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How to catch a custom exception not defined by me in python

I have the following try block:
try:
# depending on conditions, this can generate several types of errors
mymodule.do_stuff()
except Exception as e:
print("something went wrong, the error was :" + type(e).__name__)
I would like to catch potential errors from do_stuff(). After trial and error I was able to generate a list of potential errors that can be triggered by do_stuff() by printing their type(e).__name__ value:
DoStuffInsufficientMemoryException
DoStuffInsufficientCPUException
DoStuffInsufficientDiskException
but if I try do modify my except statement from except Exception as e to except DoStuffInsufficientMemoryException, I will get the error that DoStuffInsufficientMemoryException is not defined.
I tried defining a class that extends Exception for it, as most tutorials / questions in here suggest, basically:
class WAFInvalidParameterException(Exception):
pass
so now that variable is recognized, but since I can't control the error that do_sutff() will raise, I can't really raise this exception in my initial try block.
Ideally I would like to have 1 except block for each error so I would like to have something like:
try:
mymodule.do_stuff()
except DoStuffInsufficientMemoryException:
free_memory()
except DoStuffInsufficientCPUException:
kill_processes()
except DoStuffInsufficientDiskException:
free_disk_space()
but of course this doesn't work as these variables are not defined.

Just like you can't reference do_stuff without its module specifier, you have to specify in which module namespace these exceptions are defined.
try:
mymodule.do_stuff()
except mymodule.DoStuffInsufficientMemoryException:
free_memory()
except mymodule.DoStuffInsufficientCPUException:
kill_processes()
except mymodule.DoStuffInsufficientDiskException:
free_disk_space()
If free_memory is also in the mymodule namespace, of course you need to specify it there as well.
Alternatively, when you import mymodule, you can explicitly import selected symbols into the current namespace:
from mymodule import do_stuff, DoStuffInsufficientMemoryException, ...
and then, because they are in the current package, you can (or indeed must) refer to them without the package prefix mymodule.
A well-designed module will export selected symbols so you can refer to them without the package prefix, but whether this makes sense for your own package depends on its general design and intended audience. Some large packages define a separate subpackage for exceptions so you can say
import bigpackage.exceptions
to import them all. You will probably still want to explore the package's documentation (or, if it's lacking, its source code) to discover which exceptions exist and how they are organized. Many packages define a base exception class from which all its other exceptions are subclasses so that you can easily catch them all with just one symbol, like
try:
bigpackage.heavy_wizardry()
except bigpackage.BigBaseException:
print("you are turned into a frog")

EDIT : you can import other methods instead of creating your own, of course
The try/except block will try to execute the code and if an error is raised and specified in the except statement, it will stop the execution of the code located in the try block and execute the other code located in the except block. So, to catch your custom error, you have to raise it in the first place.
If you didn't know, you can raise errors using the raise statement. Here, I've made a simple chunk of code. I have a custom error, a variable x initialized at 2, and a method that adds 1 to the variable given in argument. The method will raise a CustomError if the variable becomes 3.
# Here, I define the custom error and allow a custom message to be displayed
# using the super() method
class CustomError(Exception):
def __init__(self, msg):
super().__init__(msg)
# I initialize x at 2
x = 2
# I create the method that will add 1 to the variable given in argument
# and raise a CustomError if the variable becomes 3
# This is completely random, and you can make whatever checks you want before raising
# Your custom error
def do_stuff(x):
x += 1
if x == 3:
raise CustomError("x is 3")
# Now, I write the try/except block. I raise e (the CustomError) if it is
# being raised in the method do_stuff(x)
try:
do_stuff(x)
except CustomError as e:
raise e
Feel free to try the code out to get a better understanding of it !

Usually if a function
module.foo()
throws an exception DoStuffInsufficientMemoryException it would be also importable as
from module import DoStuffInsufficientMemoryException
If this results in ImportError then you need the fullname function from this answer; use it with e (it takes an instance and returns the class name). If it gives
foo.bar.DoStuffInsufficientMemoryException
then you can import the exception as
from foo.bar import DoStuffInsufficientMemoryException
The above might not work for all cases. One notable case is Boto 3 AWS client library that does not make the exceptions importable - instead they will be attributes on the client instance.

strange behavior of constant 'e' in Python

I experienced very strange behavior in Python.
I am an electronics engineer. So I use constant 'e' in the equation sometimes.
from math import *
try:
eval('print(log(e))')
except Exception as e:
print(e)
try:
eval('print(long(e))')
except Exception as e:
print(e)
try:
eval('log(e)')
except Exception as e:
print(e)
the output is
1.0
name 'long' is not defined
name 'e' is not defined
I miss typed log(e) to long(e). Before that equation log(e) works well, but the second log(e) after long(e), the Python does not understand 'e'.
Do you have any idea what's going on there?
I am using python3.8.2 on Windows.

