Tutorial: Debugging

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The example1.py program you have just created connects to python.org via HTTP, reads and parses the Python-related news feed in RDF format, and then prints the most recent five items as text and HTML. Don't worry if you are working offline. The script has canned data it will use when it cannot connect to python.org.

To start debugging, set a breakpoint on the line that reads return 5 in the GetItemCount function. This can be done by clicking on the line and selecting the break-set Break toolbar item, or by clicking on the left-most margin to the left of the line. The breakpoint should appear as a filled red circle:


Next start the debugger with continue Debug in the toolbar or the Start/Continue item in the Debug menu. Wing will show the Debug Properties dialog with the properties that will be used during the debug run. Just ignore this for now, uncheck the Show this dialog before each run checkbox at the bottom, and press OK.

System Message: WARNING/2 (<string>, line 25); backlinks:

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Note: In Wing 101, the toolbar icon to start debugging is instead a bug icon, and the green arrow is used to evaluate the current file in the Python Shell.

Wing will run to the breakpoint and stop, placing a red indicator on the line. Notice that the toolbar changes to include additional debug tools, as shown below:


Your display may vary depending on the size of your screen, or if you have altered the toolbar's configuration. Wing displays tooltips explaining what the items do when you hover the mouse over them.

Now you can inspect the program state at that point with the Stack Data tool and by going up and down the stack with up-stack Up Stack and down-stack Down Stack in the toolbar or from the Debug menu. The stack can also be viewed as a list using the Call Stack tool:


Notice that the debug status indicator in the lower left of Wing's main window changes color depending on the state of the debug process. Hover the mouse over the indicator to see detailed status in a tooltip.

Next, try stepping out to the enclosing call to ReadPythonNews. In this particular context, you can achieve this in a single click with the step-out Step Out in the toolbar or Debug menu. Two clicks on step-over Step Over also work. ReadPythonNews is a good function to step through in order to try out the basic debugger features described above.

Try stepping or running to a breakpoint on the last line of this function, which reads return news[:count]. In this context, right-clicking on news under locals in Stack Data allows viewing the value in textual form or as an array. The latter loads data incrementally for only the visible portion of the value, which is useful with numpy arrays, pandas DataFrames, sqlite query results, and other larger data sets.

Data can also be viewed in tooltips on the editor by hovering the mouse over a value. Try this with count to see the value 5. In Wing Pro, pressing Shift-Space displays tooltips for all values visible in the editor, if they are defined in the current stack frame. The last line of ReadPythonNews is also a good place to try that.

Finally, try step-over Step Over to reach the return event in ReadPythonNews, which is indicated by a change from the solid debug line marker to an underline. Notice that hovering the mouse over return in the editor displays the value that is being returned from the function. Similarly, <return value> is added to the locals shown in the Stack Data tool.

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