python from 2014b matlab - debug challanges - where is python stdout, stderr?

When running python from 2014b matlab, where does the output from the python program go?

The greater problem is I'm at a loss as to how to debug my python script. It runs well outside of matlab, but has started to fail when run inside matlab in an seemingly un-tracable manner as it has grown. I've added extra exception handlers, but can't see the output.

 except :
    exc_type, exc_value, exc_traceback = sys.exc_info()
    traceback.print_tb(exc_traceback, limit=1, file=sys.stdout)
    traceback.print_exception(exc_type, exc_value, exc_traceback, limit=5, file=sys.stdout)

(I'll change that file=sys.stdout to a real file which can help)

Any debug tips, or where I can find stdout?

Answer 1

[Robert Snoeberger]

sys.stdout should be redirected to the MATLAB command window. Do you see output with Python's print function?

Example

>> py.print('hello!')
hello!
>>

sys.stderr is not redirected. When you use the functions print_tb and print_exception, you need to tell the functions to write to sys.stdout.

As a debug tip, the exception you catch in MATLAB due to a Python error is a PyException. The PyException has a property ExceptionObject, which is the same result you get from calling sys.exc_info.

>> try
py.fractions.Fraction(1,0)
catch e
end
>> e

e = 

    PyException with properties:

      ExceptionObject: [1x1 py.tuple]
           identifier: 'MATLAB:Python:PyException'
              message: 'Python Error: both arguments should be Rational instances'
                cause: {}
                stack: [0x1 struct]

>> e.ExceptionObject

ans = 

    Python tuple with no properties.

      (<type 'exceptions.TypeError'>, TypeError('both arguments should be Rational instances',), <traceback object at 0x0000000012AE9A48>)

>> py.traceback.extract_tb(py.operator.getitem(e.ExceptionObject, int32(2)))

ans = 

    Python list with no properties.

      [('C:\\Python27\\lib\\fractions.py', 158, '__new__', 'raise TypeError("both arguments should be "')]

>>

[Chris Barnhart]

Re pyexception, I'm seeing something different. I wrote a script:

try
    py.fractions.Fraction(int16(123),int16(0))
catch e
      disp(e)
  end

    MException with properties:

      identifier: 'MATLAB:class:InvalidClass'
         message: 'The class 'matlab.exception.PyException' contains a parse error, cannot be found on MATLAB...'
           cause: {0x1 cell}
           stack: [1x1 struct]

Regarding print, I agree, I should see output, but actually I do not.

I wrote up what I've found here: 2014b python from Matlab stdout oddity (bug?)

I'm using python 2.7.9 64 bit with Matlab 64 bit (home edition). Are you using 64 bit? Possibly it is my home edition that is the problem.

[Robert Snoeberger]

Debugging will be very difficult without a valid matlab.exception.PyException class. The message says that matlab.exception.PyException contains a parse error or cannot be found. Use the which function to see if can be found.

>> which matlab.exception.PyException
C:\Program Files\MATLAB\R2014b\toolbox\matlab\external\interfaces\python\+matlab\+exception\PyException.m  % matlab.exception.PyException constructor
>>

If it is found, then try to create a PyException to check for a parse error.

>> e = matlab.exception.PyException('MATLAB:Py:Test', 'testing', [])

e = 

    PyException with properties:

      ExceptionObject: []
           identifier: 'MATLAB:Py:Test'
              message: 'testing'
                cause: {}
                stack: [0x1 struct]

>>

[Chris Barnhart]

"Debugging will be very difficult without a valid matlab.exception.PyException class. " It is very difficult!

PyException wasn't found, so I added 'C:\Program Files\MATLAB\R2014b\toolbox\matlab\external' and subfolders with pathtool. Your fraction exception now works. Thank you.

Wonder if some other config issue can explain the py.print() issue....