Setting:
except Exception as e:
shadows the imported name from math, but only if the except block is entered (i.e. an error is thrown). The reference is cleared outside the except block, but that removes any reference to e. It's equivalent to:
>>> from math import e
>>> e
2.718281828459045
>>> e = "foo" # shadow the imported name
>>> e
'foo'
>>> del e # try to return to previous value
>>> e
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'e' is not defined
This is also mentioned in the documentation for the try statement (emphasis mine):
When an exception has been assigned using as target, it is cleared
at the end of the except clause. This is as if
except E as N:
foo
was translated to
except E as N:
try:
foo
finally:
del N
The simplest fixes are to either:
rename the exception to something other than e; or
import math then refer to math.e (note wildcard imports are discouraged by PEP-0008).

So we are shadowing the variable e from the math module. In the second try-catch you are catching the exception as e, so, the e from math module get replaced by this e (exception).
You should always keep in mind that never use a variable name that is already being used by some module that you are importing etc. For example, don't use variable name list as it will shadow the built-in type list and you program will behave strangely.
In order to fix this problem, replace this:
except Exception as e:
to this:
except Exception as ex:

pylint syntax error on valid program (raise with three arguments / expressions)

I'm looking at this minimal valid(?) program:
import sys
def f():
try:
raise Exception()
except Exception:
raise Exception(), None, sys.exc_info()[2]
f()
This program executes and behaves as expected , preserving the stack trace of the inner exception, as documented by help("raise"). However, when I run pylint on it, this is what I get:
$ pylint program.py
************* Module tmp
E: 7, 0: invalid syntax (<string>, line 7) (syntax-error)
The syntax-error disappears when I remove the second and third expressions to raise.
Is this a bug in pylint, or am I overlooking something?

Your pylint binary testing for Python 3 syntax, your code is valid for Python 2 only. Pylint tests code following the syntax of the Python binary you installed it with (it uses Python's own parser).
In Python 3, you'd use:
raise Exception().with_traceback(sys.exc_info()[2])
See the raise statement documentation for Python 3.
While your syntax may be correct for Python 2, you are technically using raise wrong. When passing in 3 elements, the first must be a class, not an instance. The second is an instance of that class, the third the traceback:
raise Exception, Exception(), sys.exc_info()[2]
or you can pass in None for an empty argument list passed to the first (the class) to create an instance:
raise Exception, None, sys.exc_info()[2]
Your code still happens to work, but only because Python isn't being too strict and takes that first argument as the instance when it is not a class.
If you want to test Python 2 code with pylint, install a copy into your Python 2 binary, and run that version. See Specify which python version pylint should evaluate for

Catch Built-in Exception (rather than self defined)

I want to patch a library to catch the built-in ConnectionError (which inherits from OSError).
So far so good. As it happens, the library has a "self-defined" Exception that is also called ConnectionError:
class LibraryError(Exception):
pass
class ConnectionError(LibraryError):
pass
I guess, if I now tried to catch a ConnectionError, doing something like
try:
do_something()
except ConnectionError as e:
try_to_get_it_right_again()
I would only catch the self-defined ConnectionError, which inherits from LibraryError. (Disclaimer: I have to admit, I haven't tested that myself, as I didn't know how).
How would I get Python to catch the built-in ConnectionError?

Use the builtins module, the explicit name for the namespace where built-in names like int and ConnectionError live.
import builtins
try:
...
except builtins.ConnectionError:
...
In Python 2, this would be __builtin__, although Python 2 doesn't have ConnectionError. Note that __builtins__ is its own weird thing; even if it looks like what you want, it's not.
If you want code that works in both Python 2 and Python 3... well, the exception hierarchy looks pretty different in Python 2, and ConnectionError doesn't even exist, so it's not as simple as deciding whether to use builtins or __builtin__. The builtins/__builtin__ thing is easy enough to solve, at least.
To import the right module depending on Python version, you can catch the ImportError and import the other module:
try:
import builtins
except ImportError:
import __builtin__ as builtins
Pretending for a moment that Python 2 has ConnectionError, you could save a reference to the built-in ConnectionError before shadowing the name:
_builtin_ConnectionError = ConnectionError
class ConnectionError(LibraryError):
...

Use the the ConnectionError defined along with the other exceptions in the builtins library:
import builtins
try:
# connection error raised
except builtins.ConnectionError as conerr:
# handle stuff

how can I use assertRaises() in python's unittest to catch syntaxerror?

For example, I want to test a function which has a syntax, in my unittest class's method, can I use code as the following?
self.assertRaises(SyntaxError, my_function)
When I use this, it just appears traceback of syntax error rather than showing how many tests have passed.

In order for the test to run, the code must be byte compiled by the Python interpreter. This happens when the module containing your function is imported, before the function is ever run. It is during the compilation that the SyntaxError is generated.
In your test module, you could wrap the import statement in a try/except:
raised = False
try:
import foo
except SyntaxError:
# A syntax error was generated during the import...
raised = True
self.assert_(raised, "'import foo' failed to raise a SyntaxError.")
or use one of the methods suggested by #alecxe, which look simpler and cleaner.

Following Warren Weckesser's explanation, you can test that an import function is throwing an error:
self.assertRaises(SyntaxError, __import__, "error_library")
For Python 2.7 and above, importlib.import_module() can/should be used instead:
self.assertRaises(SyntaxError, importlib.import_module, "error_library")