Stop propagating if import wx fails

[Boštjan Mejak]

The code is worth a thousand words, so here are my thousand words:

try:

    import wx_  # Intentionally added the trailing underscore to invoke ImportError

except ImportError:

    print("You don't seem to have wxPython installed.")

class MyFrame(wx.Frame):

    pass

I have a problem that even though the ImportError exception is properly handled the propagation does not stop there but propagates to  class MyFrame(wx.Frame)  and then throws the NameError exception at me, saying "Name 'wx' is not defined.". It doesn't print the "You don't seem to have wxPython installed." message at all.

How can I fix my code so that if an ImportError is caught the propagation to  class MyFrame(wx.Frame)  does not happen?

[Tim Roberts]

Boštjan Mejak wrote:
> The code is worth a thousand words, so here are my thousand words:
>
> try:
> import wx_ # Intentionally added the trailing underscore to
> invoke ImportError
> except ImportError:
> print("You don't seem to have wxPython installed.")
>
> class MyFrame(wx.Frame):
> pass
>
>
> I have a problem that even though the ImportError exception is
> properly handled the propagation does not stop there but propagates to
> class MyFrame(wx.Frame) and then throws the NameError exception at
> me, saying "Name 'wx' is not defined.". It doesn't print the "You
> don't seem to have wxPython installed." message at all.

You are misdiagnosing the situation. This is not a case of error
propagation. "Name 'wx' is not defined" is a brand-new and entirely
correct error. If you delete the first four lines completely and do
nothing but:

class MyFrame(wx.Frame):
pass

you would get exactly the same error, and for exactly the same reason.

> How can I fix my code so that if an ImportError is caught the
> propagation to class MyFrame(wx.Frame) does not happen?

If you fail to import wx, how can you possibly hope to drive a class
from wx.Frame?

--
Tim Roberts, ti...@probo.com
Providenza & Boekelheide, Inc.

[Chris Barker]

On Mon, Jul 13, 2015 at 2:58 AM, Boštjan Mejak <bostjan...@gmail.com> wrote:

The code is worth a thousand words, so here are my thousand words:

try:

    import wx_  # Intentionally added the trailing underscore to invoke ImportError

except ImportError:

    print("You don't seem to have wxPython installed.")

you have just handled the Exception, so you are telling python to keep gng and try to run the rest of the code. Which, of course, it can't do, bcause wx was not imported.

try:
    import wx_  # Intentionally added the trailing underscore to invoke ImportError
except ImportError:

    print("You don't seem to have wxPython installed.")

    raise

Should do it -- but the IMPort Error will already give a message that the wx module can't be found, so this doesn't buy you much.

Unless you provide a longer an more interesting message, like pointers to how to install wx.

-Chris

 

class MyFrame(wx.Frame):

    pass

I have a problem that even though the ImportError exception is properly handled the propagation does not stop there but propagates to  class MyFrame(wx.Frame)  and then throws the NameError exception at me, saying "Name 'wx' is not defined.". It doesn't print the "You don't seem to have wxPython installed." message at all.

How can I fix my code so that if an ImportError is caught the propagation to  class MyFrame(wx.Frame)  does not happen?

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[Mike Stover]

As Chris stated, your Try/Except block is only issuing a print when the exception occurs. I normally use something along the lines of:

    try:

        import wx

    except ImportError as e:

        print("%s" % e) # print only used for demonstration, I normally log errors like this to a file

        sys.exit(1)

-Mike S

[Boštjan Mejak]

I forgot to mention that I create a message box in which I tell the user that he/she doesn't have wxPython installed and inform the user to install it. This message box is in the except block.
ctypes.windll.user32.MessageBoxA is the thing I use as the message box. But that message box is never shown. I immediately get the NameError exception that "name 'wx' is not defined" because Python goes and sees the line class MyFrame(wx.Frame) and throws me that NameError. So how can I make this work? I wanna show that message box. Any ideas?

[Mike Stover]

See if this helps you out, it works for me with Python 2.7, wxPython 2.9.5 on Win 7 64bit.

import sys

try:

    import wx_

except ImportError as e:

    import ctypes

    messageBox = ctypes.windll.user32.MessageBoxA

    returnValue = messageBox(None, '%s' % e, 'ERROR!', 0x40|0x0)

    sys.exit(0)

class MyFrame(wx.Frame):

    pass

def RunMyProg():

    prog = wx.App()

    frame = MyFrame()

    frame.Show()

    prog.MainLoop()

if __name__ == '__main__':

    RunMyProg()

Essentially what this little code snippet does is attempt to import wx_, which we know will cause an ImportError. Then it goes ahead and import ctypes, creates the message box with the exception error and displays the message. When the user clicks ok it automatically exits the script.

- Mike S

[Boštjan Mejak]

Thanks. I'll try the code when I come back from work.

[Boštjan Mejak]

Yeah, my code works now. Thanks. I just need one more question answered. What are the keyword arguments for the MessageBoxA() function?

[Tim Roberts]

Boštjan Mejak wrote:
> Yeah, my code works now. Thanks. I just need one more question
> answered. What are the keyword arguments for the MessageBoxA() function?

That's a raw Windows API. I don't think ctypes supports keyword
arguments. If you want more information on the arguments, you can
Google the MSDN documentation.

[Mike Stover]

I don't use ctypes enough to know what can or cannot be done with the library, but as Tim suggested you should Google the MSDN documentation.

[Boštjan Mejak]

Don't bother, I used constants to make the calling function more descriptive. ;)