UnitConverter | Android Unit Converter App

Qt uses a concept called "Signal/Slot mechanism".

Every Qt object (QObject, which is inherited by all Qt widgets) is able to "emit" signals with a (possibly empty) amount of arguments, and that signal can be "registered" to slots, which are functions that will be called everytime that signal are emitted.^[1]

A typical case is a button (normally, a QPushButton), which has a clicked signal^[2] that is emitted whenever the button is clicked, meaning that the mouse cursor is over the button both when the mouse button is pressed and then released.

Every Qt widget has different signals depending on its purpose.
For instance, the QLineEdit control has a textChanged (that is emitted whenever the text is changed, even programmatically) and textEdited (which is only emitted when the user actually types something in it).
A QComboBox has the currentIndexChanged signal, emitted whenever the current item is changed (for instance, opening the popup and selecting a different item, or by using the mouse wheel over it).

In the following code, I'll be connecting to "slots" (in this case, simple python functions) by completely ignoring the arguments of the related signals, but there are cases for which those arguments are actually needed.

The idea is that whenever the user types something in any of the line edit fields, the related function tries to convert the typed value to a numeric one, and then, based on the current content of the comboboxes will do the proper conversion.

Consider that:

1. QLineEdit is not a suggested control for numeric input. A QSpinBox (for integers, otherwise a QDoubleSpinBox for floating points) is more suited for such situations; alternatively, setting a QIntValidator or QDoubleValidator on the line edit is possible.
2. Using combobox.addItem() allows setting user data for each item along with the visible label; I'm using that "hidden" data by means of currentData() for multiplicators that are used for the conversion; this also means that you should remove any existing item for those comboboxes from the UI in Designer.
3. Editing files generated by pyuic is considered bad practice (for a lot of reasons); I strongly suggest you to carefully read the official guidelines about using Designer, generate again the files from the UI and never touch them again (unless by overwriting with pyuic): those files should always be used as imports only. In the following code I'm assuming you've updated your UI (by removing the items in the first two combos) and converted them again to a file named unitConverterUi.py, which will NOT be the actual script you'll be running.
4. Using fixed geometries (manually positioning and resizing widgets) is rarely a good idea. Remember that what you see on your screen is almost always never what others will see on theirs. Read more about layout managers and how to use them in Designer. User interfaces should be able to adapt to the screen and system settings of the user (what is called "responsive design" in modern website design).