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There are several reasons why Wing may fail to stop on breakpoints or fail to show the Python source code when the breakpoint is reached:

Missing or Incorrect Source File Names -- The most common cause of failure to stop on breakpoints or to bring up source windows while stopping or stepping through code is a mismatch between the file name that is stored in the *.pyc file and the actual location of the *.py source file.

This can be caused by (1) not saving before you run in the debugger, (2) using partial path names on PYTHONPATH or when invoking a script from the command line (the partial path stored in the *.pyc file may become invalid if current directory changes), (3) moving around the *.pyc file after they are created, or (4) using compileall.py to create *.pyc files from source. The easiest way to solve this is to use only full paths on PYTHONPATH and remove any suspect *.pyc files.

Concurrent Processes -- Wing may fail to stop when debugging an application that gets invoked repeatedly in separate processes, for example a CGI script invoked multiple times from a browser as part of a page load. This is because the debugger can only debug one process at a time. If the debugger is already connected to one process, the second and later processes will not be debugged and thus may miss breakpoints.

Other Problems -- Less common causes of this problem are (1) running Python with the -O optimization option, (2) running Python with psyco or other optimizer, (3) overriding the Python __import__ routine, (4) adding breakpoints after you've started debugging an application that spends much of its time in C/C++ or other non-Python code, and (5) on Windows, using symbolic links to directories that contain your source code files.

For more information, see the Debugger Limitations section.

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Failure to Stop on Breakpoints or Show Source Code