Why does property decorator show "object has no attribute"?

Question

I have the following code: ``` import sys import platform from PyQt4.QtGui import QApplication from PyQt4.QtWebKit import QWebPage class Render(QWebPage): def __init__(self): self.app = QApplication([]) QWebPage.__init__(self) @property def html(self): return self.mainFrame().toHtml.toAscii() page = Render() print sys.version, platform.platform() print 'html attribute?', [p for p in dir(page) if 'html' in p] print page.html ``` gives this exception output: ``` stav@maia:$ python property.py 2.7.3 (default, Aug 1 2012, 05:14:39) [GCC 4.6.3] Linux-3.2.0-38-generic-x86_64-with-Ubuntu-12.04-precise html attribute? ['html'] Traceback (most recent call last): File "property.py", line 18, in <module> print page.html AttributeError: 'Render' object has no attribute 'html' ``` If I remove the `@property` decorator or I remove the `.toAscii` call, then it works. But why does the error say there is no attribute even tho `dir(page)` shows it?

Accepted Answer

The issue here is that Python gave a misleading error message. The error message one would expect in this case is this:

```
AttributeError: 'function' object has no attribute 'toAscii'
```

But instead, Python gave a misleading error message:

```
AttributeError: 'Render' object has no attribute 'html'
```

That is, an `AttributeError` generated within the property function was presented as if it were an `AttributeError` for the property itself.

This strange behavior occurs when the class with your `@property` is derived from `QObject`. It is a known issue in PyQt. In fact, the PyQt maintainers claim it is expected behavior (wrongly, IMHO). See this thread for details. (In that thread, it is claimed that `QObject` behaves identically to Python's built-in `object` class, but my own testing indicates otherwise.)

Why does property decorator show "object has no attribute"?

Solution

Problem