How to capture output of Python’s interpreter and show in a Text widget?

I assume that with “output from the interpreter”, you mean output written to the console or terminal window, such as output produced with print().

All console output produced by Python gets written to the program’s output streams sys.stdout (normal output) and sys.stderr (error output, such as exception tracebacks). These are file-like objects.

You can replace these streams with your own file-like object. All your custom implementation must provide is a write(text) function. By providing your own implementation, you can forward all output to your widget:

class MyStream(object):
    def write(self, text):
        # Add text to a QTextEdit...

sys.stdout = MyStream()
sys.stderr = MyStream()

If you ever need to reset these streams, they are still available as sys.__stdout__ and sys.__stderr__:

sys.stdout = sys.__stdout__
sys.stderr = sys.__stderr__

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Update

Here is some working code for PyQt4. First define a stream that reports data written to it with a Qt signal:

from PyQt4 import QtCore

class EmittingStream(QtCore.QObject):

    textWritten = QtCore.pyqtSignal(str)

    def write(self, text):
        self.textWritten.emit(str(text))

Now, in your GUI, install an instance of this stream to sys.stdout and connect the textWritten signal to a slot that writes the text to a QTextEdit:

# Within your main window class...

def __init__(self, parent=None, **kwargs):
    # ...

    # Install the custom output stream
    sys.stdout = EmittingStream(textWritten=self.normalOutputWritten)

def __del__(self):
    # Restore sys.stdout
    sys.stdout = sys.__stdout__

def normalOutputWritten(self, text):
    """Append text to the QTextEdit."""
    # Maybe QTextEdit.append() works as well, but this is how I do it:
    cursor = self.textEdit.textCursor()
    cursor.movePosition(QtGui.QTextCursor.End)
    cursor.insertText(text)
    self.textEdit.setTextCursor(cursor)
    self.textEdit.ensureCursorVisible()