse-workspace/junkcode/test.py", line 27, in <module>
print(second_function(argument))
File "/home/rightmire/eclipse-workspace/junkcode/test.py", line 23, in second_function
raise ValueError("This is really not a tuple")
ValueError: This is really not a tuple
As an aside...
The functional goal you're usually trying to accomplish with this type of design pattern, is to avoid the need to do checks like you're doing in first_function (where it checks if the arg is a tuple).
This design pattern is usually so you can leave the error checking to the main calling method, and all the called methods can just do their thing - raising errors if bad code is sent.
I.e.
def first_function(arg1):
return arg1.append("I should be a list")
def second_function(arg2):
try:
_list = first_function(arg2)
print("arg2 is, indeed, a list")
except Exception as e:
raise ValueError("You did not pass a list (ORIG ERROR: {}".format(e))
argument = "(1, 1)"
print(second_function(argument))
OUTPUT:
Traceback (most recent call last):
File "/home/rightmire/eclipse-workspace/junkcode/test.py", line 7, in second_function
_list = first_function(arg2)
File "/home/rightmire/eclipse-workspace/junkcode/test.py", line 2, in first_function
arg1.append("I should be a list")
AttributeError: 'str' object has no attribute 'append'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/home/rightmire/eclipse-workspace/junkcode/test.py", line 13, in <module>
print(second_function(argument))
File "/home/rightmire/eclipse-workspace/junkcode/test.py", line 10, in second_function
raise ValueError("You did not pass a list (ORIG ERROR: {}".format(e))
ValueError: You did not pass a list (ORIG ERROR: 'str' object has no attribute 'append'
Functionally, the try/except/finally is used to silence errors. As an example...
def first_function(_list, arg1):
_list.append(int(arg1))
return _list
def second_function():
_list = []
while True:
arg2 = input("Enter an integer:")
try:
_list = first_function(_list, arg2)
print("a list containing {}".format(_list))
except Exception as e:
print ("You did not pass an integer. Try again")
second_function()
OUTPUT:
Enter an integer:1
a list containing [1]
Enter an integer:2
a list containing [1, 2]
Enter an integer:3
a list containing [1, 2, 3]
Enter an integer:r
You did not pass an integer. Try again
Enter an integer:4
a list containing [1, 2, 3, 4]
Enter an integer:
suppose I have the following function:
def test():
...
if x['error']:
raise
This would raise an exception regardless if x['error'] is defined or not.
Instead if I try this, it doesn't throw any exception:
def test():
...
try:
if x['error']:
raise
except:
return
How can I test for a specific value and return an exception if it is defined, and to return successfully if it is not defined?
If you want to return error as string:
>>> def test():
try:
if x['error']:raise
except Exception as err:
return err
>>> test()
NameError("name 'x' is not defined",)
If you want to an error to occur:
>>> def test():
try:
if x['error']:raise
except:
raise
>>> test()
Traceback (most recent call last):
File "<pyshell#20>", line 1, in <module>
test()
File "<pyshell#19>", line 3, in test
if x['error']:raise
NameError: name 'x' is not defined
def test():
...
if x.get(‘error’):
raise
You can avoid unintentionally raising an error using the dictionary's built in get function. Get will return None if the value at the specified key does not exist instead of throwing an exception.
try this one
def check_not_exist(d,k):
#if keys exists in dict,raise it
if k in d:
raise
else:
return True
I hope this is not a naive question. The following is the code:
>>> print(55/0)
Traceback (most recent call last):
File "<interactive input>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero
I understand that python has the traceback object, which is a kind of exception object I guess. My question is: what is the meaning of
"in <module>" in the second line? Which module is it? Does it mean in the current module always? It does not provide any useful information, right?
Many thanks for your time and attention.
After trying the following code, I finally understand it by myself.
def get_age():
age = input("Please enter your age: ")
if not age.isdigit():
my_error = TypeError("{0} is not integer".format(age))
raise my_error
elif age < 0:
# Create a new instance of an exception
my_error = ValueError("{0} is not positive".format(age))
raise my_error
return age
def a():
get_age()
def b():
a()
def c():
b()
def d():
try:
c()
print("haha")
except ValueError:
print("no one has negative age, you fool")
#except TypeError:
#print("you should input an integer, dear")
d()
The following is the error message when I did not handle the TypeError.
RESTART: the_path_to_my_python_file.py
Please enter your age: th
Traceback (most recent call last):
File "the_path_to_my_python_file.py", line 33, in <module>
d()
File "the_path_to_my_python_file.py", line 26, in d
c()
File "the_path_to_my_python_file.py", line 22, in c
b()
File "the_path_to_my_python_file.py", line 18, in b
a()
File "the_path_to_my_python_file.py", line 14, in a
get_age()
File "the_path_to_my_python_file.py", line 5, in get_age
raise my_error
TypeError: th is not integer
You see that it does not tell you "in module" anymore. Each time it tells you the name of the function in which the error occurs. I think that is the kind of information "in module" would provide.
