try: except: not working

So I'm ru开发者_JS百科nning into a problem where the try: except: mechanism doesn't seem to be working correctly in python.

Here are the contents of my two files.

pytest1.py

import pytest2

class MyError( Exception ):
    def __init__( self, value ):
        self.value = value

    def __str__( self ):
        return repr( self.value )

def func1():
    raise MyError( 'This is an error' )

def func3():
    pytest2.func2()

if __name__ == '__main__':
    try:
        func3()
    except MyError, e:
        print 'I should catch here.'
    except:
        print 'Why caught here?'

pytest2.py

from pytest1 import func1

def func2():
    func1()

Executing the first file yields the following output:

$ python pytest1.py
Why caught here?

Basically, the exception isn't being caught. If I print out the exception type, it prints as <pytest1.MyError> instead of just <MyError>. I imagine that this is some weird cyclical reference thing, but it still seems like it should work.

The main python program is always imported as the module __main__.

When you import pytest2, it doesn't reuse the existing module because the originally imported module has the name __main__ not pytest2. The result is that pytest1 is run multiple times generating multiple exception classes. __main__.MyError and pytest1.MyError You end up throwing one and trying to catch the other.

So, don't try to import your main module from other modules.

This problem is caused by importing the script you are running as a module. This produces two separate copies of the module!

Another example:

module.py

import module

class Foo: pass

def test():
    print Foo
    print module.Foo
    print Foo is module.Foo

if __name__ == '__main__': test()

main_script.py

import module
if __name__ == '__main__': module.test()

Result

>python main_script.py
module.Foo
module.Foo
True

>python module.py
__main__.Foo
module.Foo
False

Running python somefile.py creates a module called __main__, not somefile, and runs the code in somefile.py in that module. This is why if __name__ == '__main__': is used to check if this file is being run as a script or imported from some other file.

... at a guess, you have a namespace problem which is producing a different exception.

Try replacing

except:
    print 'Why caught here?'

with

except Exception, e:
    print e

This may tell you more about what went wrong.