Why do we have a __feature__?

History

In PySide user story PYSIDE-1019, we tested certain ways to make PySide more pythonic. The first idea was to support some way to allow for snake_case function names.

This feature is possible with relatively low compatibility problems, because having the same function with different names would be not so nice, but a possible low-effort solution.

When going to true_property, things become different. When we support properties as first class objects instead of getter and setter functions, we get a conflict, because a function cannot act as a property (no braces) and be a function at the same time.

This consideration led us to the idea: Features must be selectable per-module.

Why are features selectable per-module?

Suppose you have some pre-existing code. Maybe you use some downloaded code or you generated an interface file. When you now decide to use a feature, you don’t want all this existing stuff to become incorrect. By using the statement

from __feature__ import ...

you declare that this module uses some feature. Other modules will not be influenced by this decision and can stay unchanged.

Why dunder, and why not __future__?

Especially in Python 2, but in a few cases also in Python 3, there is the future statement

from __future__ import ...

That is a statement that can only appear at the beginning of a module, and it switches how the Python parser works.

Our first idea was to mimick this behavior for PySide, although we are a bit cheating: The feature statement is not a syntactical construct, and we cannot easily forbid that it is in the middle of a module.

We then realized that the intention of Python’s __future__ import and PySide’s __feature__ import are different: While Python implies by __future__ some improvement, we do not want to associate with __feature__. We simply think that some users who come from Python may like our features, while others are used to the C++ convention and consider something that deviates from the Qt documentation as drawback.

The intention to use the from __feature__ import ... notation was the hope that people see the similarity to Python’s __future__ statement and put that import at the beginning of a module to make it very visible that this module has some special global differences.

The snake_case feature

By using the statement

from __feature__ import snake_case

all methods of all classes used in this module are changing their name.

The algorithm to change names is this:

  • if the name has less than 3 chars, or

  • if two upper chars are adjacent, or

  • if the name starts with gl (which marks OpenGL),

  • the name is returned unchanged. Otherwise

  • a single upper char C is replaced by _c

The true_property feature

By using the statement

from __feature__ import true_property

all methods of all classes used in this module which are declared in the Qt documentation as property become real properties in Python.

This feature is incompatible with the past and cannot coexist; it is the reason why the feature idea was developed at all.

Normal Properties

Normal properties have the same name as before:

QtWidgets.QLabel().color()

becomes as property

QtWidgets.QLabel().color

When there is also a setter method,

QtWidgets.QLabel().setColor(value)

becomes as property

QtWidgets.QLabel().color = value

Normal properties swallow the getter and setter functions and replace them by the property object.

Special Properties

Special properties are those with non-standard names.

QtWidgets.QLabel().size()

becomes as property

QtWidgets.QLabel().size

But here we have no setSize function, but

QtWidgets.QLabel().resize(value)

which becomes as property

QtWidgets.QLabel().size = value

In that case, the setter does not become swallowed, because so many people are used to the resize function.

Class properties

It should be mentioned that we not only support regular properties as they are known from Python. There is also the concept of class properties which always call their getter and setter:

A regular property like the aforementioned QtWidgets.QLabel has this visibility:

>>> QtWidgets.QLabel.size
<property object at 0x113a23540>
>>> QtWidgets.QLabel().size
PySide6.QtCore.QSize(640, 480)

A class property instead is also evaluated without requiring an instance:

>>> QtWidgets.QApplication.windowIcon
<PySide6.QtGui.QIcon(null) at 0x113a211c0>

You can only inspect it if you go directly to the right class dict:

>>> QtGui.QGuiApplication.__dict__["windowIcon"]
<PySide6.PyClassProperty object at 0x114fc5270>

About Property Completeness

There are many properties where the Python programmer agrees that these functions should be properties, but a few are not properties, like

>>> QtWidgets.QMainWindow.centralWidget
<method 'centralWidget' of 'PySide6.QtWidgets.QMainWindow' objects>

We are currently discussing if we should correct these rare cases, as they are probably only omissions. Having to memorize the missing properties seems to be quite cumbersome, and instead of looking all properties up in the Qt documentation, it would be easier to add all properties that should be properties and are obviously missing.

The __feature__ import

The implementation of from __feature__ import ... is built by a slight modification of the __import__ builtin. We made that explicit by assigning variables in the builtin module. This modification takes place at Qt for Python import time:

  • The original function in __import__ is kept in __orig_import__.

  • The new function is in __feature_import__ and assigned to __import__.

This function calls the Python function PySide6.support.__feature__.feature_import first, and falls back to __orig_import__ if feature import is not applicable.

Overriding __import__

This is not recommended. Import modifications should be done using import hooks, see the Python documentation on Import-Hooks.

If you would like to modify __import__ anyway without destroying the features, please override just the __orig_import__ function.

IDEs and Modifying Python stub files

Qt for Python comes with pre-generated .pyi stub files in the same location as the binary module. For instance, in the site-packages directory, you can find a QtCore.pyi file next to QtCore.abi3.so or QtCore.pyd on Windows.

When using __feature__ often with common IDEs, you may want to provide a feature-aware version of .pyi files to get a correct display. The simplest way to change them all in-place is the command

pyside6-genpyi all --feature snake_case true_property