Benefits of using pyuic vs uic.loadUi

Question

I am currently working with python and Qt which is kind of new for me coming from the C++ version and I realised that in the oficial documentation it says that an UI file can be loaded both from .ui or creating a python class and transforming the file into .py file.

I get the benefits of using .ui it is dynamically loaded so no need to transform it into python file with every change but what are the benefits of doing that?, Do you get any improvements in run time? Is it something else?

Thanks

Answer 1

Well, this question is dangerously near to the "Opinion-based" flag, but it's also a common one and I believe it deserves at least a partial answer.

Conceptually, both using the pyuic approach and the uic.loadUi() method are the same and behave in very similar ways, but with some slight differencies.
To better explain all this, I'll use the documentation about using Designer as a reference.

pyuic approach, or the "python object" method

This is probably the most popular method, especially amongst beginners. What it does is to create a python object that is used to create the ui and, if used following the "single inheritance" approach, it also behaves as an "interface" to the ui itself, since the ui object its instance creates has all widgets available as its attributes: if you create a push button, it will be available as ui.pushButton, the first label will be ui.label and so on.

In the first example of the documentation linked above, that ui object is stand-alone; that's a very basic example (I believe it was given just to demonstrate its usage, since it wouldn't provide a lot of interaction besides the connections created within Designer) and is not very useful, but it's very similar to the single inheritance method: the button would be self.ui.pushButton, etc.

IF the "multiple inheritance" method is used, the ui object will coincide with the widget subclass. In that case, the button will be self.pushButton, the label self.label, etc.

This is very important from the python point of view, because it means that those attribute names will overwrite any other instance attribute that will use the same name: if you have a function named "saveFile" and you name the button "saveFile", you won't have any [direct] access to that instance method any more as soon as setupUi is returned. In this case, using the single inheritance method might be helpful - but, in reality, you could just be more careful about function and object names.

Finally, if you don't know what the pyuic generated file does and what's it for, you might be inclined to use it to create your program. That is wrong for a lot of reasons, but, most importantly, because you might certainly realize at some point that you have to edit your ui, and merging the new changes with your modified code is clearly a PITA you don't want to face.

I recently answered a related question, trying to explain what happens when setupUi() is called in much more depth.

Using uic.loadUi

I'd say that this is a more "modular" approach, mostly because it's much more direct: as already pointed out in the question, you don't have to constantly regenerate the ui files each time they're modified.

But, there's a catch.

First of all: obviously the loading, parsing and building of an UI from an XML file is not as fast as creating the ui directly from code (which is exactly what the pyuic file does within setupUi()).

Then, there is at least one relatively small bug about layout contents margins: when using loadUi, the default system/form margins might be completely ignored and set to 0 if not explicitly set. There is a workaround about that, explained in Size of verticalLayout is different in Qt Designer and PyQt program (thanks to eyllanesc).

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A comparison

pyuic approach

Pros:

• it's faster; in a very simple test with a hundred buttons and a tablewidget with more than 1200 items I measured the following bests:

  • pyuic loading: 33.2ms
  • loadUi loading: 51.8ms
    
    

  this ratio is obviously not linear for a multitude of reasons, but you can get the idea

• if used with the single inheritance method, it can prevent accidental instance attribute overwritings, and it also means a more "contained" object structure
• using python imports ensures a more coherent project structure, especially in the deployment process (having non-python files is a common source of problems)
• the contents of those files are actually instructive, especially for beginners

Cons:

• you always must remember to regenerate the python files everytime you update an ui; we all know how easy is to forget an apparently meaningless step like this might be, expecially after hours of coding: I've seen plenty of situations for which people was banging heads on desks (hopefully both theirs) for hours because of untraceable issues, before realizing that they just forgot to run pyuic or didn't run it on the right files; my own forehead still hurts ;-)
• file tracking: you have to count two files for each ui, and you might forget one of them along the way when migrating/forking/etc, and if you forgot an ui file it possibly means that you have to recreate it completely from scratch
• n00b alert: beginners are commonly led to think that the generated python file is the one to be used to create their programs, which is obviously wrong; unfortunately, the # WARNING! message is not clear enough (I've been almost begging the head PyQt developer about this); while this is obviously not an actual problem of this approach, right now it results in being so
• some of the contents of a pyuic generated files are usually unnecessary (most importantly, the object name, which is used only for specific cases), and that's pretty obvious, since it's automatically generated ("you might need that, so better safe than sorry"); also, related to the issue above, people might be led to think that everything pyuic creates is actually needed for a GUI, resulting in unnecessary code that decreases its readability

loadUi method

Pros:

• it's direct and immediate: you edit your ui on Designer, you save it (or, at least, you remember to do it...), and when you run your code it's already there; no fuss, no muss, and desks/foreheads are safe(r)
• file tracking and deployment: it's just one file per ui, you can put all those ui files in a separate folder, you don't have to do anything else and you don't risk to forget something on the way
• direct access to widgets (but this can be achieved using the multiple inheritance approach also)

Cons:

• the layout issue mentioned above
• possible instance attribute overwriting and no "ui" object "containment"
• slightly slower loading
• path and deployment: loading is done using os relative paths and system separators, so if you put the ui in a directory different from the py file that loads that .ui you'll have to consider that; also, some package managers use to compress everything, resulting in access errors unless paths are correctly managed

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In my opinion, all considering, the loadUi method is usually the better choice. It doesn't distract me, it allows better conceptual compartmentation (which is usually good and also follows a pattern similar to MVC much more closely, conceptually speaking) and I strongly believe it as being far less prone to programmer errors, for a multitude of reasons.

But that's clearly a matter of choice.

We should also and always remember that, like every other choice we do, using ui files is an option.
There is people who completely avoids them (as there is people who uses them literally for anything), but, like everything, it all and always depends on the context.