I copied and pasted these lines of code from a Python tutorial book. Why does this code not work when I try to run it in PyCharm?
def inputNumber ():
x = input ('Pick a number: ')
if x == 17:
raise 'BadNumberError', '17 is a bad number'
return x
inputNumber()
This is what I got when I run the code:
Pick a number: 17
Traceback (most recent call last):
File "C:/Users/arman/Desktop/Scribble/Hello.py", line 153, in <module>
inputNumber()
File "C:/Users/arman/Desktop/Scribble/Hello.py", line 151, in inputNumber
raise 'BadNumberError', '17 is a bad number'
TypeError: exceptions must be old-style classes or derived from BaseException, not str
You can use standard exceptions:
raise ValueError('17 is a bad number')
Or you can define your own:
class BadNumberError(Exception):
pass
And then use it:
raise BadNumberError('17 is a bad number')
Just inherit from Exception class, then you can throw your own exceptions:
class BadNumberException(Exception):
pass
raise BadNumberException('17 is a bad number')
output:
Traceback (most recent call last):
File "<module1>", line 4, in <module>
BadNumberException: 17 is a bad number
If you want to define a your own error you have to do:
class BadNumberError(Exception):
pass
and then use it:
def inputNumber ():
x = input ('Pick a number: ')
if x == 17:
raise BadNumberError('17 is a bad number')
return x
inputNumber()
You should be raising exceptions as raise as follows BadNumberError('17 is a bad number') if you have already defined BadNumberError class exception.
If you haven't, then
class BadNumberError(Exception):
pass
And here is the docs with information about raising exceptions
I'm working on a mail-sending library, and I want to be able to catch exceptions produced by the senders (SMTP, Google AppEngine, etc.) and wrap them in easily catchable exceptions specific to my library (ConnectionError, MessageSendError, etc.), with the original traceback intact so it can be debugged. What is the best way to do this in Python 2?
The simplest way would be to reraise with the old trace object. The following example shows this:
import sys
def a():
def b():
raise AssertionError("1")
b()
try:
a()
except AssertionError: # some specific exception you want to wrap
trace = sys.exc_info()[2]
raise Exception("error description"), None, trace
Check the documentation of the raise statement for details of the three parameters. My example would print:
Traceback (most recent call last):
File "C:\...\test.py", line 9, in <module>
a()
File "C:\...\test.py", line 6, in a
b()
File "C:\...\test.py", line 5, in b
raise AssertionError("1")
Exception: error description
For completeness, in Python 3 you'd use the raise MyException(...) from e syntax.
This answer is probably a little bit late, but you can wrap the function in a python decorator.
Here is a simple cheatsheet on how different decorators.
Here is some sample code of how to do this. Just change the decorator to catch different errors in the different ways that you need.
def decorator(wrapped_function):
def _wrapper(*args, **kwargs):
try:
# do something before the function call
result = wrapped_function(*args, **kwargs)
# do something after the function call
except TypeError:
print("TypeError")
except IndexError:
print("IndexError")
# return result
return _wrapper
#decorator
def type_error():
return 1 / 'a'
#decorator
def index_error():
return ['foo', 'bar'][5]
type_error()
index_error()
Use raise_from from the future.utils package.
Relevant example copied below:
from future.utils import raise_from
class FileDatabase:
def __init__(self, filename):
try:
self.file = open(filename)
except IOError as exc:
raise_from(DatabaseError('failed to open'), exc)
Within that package, raise_from is implemented as follows:
def raise_from(exc, cause):
"""
Equivalent to:
raise EXCEPTION from CAUSE
on Python 3. (See PEP 3134).
"""
# Is either arg an exception class (e.g. IndexError) rather than
# instance (e.g. IndexError('my message here')? If so, pass the
# name of the class undisturbed through to "raise ... from ...".
if isinstance(exc, type) and issubclass(exc, Exception):
e = exc()
# exc = exc.__name__
# execstr = "e = " + _repr_strip(exc) + "()"
# myglobals, mylocals = _get_caller_globals_and_locals()
# exec(execstr, myglobals, mylocals)
else:
e = exc
e.__suppress_context__ = False
if isinstance(cause, type) and issubclass(cause, Exception):
e.__cause__ = cause()
e.__suppress_context__ = True
elif cause is None:
e.__cause__ = None
e.__suppress_context__ = True
elif isinstance(cause, BaseException):
e.__cause__ = cause
e.__suppress_context__ = True
else:
raise TypeError("exception causes must derive from BaseException")
e.__context__ = sys.exc_info()[1]
raise e